RFC 8116
Security Threats to the Optimized Link State Routing Protocol Version 2 (OLSRv2)
Pages: 26
Informational
Informational
Internet Engineering Task Force (IETF) T. Clausen Request for Comments: 8116 Category: Informational U. Herberg ISSN: 2070-1721 J. Yi Ecole Polytechnique May 2017 Security Threats to the Optimized Link State Routing Protocol Version 2 (OLSRv2)Abstract
This document analyzes common security threats to the Optimized Link State Routing Protocol version 2 (OLSRv2) and describes their potential impacts on Mobile Ad Hoc Network (MANET) operations. It also analyzes which of these security vulnerabilities can be mitigated when using the mandatory-to-implement security mechanisms for OLSRv2 and how the vulnerabilities are mitigated. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8116.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. OLSRv2 Overview . . . . . . . . . . . . . . . . . . . . . 5 1.1.1. Neighborhood Discovery . . . . . . . . . . . . . . . 5 1.1.2. MPR Selection . . . . . . . . . . . . . . . . . . . . 6 1.1.3. Link State Advertisement . . . . . . . . . . . . . . 6 1.2. Link State Vulnerability Taxonomy . . . . . . . . . . . . 6 1.3. OLSRv2 Attack Vectors . . . . . . . . . . . . . . . . . . 7 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Topology Map Acquisition . . . . . . . . . . . . . . . . . . 7 3.1. Attack on Jittering . . . . . . . . . . . . . . . . . . . 8 3.2. Hop Count and Hop Limit Attacks . . . . . . . . . . . . . 8 3.2.1. Modifying the Hop Limit . . . . . . . . . . . . . . . 8 3.2.2. Modifying the Hop Count . . . . . . . . . . . . . . . 9 4. Effective Topology . . . . . . . . . . . . . . . . . . . . . 10 4.1. Incorrect Forwarding . . . . . . . . . . . . . . . . . . 10 4.2. Wormholes . . . . . . . . . . . . . . . . . . . . . . . . 11 4.3. Sequence Number Attacks . . . . . . . . . . . . . . . . . 12 4.3.1. Message Sequence Number . . . . . . . . . . . . . . . 12 4.3.2. Advertised Neighbor Sequence Number (ANSN) . . . . . 12 4.4. Indirect Jamming . . . . . . . . . . . . . . . . . . . . 12 5. Inconsistent Topology . . . . . . . . . . . . . . . . . . . . 15 5.1. Identity Spoofing . . . . . . . . . . . . . . . . . . . . 15 5.2. Link Spoofing . . . . . . . . . . . . . . . . . . . . . . 17 5.2.1. Inconsistent Topology Maps Due to Link State Advertisements . . . . . . . . . . . . . . . . . . . 18 6. Mitigation of Security Vulnerabilities for OLSRv2 . . . . . . 19 6.1. Inherent OLSRv2 Resilience . . . . . . . . . . . . . . . 19 6.2. Resilience by Using RFC 7183 with OLSRv2 . . . . . . . . 20 6.2.1. Topology Map Acquisition . . . . . . . . . . . . . . 21 6.2.2. Effective Topology . . . . . . . . . . . . . . . . . 21 6.2.3. Inconsistent Topology . . . . . . . . . . . . . . . . 22 6.3. Correct Deployment . . . . . . . . . . . . . . . . . . . 22 7. Security Considerations . . . . . . . . . . . . . . . . . . . 22 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 8.1. Normative References . . . . . . . . . . . . . . . . . . 23 8.2. Informative References . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
The Optimized Link State Routing Protocol version 2 (OLSRv2) [RFC7181] is a successor to OLSR [RFC3626] as a routing protocol for Mobile Ad Hoc Networks (MANETs). OLSRv2 retains the same basic algorithms as its predecessor; however, it offers various improvements, e.g., a modular and flexible architecture allowing extensions (such as for security) to be developed as add-ons to the basic protocol. Such building blocks and modules include [RFC5148], [RFC5444], [RFC5497], [RFC6130], [RFC7182], [RFC7183], [RFC7187], [RFC7188], [RFC7466], etc. The developments reflected in OLSRv2 have been motivated by increased real-world deployment experiences, e.g., from networks such as FunkFeuer [FUNKFEUER], and the requirements to be addressed for continued successful operation of these networks. With participation in such networks increasing (the FunkFeuer community network has, e.g., roughly 400 individual participants at the time of publication of this document), operating under the assumption that participants can be "trusted" to behave in a non-destructive way, is naive. With deployment in the wider Internet, and a resultant increase in user numbers, an increase in attacks and abuses has followed necessitating a change in recommended practices. For example, SMTP servers, which were initially available for use by everyone on the Internet, require authentication and accounting for users today [RFC5068]. As OLSRv2 is often used in wireless environments, it is potentially exposed to different kinds of security threats, some of which are of greater significance when compared to wired networks. As radio signals can be received as well as transmitted by any compatible wireless device within radio range, there are commonly no physical constraints on the conformation of nodes and communication links that make up the network (as could be expected for wired networks). A first step towards hardening against attacks disrupting the connectivity of a network is to understand the vulnerabilities of the routing protocol managing the connectivity. Therefore, this document analyzes OLSRv2 in order to understand its inherent vulnerabilities and resilience. The authors do not claim completeness of the analysis but hope that the identified attacks, as presented, form a meaningful starting point for developing and deploying increasingly well-secured OLSRv2 networks. This document describes security vulnerabilities of OLSRv2 when it is used without the mandatory-to-implement security mechanisms, as specified in Section 23.5 of [RFC7181]. It also analyzes which of these security vulnerabilities can be mitigated when using the mandatory-to-implement security mechanisms for OLSRv2 and how the
vulnerabilities are mitigated. This separation is important since, as explicitly stated in [RFC7181]: Any deployment of OLSRv2 SHOULD use the security mechanism specified in [RFC7183] but MAY use another mechanism if more appropriate in an OLSRv2 deployment. For example, for longer-term OLSRv2 deployments, alternative security mechanisms (e.g., rekeying) SHOULD be considered. Moreover, this document is also based on the assumption that no additional security mechanism such as IPsec is used in the IP layer, or other mechanisms on lower layers, as not all MANET deployments may be able to accommodate such common protection mechanisms (e.g., because of limited resources of MANET routers). As NHDP is a fundamental component of OLSRv2, the vulnerabilities of NHDP, discussed in [RFC7186], also apply to OLSRv2. It should be noted that many OLSRv2 implementations are configurable, and so an attack on the configuration system (such as [RFC7939] and [RFC7184]) can be used to adversely affect the operation of an OLSRv2 implementation.1.1. OLSRv2 Overview
OLSRv2 contains three basic processes: neighborhood discovery, Multipoint Relay (MPR) selection, and Link State Advertisements (LSAs). They are described in the sections below with sufficient details to allow elaboration of the analyses in this document.1.1.1. Neighborhood Discovery
Neighborhood discovery is the process whereby each router discovers the routers that are in direct communication range of itself (1-hop neighbors) and detects with which of these it can establish bidirectional communication. Each router sends HELLO messages periodically, listing the identifiers of all the routers from which it has recently received a HELLO message as well as the "status" of the link (heard or verified bidirectional). A router A receiving a HELLO message from a neighbor router B, in which B indicates it has recently received a HELLO message from A, considers the link A-B to be bidirectional. As B lists identifiers of all its neighbors in its HELLO message, A learns the "neighbors of its neighbors" (2-hop neighbors) through this process. HELLO messages are sent periodically; however, certain events may trigger non-periodic HELLOs. OLSRv2 [RFC7181] uses NHDP [RFC6130] as its neighborhood discovery mechanism. The vulnerabilities of NHDP are analyzed in [RFC7186].
1.1.2. MPR Selection
Multipoint Relay (MPR) selection is the process whereby each router is able to identify a set of relays for efficiently conducting network-wide broadcasts. Each router designates, from among its bidirectional neighbors, a subset (the "MPR set") such that an OLSRv2-specific multicast message transmitted by the router and relayed by the MPR set can be received by all its 2-hop neighbors. MPR selection is encoded in outgoing NHDP HELLO messages. In their HELLO messages, routers may express their "willingness" to be selected as an MPR using an integer between 0 and 7 ("will never" to "will always"). This is taken into consideration for the MPR calculation and is useful, for example, when an OLSRv2 network is "planned". The set of routers having selected a given router as an MPR is the MPR selector set of that router. A study of the MPR flooding algorithm can be found in [MPR-FLOODING].1.1.3. Link State Advertisement
Link State Advertisement (LSA) is the process whereby routers determine which link state information to advertise through the network. Each router must advertise, at least, all links between itself and its MPR selectors in order to allow all routers to calculate shortest paths. Such LSAs are carried in Topology Control (TC) messages, which are broadcast through the network using the MPR flooding process described in Section 1.1.2. As a router selects MPRs only from among bidirectional neighbors, links advertised in TC are also bidirectional and routing paths calculated by OLSRv2 contain only bidirectional links. TCs are sent periodically; however, certain events may trigger non-periodic TCs.1.2. Link State Vulnerability Taxonomy
Proper functioning of OLSRv2 assumes that: o each router signals its presence in the network and the topology information that it obtained correctly; o each router can acquire and maintain a topology map that accurately reflects the effective network topology; and, o that the network converges, i.e., that all routers in the network will have sufficiently identical topology maps. An OLSRv2 network can be disrupted by breaking any of these assumptions, specifically that (a) routers may be prevented from acquiring a topology map of the network, (b) routers may acquire a
topology map that does not reflect the effective network topology, and (c) two or more routers may acquire inconsistent topology maps.1.3. OLSRv2 Attack Vectors
Besides "radio jamming", attacks on OLSRv2 consist of a compromised OLSRv2 router injecting apparently correct, but invalid, control traffic (TCs, HELLOs) into the network. A compromised OLSRv2 router can either (a) advertise erroneous information about itself (its identification and its willingness to serve as an MPR), henceforth called identity spoofing, or (b) advertise erroneous information about its relationship to other routers (pretend existence of links to other routers), henceforth called link spoofing. Such attacks may disrupt the LSA process by targeting the MPR flooding mechanism or by causing incorrect link state information to be included in TCs, causing routers to have incomplete, inaccurate, or inconsistent topology maps. In a different class of attacks, a compromised OLSRv2 router injects control traffic designed so as to cause an in-router resource exhaustion, e.g., by causing the algorithms calculating routing tables or MPR sets to be invoked continuously, preventing the internal state of a router from converging, which depletes the energy of battery-driven routers, etc.2. Terminology
This document uses the terminology and notation defined in [RFC5444], [RFC6130], and [RFC7181]. Additionally, it defines the following terminology: Compromised OLSRv2 router: An attacker that eavesdrops on the network traffic and/or generates syntactically correct OLSRv2 control messages. Control messages emitted by a compromised OLSRv2 router may contain additional information or omit information, as compared to a control message generated by a non- compromised OLSRv2 router located in the same topological position in the network. Legitimate OLSRv2 router: An OLSRv2 router that is not a compromised OLSRv2 router.3. Topology Map Acquisition
Topology Map Acquisition relates to the ability for any given router in the network to acquire a representation of the network connectivity. A router that is unable to acquire a topology map is incapable of calculating routing paths and participating in forwarding data. Topology map acquisition can be hindered by (i) TCs
not being delivered to (all) routers in the network, such as what happens in case of flooding disruption, or (ii) in case of "jamming" of the communication channel. The jamming and flooding disruption due to identity spoofing and link spoofing have been discussed in [RFC7186].3.1. Attack on Jittering
OLSRv2 incorporates a jittering mechanism: a random, but bounded, delay on outgoing control traffic [RFC5148]. This may be necessary when link layers (such as 802.11 [IEEE802.11]) are used that do not guarantee collision-free delivery of frames and where jitter can reduce the probability of collisions of frames on lower layers. In OLSRv2, TC forwarding is jittered by a value between 0 and MAX_JITTER. In order to reduce the number of transmissions, when a control message is due for transmission, OLSRv2 piggybacks all queued messages into a single transmission. Thus, if a compromised OLSRv2 router sends many TCs within a very short time interval, the jitter time of the attacked router tends towards 0. This renders jittering ineffective and can lead to collisions on the link layer. In addition to causing more collisions, forwarding a TC with little or no jittering can make sure that the TC message forwarded by a compromised router arrives before the message forwarded by legitimate routers. The compromised router can thus inject malicious content in the TC: for example, if the message identification is spoofed, the legitimate message will be discarded as a duplicate message. This preemptive action is important for some of the attacks introduced in the following sections.3.2. Hop Count and Hop Limit Attacks
The hop count and hop limit fields are the only parts of a TC that are modified when forwarding; therefore, they are not protected by integrity check mechanisms. A compromised OLSRv2 router can modify either of these when forwarding TCs.3.2.1. Modifying the Hop Limit
A compromised OLSRv2 router can decrease the hop limit when forwarding a TC. This will reduce the scope of forwarding for the message and may lead to some routers in the network not receiving that TC. Note that this is not necessarily the same as not relaying the message (i.e., setting the hop limit to 0), as is illustrated in Figure 1.
.---. | X | --'---' __ / \ / \ .---. .---. TC -----> | A | | C | '---' '---' \ .---. / \-- | B |__/ '---' Figure 1: Hop Limit Attack A TC arrives at and is forwarded by router A such that it is received by both B and the malicious X. X can forward the TC without any delay (including without jitter) such that its transmissions arrive before that of B at C. Before forwarding, it significantly reduces the hop limit of the message. Router C receives the TC, processes (and forwards) it, and marks it as already received -- causing it to discard further copies received from B. Thus, if the TC is forwarded by C, it has a very low hop limit and will not reach the whole network.3.2.2. Modifying the Hop Count
A compromised OLSRv2 router can modify the hop count when forwarding a TC. This may have two consequences: (i) if the hop count is set to the maximum value, then the TC will be forwarded no further or (ii) artificially manipulating the hop count may affect the validity time as calculated by recipients, when using distance-dependent validity times as defined in [RFC5497] (e.g., as part of a Fish Eye extension to OLSR2 [OLSR-FSR] [OLSR-FSR-Scaling]). v_time(3hops)=9s v_time(4hops)=12s v_time(5hops)=15s .---. .---. .---. .---. | A |-- ... --> | B | -------> | X |---------->| C | `---' `---' `---' `---' Figure 2: Different Validity Times Based on the Distance in Hops In Figure 2, router A sends a TC with a validity time of 9 seconds for routers in a 3-hop distance, 12 seconds for routers in a 4-hop distance, and 15 seconds in a 5-hop distance. If X is a compromised OLSRv2 router and modifies the hop count (say, by decreasing it to 3), then C will calculate the validity time of received information to 9 seconds -- after which it expires unless refreshed. If TCs from
A are sent less frequently than that up to 4 hops, this causes links advertised in such TCs to be only intermittently available to C.4. Effective Topology
Link state protocols assume that each router can acquire an accurate topology map that reflects the effective network topology. This implies that the routing protocol is able to identify a path from a source to a destination, and this path is valid for forwarding data traffic. If an attacker disturbs the correct protocol behavior, the perceived topology map of a router can permanently differ from the effective topology. Consider the example in Figure 3(a), which illustrates the topology map as acquired by router S. This topology map indicates that the routing protocol has identified that for S, a path exists to D via B, which it therefore assumes can be used for transmitting data. If B does not forward data traffic from S, then the topology map in S does not accurately reflect the effective network topology. Rather, the effective network topology from the point of view of S would be as indicated in Figure 3(b): D is not part of the network reachable from router S. .---. .---. .---. .---. .---. | S |----| B |----| D | | S |----| B | `---' `---' `---' `---' `---' (a) (b) Figure 3: Incorrect Data Traffic Forwarding Some of the attacks related to NHDP, such as message timing attacks and indirect channel overloading, are discussed in [RFC7186]. Other threats specific to OLSRv2 are further detailed in this section.4.1. Incorrect Forwarding
OLSRv2 routers exchange information using link-local transmissions (link-local multicast or limited broadcast) for their control messages, with the routing process in each router retransmitting received messages destined for network-wide diffusion. Thus, if the operating system in a router is not configured to enable forwarding, this will not affect the operating of the routing protocol or the topology map acquired by the routing protocol. It will, however, cause a discrepancy between the effective topology and the topology map, as indicated in Figures 3(a) and 3(b).
This situation is not hypothetical. A common error seen when deploying OLSRv2-based networks using a Linux-based computer as a router is to neglect enabling IP forwarding, which effectively becomes an accidental attack of this type.4.2. Wormholes
A wormhole, depicted in the example in Figure 4, may be established between two collaborating devices that are connected by an out-of- band channel. These devices send traffic through the "tunnel" to their alter ego, which "replays" the traffic. Thus, routers D and S appear as if direct neighbors and are reachable from each other in 1 hop through the tunnel, with the path through the MANET being 100 hops long. .---. .---. | S |---- ....100-hop-long path ... ---| D | `---. / `---' \ / \ / \X=============================X 1-hop path via wormhole Figure 4: Wormholing between Two Collaborating Devices Not Participating in the Routing Protocol The consequences of such a wormhole in the network depend on the detailed behavior of the wormhole. If the wormhole relays only control traffic, but not data traffic, the same considerations as in Section 4.1 apply. If, however, the wormhole relays all traffic (control and data alike), it is identical, connectivity wise, to a usable link - and the routing protocol will correctly generate a topology map reflecting the effective network topology. The efficiency of the topology obtained depends on (i) the wormhole characteristics, (ii) how the wormhole presents itself, and (iii) how paths are calculated. Assuming that paths are calculated with unit cost for all links, including the "link" presented by the wormhole, if the real characteristics of the wormhole are as if it were a path of more than 100 hops (e.g., with respect to delay, bandwidth, etc.), then the presence of the wormhole results in a degradation in performance as compared to using the non-wormhole path. Conversely, if the "link" presented by the wormhole has better characteristics, the wormhole results in improved performance.
If paths are calculated using non-unit-costs for all links, and if the cost of the "link" presented by the wormhole correctly represents the actual cost (e.g., if the cost is established through measurements across the wormhole), then the wormhole may, in the worst case, cause no degradation in performance or, in the best case, improve performance by offering a better path. If the cost of the "link" presented by the wormhole is misrepresented, then the same considerations as for unit-cost links apply. An additional consideration with regard to wormholes is that they may present topologically attractive paths for the network; however, it may be undesirable to have data traffic transit such a path. An attacker could, by virtue of introducing a wormhole, acquire the ability to record and inspect transiting data traffic.4.3. Sequence Number Attacks
OLSRv2 uses two different sequence numbers in TCs to (i) avoid processing and forwarding the same message more than once (Message Sequence Number) and to (ii) ensure that old information, arriving late due to, e.g., long paths or other delays, is not allowed to overwrite more recent information generated (Advertised Neighbor Sequence Number (ANSN)).4.3.1. Message Sequence Number
An attack may consist of a compromised OLSRv2 router spoofing the identity of another router in the network and transmitting a large number of TCs, each with different Message Sequence Numbers. Subsequent TCs with the same sequence numbers, originating from the router whose identity was spoofed, would hence be ignored until eventually information concerning these "spoofed" TCs expires.4.3.2. Advertised Neighbor Sequence Number (ANSN)
An attack may consist of a compromised OLSRv2 router spoofing the identity of another router in the network and transmitting a single TC with an ANSN significantly larger than that which was last used by the legitimate router. Routers will retain this larger ANSN as "the most recent information" and discard subsequent TCs with lower sequence numbers as being "old".
4.4. Indirect Jamming
Indirect jamming is an attack in which a compromised OLSRv2 router is, by its actions, causing legitimate routers to generate inordinate amounts of control traffic, thereby increasing both channel occupation and the overhead incurred in each router for processing this control traffic. This control traffic will be originated from legitimate routers; thus, to the wider network, the malicious device may remain undetected. The general mechanism whereby a malicious router can cause indirect jamming is for it to participate in the protocol by generating plausible control traffic and to tune this control traffic to in turn trigger receiving routers to generate additional traffic. For OLSRv2, such an indirect attack can be directed at the neighborhood discovery mechanism and the LSA mechanism, respectively. One efficient indirect jamming attack in OLSRv2 is to target control traffic destined for network-wide diffusion. This is illustrated in Figure 5. The malicious router X selects router A as an MPR at time t0 in a HELLO. This causes X to appear as MPR selector for A and, consequently, A sets X to be advertised in its "Neighbor Set" and increments the associated "Advertised Neighbor Sequence Number" (ANSN). Router A must then advertise the link between itself and X in subsequent outgoing TCs (t1), also including the ANSN in such TCs. Upon X having received this TC, it declares the link between itself and A as no longer valid (t2) in a HELLO (indicating the link to A as LOST). Since only symmetric links are advertised by OLSRv2 routers, A will (upon receipt hereof) remove X from the set of advertised neighbors and increment the ANSN. Router A will then, in subsequent TCs, advertise the remaining set of advertised neighbors (i.e., with X removed) and the corresponding ANSN (t3). Upon X having received this information in another TC from A, it may repeat this cycle, alternating advertising the link A-X as "LOST" and as "MPR".
broadcast TC ANS={} TC:() (X-A) ANSN ANSN++ ANSN .---. .---. .---. .---. | A | | A | | A | | A | '---' '---' '---' '---' ^ | ^ | | | | | | select | |indicate | | as MPR | |as LOST | .---. .---. .---. .---. | X | | X | | X | | X | '---' '---' '---' '---' t0 t1 t2 t3 Description: The malicious X flips between link status MPR and LOST. Figure 5: Indirect Jamming in Link State Advertisement Routers receiving a TC message will parse and process this message, specifically updating their topology map as a consequence of successful receipt. If the ANSN between two successive TCs from the same router has incremented, then the topology has changed and routing sets are to be recalculated. This has the potential to be a computationally costly operation. A compromised OLSRv2 router may chose to conduct this attack against all its neighbors, thus maximizing its disruptive impact on the network with relatively little overhead of its own: other than participating in the neighborhood discovery procedure, the compromised OLSRv2 router will monitor TCs generated by its neighbors and alternate the advertised status for each such neighbor between "MPR" and "LOST". The compromised OLSRv2 router will indicate its willingness to be selected as an MPR as 0 (thus avoiding selection as an MPR) and may ignore all other protocol operations while still remaining effective as an attacker. The basic operation of OLSRv2 employs periodic message emissions, and by this attack it can be ensured that each such periodic message will entail routing table recalculation in all routers in the network. If the routers in the network have "triggered TCs" enabled, this attack may also cause an increased TC frequency. Triggered TCs are intended to allow a (stable) network to have relatively low TC emission frequencies yet still allow link breakage or link emergence to be advertised through the network rapidly. A minimum message interval (typically much smaller than the regular periodic message interval) is imposed to rate-limit worst-case message emissions.
This attack can cause the TC interval to permanently become equal to the minimum message interval. [RFC7181] proposes as default that the minimum TC interval be 0.25 x TC_INTERVAL (TC_INTERVAL being the maximum interval between two TC messages from the same OLSRv2 router). Indirect jamming by a compromised OLSRv2 router can thus have two effects: (i) it may cause increased frequency of TC generation and transmission, and (ii) it will cause additional routing table recalculation in all routers in the network.5. Inconsistent Topology
Inconsistent topology maps can occur by a compromised OLSRv2 router employing either identity spoofing or link spoofing for conducting an attack against an OLSRv2 network. The threats related to NHDP, such as identity spoofing in NHDP, link spoofing in NHDP, and creating loops, have been illustrated in [RFC7186]. This section mainly addresses the vulnerabilities in [RFC7181].5.1. Identity Spoofing
Identity spoofing can be employed by a compromised OLSRv2 router via the neighborhood discovery process and via the LSA process. Either of them causes inconsistent topology maps in routers in the network. The creation of inconsistent topology maps due to neighborhood discovery has been discussed in [RFC7186]. For OLSRv2, the attack on the LSA process can also cause inconsistent topology maps. An inconsistent topology map may occur when the compromised OLSRv2 router takes part in the LSA process by selecting a neighbor as an MPR, which in turn advertises the spoofed identities of the compromised OLSRv2 router. This attack will alter the topology maps of all routers of the network. A -- B -- C -- D -- E -- F -- X (X spoofs A) Description: A compromised OLSRv2 router X spoofs the identity of A, leading to a wrongly perceived topology. Figure 6: Identity Spoofing In Figure 6, router X spoofs the address of router A. If X selects F as an MPR, all routers in the network will be informed about the link F-A by the TCs originating from F. Assuming that (the real) A
selects B as an MPR, the link B-A will also be advertised in the network. When calculating paths, B and C will calculate paths to A via B, as illustrated in Figure 7(a); for these routers, the shortest path to A is via B. E and F will calculate paths to A via F, as illustrated in Figure 7(b); for these routers, the shortest path to A is via the compromised OLSRv2 router X, and these are thus disconnected from the real A. D will have a choice, as the path calculated to A via B is of the same length as the path via the compromised OLSRv2 router X, as illustrated in Figure 7(c). In general, the following observations can be made: o The network will be split in two, with those routers closer to B than to X reaching A, whereas those routers closer to X than to B will be unable to reach A. o Routers beyond B, i.e., routers beyond 1 hop away from A, will be unable to detect this identity spoofing. The identity spoofing attack via the LSA procedure has a higher impact than the attack on the neighborhood discovery procedure since it alters the topology maps of all routers in the network and not only in the 2-hop neighborhood. However, the attack is easier to detect by other routers in the network. Since the compromised OLSRv2 router is advertised in the whole network, routers whose identities are spoofed by the compromised OLSRv2 router can detect the attack. For example, when A receives a TC from F advertising the link F-A, it can deduce that some entity is injecting incorrect link state information as it does not have F as one of its direct neighbors. (X spoofs A) A < ---- B < ---- C E ----> F ----> X (a) Routers B and C (b) Routers E and F A < --- B < --- C < --- D ---> E ---> F ----> X (X spoofs A) Description: These paths appear as calculated by the different routers in the network in presence of a compromised OLSRv2 router X, spoofing the address of A. Figure 7: Routing Paths towards A
As the compromised OLSRv2 router X does not itself send the TCs, but rather, by virtue of MPR selection, ensures that the addresses it spoofs are advertised in TCs from its MPR selector F, the attack may be difficult to counter. Simply ignoring TCs that originate from F may also suppress the link state information for other, legitimate, MPR selectors of F. Thus, identity spoofing by a compromised OLSRv2 router, participating in the LSA process by selecting MPRs only, creates a situation wherein two or more routers have substantially inconsistent topology maps: traffic for an identified destination is, depending on where in the network it appears, delivered to different routers.5.2. Link Spoofing
Link spoofing is a situation in which a router advertises non- existing links to another router (possibly not present in the network). Essentially, TCs and HELLOs both advertise links to direct neighbor routers with the difference being the scope of the advertisement. Thus, link spoofing consists of a compromised OLSRv2 router reporting that it has neighbors routers that are either not present in the network or are effectively not neighbors of the compromised OLSRv2 router. It can be noted that a situation similar to link spoofing may occur temporarily in an OLSR or OLSRv2 network without compromised OLSRv2 routers: if A was, but is no more, a neighbor of B, then A may still be advertising a link to B for the duration of the time it takes for the neighborhood discovery process to determine this changed neighborhood. In the context of this document, link spoofing refers to a persistent situation where a compromised OLSRv2 router intentionally advertises links to other routers for which it is not a direct neighbor.
5.2.1. Inconsistent Topology Maps Due to Link State Advertisements
Figure 8 illustrates a network in which the compromised OLSRv2 router X spoofs links to an existing router A by participating in the LSA process and including this non-existing link in its advertisements. A --- B --- C --- D --- E --- F --- G --- H --- X (X spoofs the link to A) Description: The compromised OLSRv2 router X advertises a spoofed link to A in its TCs; thus, all routers will record both of the links X-A and B-A. Figure 8: Link Spoofing As TCs are flooded through the network, all routers will receive and record information describing a link X-A in this link state information. If A has selected router B as an MPR, B will likewise flood this link state information through the network; thus, all routers will receive and record information describing a link B-A. When calculating routing paths, B, C, and D will calculate paths to A via B, as illustrated in Figure 9(a); for these routers, the shortest path to A is via B. F and G will calculate paths to A via X, as illustrated in Figure 9(b); for these routers, the shortest path to A is via X, and these are thus disconnected from the real router A. E will have a choice: the path calculated to A via B is of the same length as the path via X, as illustrated in Figure 9(b). A < --- B < --- C < --- D F ---> G ---> X ---> A (a) Routers B, C, and D (b) Routers F and G A < --- B < --- C < --- D < --- E ---> F ---> G ---> X ---> A (c) Router E Description: These paths appear as calculated by the different routers in the network in the presence of a compromised OLSRv2 router X, spoofing a link to router A. Figure 9: Routing Paths towards Router A
In general, the following observations can be made:
o The network will be separated in two: routers closer to B than X
will reach A, whereas routers closer to X than B will be unable to
reach A.
o Routers beyond B, i.e., routers beyond 1 hop away from A, will be
unable to detect this link spoofing.
6. Mitigation of Security Vulnerabilities for OLSRv2
As described in Section 1, [RFC7183] specifies a security mechanism
for OLSRv2 that is mandatory to implement. However, deployments may
choose to use different security mechanisms if more appropriate.
Therefore, it is important to understand both the inherent resilience
of OLSRv2 against security vulnerabilities when not using the
mechanisms specified in [RFC7183] and the protection that [RFC7183]
provides when used in a deployment.
6.1. Inherent OLSRv2 Resilience
OLSRv2 (even when used without the mandatory-to-implement security
mechanisms in [RFC7183]) provides some inherent resilience against
part of the attacks described in this document. In particular, it
provides the following resilience:
o Sequence numbers: OLSRv2 employs message sequence numbers, which
are specific per the router identity and message type. Routers
keep an "information freshness" number (ANSN) incremented each
time the content of an LSA from a router changes. This allows
rejecting both "old" information and duplicate messages, and it
provides some protection against "message replay". However, this
also presents an attack vector (Section 4.3).
o Ignoring unidirectional links: The neighborhood discovery process
detects and admits only bidirectional links for use in MPR
selection and LSA. Jamming attacks may affect only reception of
control traffic; however, OLSRv2 will correctly recognize, and
ignore, such a link that is not bidirectional.
o Message interval bounds: The frequency of control messages, with
minimum intervals imposed for HELLO and TCs. This may limit the
impact from an indirect jamming attack (Section 4.4).
o Additional reasons for rejecting control messages: The OLSRv2
specification includes a list of reasons for which an incoming
control message should be rejected as malformed -- and allows that
a protocol extension may recognize additional reasons for OLSRv2
to consider a message malformed. Together with the flexible
message format [RFC5444], this allows addition of security
mechanisms, such as digital signatures, while remaining compliant
with the OLSRv2 standard specification.
6.2. Resilience by Using RFC 7183 with OLSRv2
[RFC7183] specifies mechanisms for integrity and replay protection
for NHDP and OLSRv2 using the generalized packet/message format
described in [RFC5444] and the TLV definitions in [RFC7182]. The
specification describes how to add an Integrity Check Value (ICV) in
a TLV to each control message, providing integrity protection of the
content of the message using Hashed Message Authentication Code
(HMAC) / SHA-256. In addition, a timestamp TLV is added to the
message prior to creating the ICV, enabling replay protection of
messages. The document specifies how to sign outgoing messages and
how to verify incoming messages, as well as under which circumstances
an invalid message is rejected. Because of the HMAC/SHA-256 ICV, a
shared key between all routers in the MANET is assumed. A router
without valid credentials is not able to create an ICV that can be
correctly verified by other routers in the MANET; therefore, such an
incorrectly signed message will be rejected by other MANET routers,
and the router cannot participate in the OLSRv2 routing process
(i.e., the malicious router will be ignored by other legitimate
routers). [RFC7183] does not address the case where a router with
valid credentials has been compromised. Such a compromised router
will not be excluded from the routing process, and other means of
detecting such a router are necessary if required in a deployment:
for example, using an asymmetric key extension to [RFC7182] that
allows revocation of the access of one particular router.
In the following sections, each of the vulnerabilities described
earlier in this document will be evaluated in terms of whether OLSRv2
with the mechanisms in [RFC7183] provides sufficient protection
against the attack. It is implicitly assumed in each of the
following sections that [RFC7183] is used with OLSRv2.
6.2.1. Topology Map Acquisition
Attack on Jittering: As only OLSRv2 routers with valid credentials can participate in the routing process, a malicious router cannot reduce the jitter time of an attacked router to 0 by sending many TC messages in a short time. The attacked router would reject all the incoming messages as "invalid" and not forward them. The same applies for the case where a malicious router wants to assure that by forcing a 0 jitter interval, the message arrives before the same message forwarded by legitimate routers. Modifying the Hop Limit and the Hop Count: As the hop limit and hop count are not protected by [RFC7183] (since they are mutable fields that change at every hop), this attack is still feasible. It is possible to apply [RFC5444] packet-level protection by using ICV Packet TLV defined in [RFC7182] to provide hop-by-hop integrity protection -- at the expense of a requirement of pairwise trust between all neighbor routers.6.2.2. Effective Topology
Incorrect Forwarding: As only OLSRv2 routers with valid credentials can participate in the routing process, a malicious router will not be part of the topology of other legitimate OLSRv2 routers. Therefore, no data traffic will be sent to the malicious router for forwarding. Wormholes: Since a wormhole consists of at least two devices forwarding (unmodified) traffic, this attack is still feasible and undetectable by the OLSRv2 routing process since the attack does not involve the OLSRv2 protocol itself (but rather lower layers). By using [RFC7183], it can at least be assured that the content of the control messages is not modified while being forwarded via the wormhole. Moreover, the timestamp TLV assures that the forwarding can only be done in a short time window after the actual TC message has been sent. Message Sequence Number: As the message sequence number is included in the ICV calculation, OLSRv2 is protected against this attack. Advertised Neighbor Sequence Number (ANSN): As the ANSN is included in the ICV calculation, OLSRv2 is protected against this attack. Indirect Jamming: Since the control messages of a malicious router will be rejected by other legitimate OLSRv2 routers in the MANET, this attack is mitigated.
6.2.3. Inconsistent Topology
Identity Spoofing: Since the control messages of a malicious router will be rejected by other legitimate OLSRv2 routers in the MANET, a router without valid credentials may spoof its identity (e.g., IP source address or message originator address), but the messages will be ignored by other routers. As the mandatory mechanism in [RFC7183] uses shared keys amongst all MANET routers, a single compromised router may spoof its identity and cause harm to the network stability. Removing this one malicious router, once detected, implies rekeying all other routers in the MANET. Asymmetric keys, particularly when using identity-based signatures (such as those specified in [RFC7859]), may give the possibility of revoking single routers and verifying their identity based on the ICV itself. Link Spoofing: Similar to identity spoofing, a malicious router without valid credentials may spoof links, but its control messages will be rejected by other routers, thereby mitigating the attack. Inconsistent Topology Maps Due to LSAs: The same considerations for link spoofing apply.6.3. Correct Deployment
Other than implementing OLSRv2, including appropriate security mechanisms, the way in which the protocol is deployed is also important to ensure proper functioning and threat mitigation. For example, Section 4.1 discussed considerations due to an incorrect forwarding-policy setting, and Section 4.2 discussed considerations for when intentional wormholes are present in a deployment.7. Security Considerations
This document does not specify a protocol or a procedure but reflects on security considerations for OLSRv2 and for its constituent parts, including NHDP. The document initially analyses threats to topology map acquisition, with the assumption that no security mechanism (including the mandatory-to-implement mechanisms from [RFC7182] and [RFC7183]) is in use. Then, it proceeds to discuss how the use of [RFC7182] and [RFC7183] mitigate the identified threats. When [RFC7183] is used with routers using a single shared key, the protection offered is not effective if a compromised router has valid credentials.
8. References
8.1. Normative References
[RFC6130] Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)", RFC 6130, DOI 10.17487/RFC6130, April 2011, <http://www.rfc-editor.org/info/rfc6130>. [RFC7181] Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg, "The Optimized Link State Routing Protocol Version 2", RFC 7181, DOI 10.17487/RFC7181, April 2014, <http://www.rfc-editor.org/info/rfc7181>. [RFC7186] Yi, J., Herberg, U., and T. Clausen, "Security Threats for the Neighborhood Discovery Protocol (NHDP)", RFC 7186, DOI 10.17487/RFC7186, April 2014, <http://www.rfc-editor.org/info/rfc7186>.8.2. Informative References
[FUNKFEUER] Funkfeuer, "Funkfeuer", <https://www.funkfeuer.at/>. [IEEE802.11] IEEE, "IEEE Standard for Information technology - Telecommunications and information exchange between systems Local and metropolitan area networks - Specfic requirements Part 11: Wireless LAN Medium Access Control and Physical (PHY) Specifications", IEEE Std 802.11-2016, DOI 10.1109/IEEESTD.2016.7786995, December 2016. [MPR-FLOODING] Qayyum, A., Viennot, L., and A. Laouiti, "Multipoint Relaying: An Efficient Technique for Flooding in Mobile Wireless Networks", Proceedings of the 35th Annual Hawaii International Conference on System Sciences (HICSS '01), IEEE Computer Society, 2001. [OLSR-FSR] Clausen, T., "Combining Temporal and Spatial Partial Topology for MANET routing - Merging OLSR and FSR", Proceedings of the 2003 IEEE Conference of Wireless Personal Multimedia Communications (WPMC '03), 2003.
[OLSR-FSR-Scaling] Adjih, C., Baccelli, E., Clausen, T., Jacquet, P., and G. Rodolakis, "Fish Eye OLSR Scaling Properties", IEEE Journal of Communication and Networks (JCN), Special Issue on Mobile Ad Hoc Networks, December 2004. [RFC3626] Clausen, T., Ed. and P. Jacquet, Ed., "Optimized Link State Routing Protocol (OLSR)", RFC 3626, DOI 10.17487/RFC3626, October 2003, <http://www.rfc-editor.org/info/rfc3626>. [RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T. Finch, "Email Submission Operations: Access and Accountability Requirements", BCP 134, RFC 5068, DOI 10.17487/RFC5068, November 2007, <http://www.rfc-editor.org/info/rfc5068>. [RFC5148] Clausen, T., Dearlove, C., and B. Adamson, "Jitter Considerations in Mobile Ad Hoc Networks (MANETs)", RFC 5148, DOI 10.17487/RFC5148, February 2008, <http://www.rfc-editor.org/info/rfc5148>. [RFC5444] Clausen, T., Dearlove, C., Dean, J., and C. Adjih, "Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format", RFC 5444, DOI 10.17487/RFC5444, February 2009, <http://www.rfc-editor.org/info/rfc5444>. [RFC5497] Clausen, T. and C. Dearlove, "Representing Multi-Value Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497, DOI 10.17487/RFC5497, March 2009, <http://www.rfc-editor.org/info/rfc5497>. [RFC7182] Herberg, U., Clausen, T., and C. Dearlove, "Integrity Check Value and Timestamp TLV Definitions for Mobile Ad Hoc Networks (MANETs)", RFC 7182, DOI 10.17487/RFC7182, April 2014, <http://www.rfc-editor.org/info/rfc7182>. [RFC7183] Herberg, U., Dearlove, C., and T. Clausen, "Integrity Protection for the Neighborhood Discovery Protocol (NHDP) and Optimized Link State Routing Protocol Version 2 (OLSRv2)", RFC 7183, DOI 10.17487/RFC7183, April 2014, <http://www.rfc-editor.org/info/rfc7183>. [RFC7184] Herberg, U., Cole, R., and T. Clausen, "Definition of Managed Objects for the Optimized Link State Routing Protocol Version 2", RFC 7184, DOI 10.17487/RFC7184, April 2014, <http://www.rfc-editor.org/info/rfc7184>.
[RFC7187] Dearlove, C. and T. Clausen, "Routing Multipoint Relay Optimization for the Optimized Link State Routing Protocol Version 2 (OLSRv2)", RFC 7187, DOI 10.17487/RFC7187, April 2014, <http://www.rfc-editor.org/info/rfc7187>. [RFC7188] Dearlove, C. and T. Clausen, "Optimized Link State Routing Protocol Version 2 (OLSRv2) and MANET Neighborhood Discovery Protocol (NHDP) Extension TLVs", RFC 7188, DOI 10.17487/RFC7188, April 2014, <http://www.rfc-editor.org/info/rfc7188>. [RFC7466] Dearlove, C. and T. Clausen, "An Optimization for the Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)", RFC 7466, DOI 10.17487/RFC7466, March 2015, <http://www.rfc-editor.org/info/rfc7466>. [RFC7859] Dearlove, C., "Identity-Based Signatures for Mobile Ad Hoc Network (MANET) Routing Protocols", RFC 7859, DOI 10.17487/RFC7859, May 2016, <http://www.rfc-editor.org/info/rfc7859>. [RFC7939] Herberg, U., Cole, R., Chakeres, I., and T. Clausen, "Definition of Managed Objects for the Neighborhood Discovery Protocol", RFC 7939, DOI 10.17487/RFC7939, August 2016, <http://www.rfc-editor.org/info/rfc7939>.
Authors' Addresses
Thomas Clausen Phone: +33-6-6058-9349 Email: T.Clausen@computer.org URI: http://www.thomasclausen.org Ulrich Herberg Email: ulrich@herberg.name URI: http://www.herberg.name Jiazi Yi Ecole Polytechnique 91128 Palaiseau Cedex France Phone: +33 1 77 57 80 85 Email: jiazi@jiaziyi.com URI: http://www.jiaziyi.com/