# CVE-2026-80634: netfilter: flowtable: avoid num_encaps underflow on bridge VLAN untag

CVE-2026-80634 is fixed in linux on 1 platform, first by DEBIAN-CVE-2026-80634, released August 29, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-80634
- Priority: High
- CVSS: 9.8 (Low)
- EPSS: 0.0060
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Aug 28, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-80634

## Description

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 ].

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 13.5 (trixie) | 6.12.115-2 | [DEBIAN-CVE-2026-80634](https://patchblox.com/security/data/fix/linux/debian-cve-2026-80634.md) | Aug 29, 2026 |
| Linux | linux Debian 14 | 7.1.5-1 | [DEBIAN-CVE-2026-80634](https://patchblox.com/security/data/fix/linux/debian-cve-2026-80634.md) | Aug 29, 2026 |

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Data as of 2026-11-09T01:18:40Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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