11. Security Considerations

This section analyzes the possible threats to the protocol. It is meant to inform protocol and application developers about the security limitations of CoAP as described in this document. As CoAP realizes a subset of the features in HTTP/1.1, the security considerations in Section 15 of [RFC2616] are also pertinent to CoAP. This section concentrates on describing limitations specific to CoAP.

11.1. Parsing the Protocol and Processing URIs

A network-facing application can exhibit vulnerabilities in its processing logic for incoming packets. Complex parsers are well- known as a likely source of such vulnerabilities, such as the ability to remotely crash a node, or even remotely execute arbitrary code on it. CoAP attempts to narrow the opportunities for introducing such vulnerabilities by reducing parser complexity, by giving the entire range of encodable values a meaning where possible, and by aggressively reducing complexity that is often caused by unnecessary choice between multiple representations that mean the same thing. Much of the URI processing has been moved to the clients, further reducing the opportunities for introducing vulnerabilities into the servers. Even so, the URI processing code in CoAP implementations is likely to be a large source of remaining vulnerabilities and should be implemented with special care. CoAP access control implementations need to ensure they don't introduce vulnerabilities through discrepancies between the code deriving access control decisions from a URI and the code finally serving up the resource addressed by the URI. The most complex parser remaining could be the one for the CoRE Link Format, although this also has been designed with a goal of reduced implementation complexity [RFC6690]. (See also Section 15.2 of [RFC2616].)

11.2. Proxying and Caching

As mentioned in Section 15.7 of [RFC2616], proxies are by their very nature men-in-the-middle, breaking any IPsec or DTLS protection that a direct CoAP message exchange might have. They are therefore interesting targets for breaking confidentiality or integrity of CoAP message exchanges. As noted in [RFC2616], they are also interesting targets for breaking availability.

The threat to confidentiality and integrity of request/response data is amplified where proxies also cache. Note that CoAP does not define any of the cache-suppressing Cache-Control options that HTTP/1.1 provides to better protect sensitive data.

For a caching implementation, any access control considerations that would apply to making the request that generated the cache entry also need to be applied to the value in the cache. This is relevant for clients that implement multiple security domains, as well as for proxies that may serve multiple clients. Also, a caching proxy MUST NOT make cached values available to requests that have lesser transport-security properties than those the proxy would require to perform request forwarding in the first place.

Unlike the "coap" scheme, responses to "coaps" identified requests are never "public" and thus MUST NOT be reused for shared caching, unless the cache is able to make equivalent access control decisions to the ones that led to the cached entry. They can, however, be reused in a private cache if the message is cacheable by default in CoAP.

Finally, a proxy that fans out Separate Responses (as opposed to piggybacked Responses) to multiple original requesters may provide additional amplification (see Section 11.3).

11.3. Risk of Amplification

CoAP servers generally reply to a request packet with a response packet. This response packet may be significantly larger than the request packet. An attacker might use CoAP nodes to turn a small attack packet into a larger attack packet, an approach known as amplification. There is therefore a danger that CoAP nodes could become implicated in denial-of-service (DoS) attacks by using the amplifying properties of the protocol: an attacker that is attempting to overload a victim but is limited in the amount of traffic it can generate can use amplification to generate a larger amount of traffic.

This is particularly a problem in nodes that enable NoSec access, are accessible from an attacker, and can access potential victims (e.g., on the general Internet), as the UDP protocol provides no way to verify the source address given in the request packet. An attacker need only place the IP address of the victim in the source address of a suitable request packet to generate a larger packet directed at the victim.

As a mitigating factor, many constrained networks will only be able to generate a small amount of traffic, which may make CoAP nodes less attractive for this attack. However, the limited capacity of the constrained network makes the network itself a likely victim of an amplification attack.

Therefore, large amplification factors SHOULD NOT be provided in the response if the request is not authenticated. A CoAP server can reduce the amount of amplification it provides to an attacker by using slicing/blocking modes of CoAP [BLOCK] and offering large resource representations only in relatively small slices. For example, for a 1000-byte resource, a 10-byte request might result in an 80-byte response (with a 64-byte block) instead of a 1016-byte response, considerably reducing the amplification provided.

CoAP also supports the use of multicast IP addresses in requests, an important requirement for M2M. Multicast CoAP requests may be the source of accidental or deliberate DoS attacks, especially over constrained networks. This specification attempts to reduce the amplification effects of multicast requests by limiting when a response is returned. To limit the possibility of malicious use, CoAP servers SHOULD NOT accept multicast requests that can not be authenticated in some way, cryptographically or by some multicast boundary limiting the potential sources. If possible, a CoAP server SHOULD limit the support for multicast requests to the specific resources where the feature is required.

On some general-purpose operating systems providing a POSIX-style API [IEEE1003.1], it is not straightforward to find out whether a packet received was addressed to a multicast address. While many implementations will know whether they have joined a multicast group, this creates a problem for packets addressed to multicast addresses of the form FF0x::1, which are received by every IPv6 node. Implementations SHOULD make use of modern APIs such as IPV6_RECVPKTINFO [RFC3542], if available, to make this determination.

11.4. IP Address Spoofing Attacks

Due to the lack of a handshake in UDP, a rogue endpoint that is free to read and write messages carried by the constrained network (i.e., NoSec or PreSharedKey deployments with a nodes/key ratio > 1:1), may easily attack a single endpoint, a group of endpoints, as well as the whole network, e.g., by:

spoofing a Reset message in response to a Confirmable message or Non-confirmable message, thus making an endpoint "deaf"; or
spoofing an ACK in response to a CON message, thus potentially preventing the sender of the CON message from retransmitting, and drowning out the actual response; or
spoofing the entire response with forged payload/options (this has different levels of impact: from single-response disruption, to much bolder attacks on the supporting infrastructure, e.g., poisoning proxy caches, or tricking validation/lookup interfaces in resource directories and, more generally, any component that stores global network state and uses CoAP as the messaging facility to handle setting or updating state is a potential target.); or
spoofing a multicast request for a target node; this may result in network congestion/collapse, a DoS attack on the victim, or forced wake-up from sleeping; or
spoofing observe messages, etc.

Response spoofing by off-path attackers can be detected and mitigated even without transport layer security by choosing a nontrivial, randomized token in the request (Section 5.3.1). [RFC4086] discusses randomness requirements for security.

In principle, other kinds of spoofing can be detected by CoAP only in case Confirmable message semantics is used, because of unexpected Acknowledgement or Reset messages coming from the deceived endpoint. But this imposes keeping track of the used Message IDs, which is not always possible, and moreover detection becomes available usually after the damage is already done. This kind of attack can be prevented using security modes other than NoSec.

With or without source address spoofing, a client can attempt to overload a server by sending requests, preferably complex ones, to a server; address spoofing makes tracing back, and blocking, this attack harder. Given that the cost of a CON request is small, this attack can easily be executed. Under this attack, a constrained node

with limited total energy available may exhaust that energy much more quickly than planned (battery depletion attack). Also, if the client uses a Confirmable message and the server responds with a Confirmable separate response to a (possibly spoofed) address that does not respond, the server will have to allocate buffer and retransmission logic for each response up to the exhaustion of MAX_TRANSMIT_SPAN, making it more likely that it runs out of resources for processing legitimate traffic. The latter problem can be mitigated somewhat by limiting the rate of responses as discussed in Section 4.7. An attacker could also spoof the address of a legitimate client; this might cause the server, if it uses separate responses, to block legitimate responses to that client because of NSTART=1. All these attacks can be prevented using a security mode other than NoSec, thus leaving only attacks on the security protocol.

11.5. Cross-Protocol Attacks

The ability to incite a CoAP endpoint to send packets to a fake source address can be used not only for amplification, but also for cross-protocol attacks against a victim listening to UDP packets at a given address (IP address and port). This would occur as follows:

o The attacker sends a message to a CoAP endpoint with the given address as the fake source address.

o The CoAP endpoint replies with a message to the given source address.

o The victim at the given address receives a UDP packet that it interprets according to the rules of a different protocol.

This may be used to circumvent firewall rules that prevent direct communication from the attacker to the victim but happen to allow communication from the CoAP endpoint (which may also host a valid role in the other protocol) to the victim.

Also, CoAP endpoints may be the victim of a cross-protocol attack generated through an endpoint of another UDP-based protocol such as DNS. In both cases, attacks are possible if the security properties of the endpoints rely on checking IP addresses (and firewalling off direct attacks sent from outside using fake IP addresses). In general, because of their lack of context, UDP-based protocols are relatively easy targets for cross-protocol attacks.

Finally, CoAP URIs transported by other means could be used to incite clients to send messages to endpoints of other protocols.

One mitigation against cross-protocol attacks is strict checking of the syntax of packets received, combined with sufficient difference in syntax. As an example, it might help if it were difficult to incite a DNS server to send a DNS response that would pass the checks of a CoAP endpoint. Unfortunately, the first two bytes of a DNS reply are an ID that can be chosen by the attacker and that map into the interesting part of the CoAP header, and the next two bytes are then interpreted as CoAP's Message ID (i.e., any value is acceptable). The DNS count words may be interpreted as multiple instances of a (nonexistent but elective) CoAP option 0, or possibly as a Token. The echoed query finally may be manufactured by the attacker to achieve a desired effect on the CoAP endpoint; the response added by the server (if any) might then just be interpreted as added payload.

                               1  1  1  1  1  1
 0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5

 Figure 15: DNS Header ([RFC1035], Section 4.1.1) vs. CoAP Message

In general, for any pair of protocols, one of the protocols can very well have been designed in a way that enables an attacker to cause the generation of replies that look like messages of the other protocol. It is often much harder to ensure or prove the absence of viable attacks than to generate examples that may not yet completely enable an attack but might be further developed by more creative minds. Cross-protocol attacks can therefore only be completely mitigated if endpoints don't authorize actions desired by an attacker just based on trusting the source IP address of a packet. Conversely, a NoSec environment that completely relies on a firewall for CoAP security not only needs to firewall off the CoAP endpoints but also all other endpoints that might be incited to send UDP messages to CoAP endpoints using some other UDP-based protocol.

In addition to the considerations above, the security considerations for DTLS with respect to cross-protocol attacks apply. For example, if the same DTLS security association ("connection") is used to carry data of multiple protocols, DTLS no longer provides protection against cross-protocol attacks between these protocols.

11.6. Constrained-Node Considerations

Implementers on constrained nodes often find themselves without a good source of entropy [RFC4086]. If that is the case, the node MUST NOT be used for processes that require good entropy, such as key generation. Instead, keys should be generated externally and added to the device during manufacturing or commissioning.

Due to their low processing power, constrained nodes are particularly susceptible to timing attacks. Special care must be taken in implementation of cryptographic primitives.

Large numbers of constrained nodes will be installed in exposed environments and will have little resistance to tampering, including recovery of keying materials. This needs to be considered when defining the scope of credentials assigned to them. In particular, assigning a shared key to a group of nodes may make any single constrained node a target for subverting the entire group.

11.1. Parsing the Protocol and Processing URIs​

11.2. Proxying and Caching​

11.3. Risk of Amplification​

11.4. IP Address Spoofing Attacks​

11.5. Cross-Protocol Attacks​

11.6. Constrained-Node Considerations​