CVE-2026-99076 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
ipv6: udp: Create exceptions before socket matching
Currently, when ICMPv6 Packet Too Big and Redirect Message packets are locally delivered and quote a UDP packet, an exception is only created in the IPv6 exception cache if the kernel can match the UDP packet to an existing socket.
This behavior allows off-path attackers to conduct a side-channel attack on the exception cache in order to discover the ephemeral port used by a connected UDP socket.
Commit 4785305c05b2 ("ipv6: use siphash in rt6_exception_hash()") and commit a00df2caffed ("ipv6: make exception cache less predictible") tried to mitigate such attacks by making it harder for attackers to discover hash collisions in the exception cache and by randomizing the number of exceptions a hash bucket can hold, respectively. Unfortunately, both of the mitigations can be bypassed.
Instead, mitigate such attacks by always creating an exception, even before trying to find a matching socket. Do that by calling ip6_update_pmtu() and ip6_redirect(), the helpers used when the quoted packet did not originate from a socket.
This means that guesses (right or wrong) from an off-path attacker will always result in an exception being created or updated in the cache that the attacker can observe.
Pass the ifindex of the ingress device and the default uid, in a similar fashion to icmpv6_err(). Unlike IPv4, an oif of 0 would not match any nexthop in ip6_redirect_nh_match() and no exception would be created in response to a Redirect Message.
Note that this does not allow attackers to create exceptions that they could not create before, as both helpers can already be reached with little to no validation. For example, by sending an ICMPv6 error that quotes an ICMPv6 Echo Reply or one that quotes a UDP source port that matches a wildcard socket.
Also note that in the good case (matched socket) the above scheme comes at the cost of an extra route lookup, as the no socket helpers perform their own lookup before the one performed by ip6_sk_update_pmtu() / ip6_sk_redirect(). When the two resolve to different nexthops, it also results in two exceptions being created for the same destination IP. One in the exception cache of the nexthop resolved by the no socket helpers and another in the exception cache of the nexthop used by the socket.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel contained a vulnerability within its IPv6 UDP implementation that allowed off-path attackers to conduct side-channel attacks against the network stack's exception caching mechanism. The core issue stemmed from the conditional creation of Path Maximum Transmission Unit (PMTU) and redirect exceptions in response to ICMPv6 error messages, specifically Packet Too Big and Redirect Message packets. When these ICMPv6 errors were locally delivered and quoted a UDP packet, the kernel only created an entry in the IPv6 exception cache if it could successfully match the quoted packet to an existing active socket. This conditional logic introduced a timing-based side channel because the presence or absence of an exception cache entry depended on whether the attacker guessed the correct ephemeral port used by a connected UDP socket. If the guess was incorrect, no exception would be created; if correct, one would appear, allowing the attacker to infer private network state information such as active connection details and internal IP configurations.
Previous mitigation attempts had focused on obfuscating the hash function and randomizing bucket sizes to make collision discovery more difficult for attackers. Specifically, commits introducing siphash into rt6_exception_hash() and making exception cache buckets less predictable were intended to raise the barrier for exploitation by increasing computational complexity and reducing predictability of hash collisions. However, these measures proved insufficient because they did not address the fundamental logic flaw that tied exception creation to socket matching success. Attackers could still bypass these mitigations by observing whether an entry existed in the cache after sending crafted ICMPv6 packets with varying port guesses. The lack of uniform behavior for all guessed ports meant that the side channel remained viable, enabling reconnaissance attacks against systems where UDP sockets were actively used for communication.
The resolution to this vulnerability involved a structural change to how ICMPv6 errors are processed by ensuring that exceptions are always created in the cache regardless of whether a matching socket is found. This was achieved by invoking ip6_update_pmtu() and ip6_redirect(), helper functions typically reserved for cases where the quoted packet did not originate from a local socket, before attempting any socket lookup logic. By decoupling exception creation from socket existence, the kernel ensures that every guess made by an attacker results in a cache update or insertion. This uniformity eliminates the side channel because the observable state of the cache no longer reveals whether a specific port is bound to an active socket. The implementation also correctly passes interface index and default user ID parameters to these helpers, ensuring proper routing resolution for redirect messages which require valid outgoing interface specifications to function correctly in IPv6 environments.
This fix introduces minor performance overhead due to additional route lookups performed by the no-socket helper functions before the standard socket-based lookup occurs. In scenarios where a matching socket exists and both lookups resolve to different next hops, two separate exceptions may be created for the same destination IP address—one associated with the routing decision of the helper function and another linked to the actual socket's path. While this results in redundant cache entries under normal operation, it is an acceptable trade-off given that these helpers are already reachable with minimal validation constraints. Attackers could previously trigger exception creation through other vectors such as ICMPv6 Echo Replies or wildcard UDP port matches, meaning the fix does not expand the attack surface but rather neutralizes the specific side-channel exploitation path related to ephemeral port discovery.
From a classification perspective, this vulnerability aligns with CWE-200: Information Exposure and CWE-358: Improperly Implemented Security Check for Conditions within the context of network protocol handling. The exploit technique corresponds to ATT&CK T1046: Network Service Discovery, specifically leveraging ICMPv6 error messages as a probe mechanism to infer internal network topology and active service ports. Defense in depth strategies should include monitoring for anomalous patterns of ICMPv6 traffic directed at local addresses, particularly those containing quoted UDP packets with sequential or randomized port numbers indicative of scanning activity. Additionally, deploying host-based intrusion detection systems capable of detecting side-channel timing variations can provide early warning indicators of such reconnaissance attempts before they lead to successful exploitation.