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ipv6: udp: Create exceptions before socket matching

Assessment

Severity
Important
CVSS base score
7.4CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
Impact
Not stated
EPSS, next 30 days
Not scored yet
Public exploit code
No Nuclei template lists it
Exploitability
Not assessed
Publicly disclosed
No
Customer action
Required: apply the update
Component
Mariner
Weakness
—
Issued by
Linux
Published
25 Oct 2026 · October 2026
Last revised
26 Oct 2026Information published.
CVSS 3.1 base metricsRed raises the risk · green lowers it
Attack vector
NetworkAV:N
Attack complexity
HighAC:H
Privileges required
NonePR:N
User interaction
NoneUI:N
Scope
UnchangedS:U
Confidentiality
HighC:H
Integrity
HighI:H
Availability
NoneA:N

Updates that fix it

0 KBs

No KB listed

MSRC lists no downloadable update, which usually means a service-side fix or a release-notes update. Check the MSRC advisory.

Affected products

1 affected product
  • azl3 kernel 6.6.157.1-1 on Azure Linux 3.0
Page history

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Explore global source statements, affected packages and advisory history for CVE-2026-99076. The catalog has a separate collection scope from Microsoft release records.

Collected exploitation and prediction evidence

Risk source coverage

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

cve-program · CVE-2026-99076

Published

ipv6: udp: Create exceptions before socket matching

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.

Source published
23 Oct 2026 12:23:19 UTC
Source modified
23 Oct 2026 12:23:19 UTC
Observed here
23 Oct 2026 16:01:20 UTC
Source terms
CVE Program Terms of Use

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An IT change radar: releases, security, known issues, retirements, documentation changes, and service status from public sources. Every item links to supporting evidence; dates and statuses can change after they are read.

Sources read 28 Oct 01:59 UTC · 272 of 272 readable · 270 fully read · documentation 469/469 current