Bug 2525478 (CVE-2026-80634)

Summary: CVE-2026-80634 kernel: netfilter: flowtable: avoid num_encaps underflow on bridge VLAN untag
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's netfilter flowtable component. An integer underflow can occur during bridge Virtual Local Area Network (VLAN) untagging operations. If the `num_encaps` value is zero, a decrement operation can cause it to wrap to 255. This can lead to an out-of-bounds stack read when processing network paths, potentially resulting in information disclosure or system instability if a malformed bridge path stack is processed.
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Description OSIDB Bzimport 2026-08-28 07:47:03 UTC 
In the Linux kernel, the following vulnerability has been resolved:

netfilter: flowtable: avoid num_encaps underflow on bridge VLAN untag

The DEV_PATH_BR_VLAN_UNTAG case post-decrements info->num_encaps
inside WARN_ON_ONCE(). num_encaps is u8, so if it's already 0 the
decrement still happens and wraps it to 255. The break only leaves
the inner switch -- a later path entry can set info->indev back to
a real device, and we end up returning with num_encaps == 255.

nft_dev_forward_path() then walks info.encap[] (size 2) up to
num_encaps, which means an OOB stack read and a bogus count copied
into the route descriptor.

Should only happen on a malformed bridge path stack, hence the WARN,
but worth handling sanely. Move the decrement out of the WARN.

[ While at this, remove the WARN_ON_ONCE since this can only happen
  with a buggy bridge path stack --pablo ].
Comment 1 gkamathe 2026-09-01 08:19:20 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026082839-CVE-2026-80634-0f62@gregkh/T