# CVE-2026-99076: ipv6: udp: Create exceptions before socket matching


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 23 Oct 2026
- **Updated:** 23 Oct 2026
- **Also known as:** AZL-110969, BELL-CVE-2026-99076, DEBIAN-CVE-2026-99076, GHSA-r224-867j-vfxf, UBUNTU-CVE-2026-99076

## What is CVE-2026-99076?

CVE-2026-99076 is a vulnerability in Linux, published on 23 Oct 2026. 

### Official description

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.

## Which versions are affected by CVE-2026-99076?


## How do I fix CVE-2026-99076?

Upgrade to a fixed version.

## Is CVE-2026-99076 being exploited?

There is no known exploitation of CVE-2026-99076 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99076 and fix it if it is affected.

What it is: CVE-2026-99076 is a vulnerability in Linux, published on 23 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99076 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-99076/ (Markdown: https://insights.sbomify.com/CVE-2026-99076/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99076
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99076
- OSV: https://osv.dev/vulnerability/CVE-2026-99076

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99076/
Summaries are generated and may contain mistakes; the official description above is the record.