UBUNTU-CVE-2026-99077: High severity ubuntu/linux vulnerability
In the Linux kernel, the following vulnerability has been resolved: ipv4: udp: Create exceptions before socket matching Currently, when ICMP Fragmentation Needed and Redirect Message packets are locally delivered and quote a UDP packet, a FIB nexthop exception (FNHE) is only created 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 FNHE cache in order to discover the ephemeral port used by a connected UDP socket. Commit 6457378fe796 ("ipv4: use siphash instead of Jenkins in fnhehashfun()") and commit 67d6d681e15b ("ipv4: make exception cache less predictible") tried to mitigate such attacks by making it harder for attackers to discover hash collisions in the FNHE 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 a FNHE, even before trying to find a matching socket. Do that by calling ipv4updatepmtu() and ipv4redirect(), 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 a FNHE being created or updated in the cache that the attacker can observe. Pass an oif of 0, in a similar fashion to icmperr(). This is also the oif used by the socket path for sockets that are not bound to a device. Note that this does not allow attackers to create FNHEs that they could not create before, as both helpers can already be reached with little to no validation. For example, by sending an ICMP error that quotes an ICMP 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 ipv4skupdatepmtu() / ipv4skredirect(). When the two resolve to different nexthops, it also results in two exceptions being created for the same destination IP. One in the FNHE cache of the nexthop resolved by the no socket helpers and another in the FNHE cache of the nexthop used by the socket.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Compensating control
Always create or update a FIB nexthop exception (FNHE) before attempting UDP socket matching for locally delivered ICMP Fragmentation Needed and Redirect Message packets; call ipv4_update_pmtu() and ipv4_redirect(), passing an oif of 0.
Event History
Frequently Asked Questions
Which systems are in scope?
The affected software list includes Ubuntu linux, linux-fips, linux-aws, linux-azure, linux-gcp, and their listed FIPS variants. The supplied information does not identify specific affected package versions or Ubuntu releases.
What does an attacker need to exploit this issue?
The attack is network-based and requires no privileges or user interaction. An off-path attacker must be able to use locally delivered ICMP Fragmentation Needed or Redirect packets that quote UDP traffic and exploit the FNHE-cache side channel; the attack complexity is rated high.
What could an attacker learn through the side channel?
The described outcome is discovery of the ephemeral port used by a connected UDP socket. The issue is rated high severity with high confidentiality and integrity impact in the supplied vector.