GoBGP: BGP OPEN capability parser may read capability values outside declared CapLen boundaries

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.9 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS

• CVE-2026-49837: 0.33% estimated exploitation activity in the next 30 days; percentile 26.45 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary GoBGP contains a BGP OPEN capability parsing issue where several concrete capability decoders may parse data from the full remaining capability buffer instead of the slice bounded by the declared capability length, CapLen. A malformed BGP OPEN message can cause bytes from a following capability to be interpreted as part of the current capability. The most security-relevant case is the 4-octet AS capability, where a capability with CapLen == 0 may cause the parser to read bytes from the following capability as the 4-octet AS value. This parsed value may later affect peer AS validation during BGP session establishment. Details The issue is in the BGP OPEN capability parser under: - pkg/packet/bgp/bgp.go - pkg/packet/bgp/validate.go - The BGP OPEN optional parameter capability format includes a capability code, a capability length field, and a capability value. Each concrete capability decoder should only parse bytes inside the declared capability value boundary. In affected…

Published by the source

9 Jul 2026, 23:20 UTC

Last modified by the source

9 Jul 2026, 23:20 UTC

Source last checked

26 Oct 2026, 15:24 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-gjrg-jjr3-56cm

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# Patch Next advisory brief: CVE-2026-49837

Advisory: GoBGP: BGP OPEN capability parser may read capability values outside declared CapLen boundaries
IDs: CVE-2026-49837, GHSA-gjrg-jjr3-56cm
Record: https://patchnext.com/advisories/cve-2026-49837 (revision 1, source checked 26 Oct 2026, 15:24 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/osrg/gobgp/v4 4.6.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/osrg/gobgp/v4: affected <= 4.5.0; fixed in 4.6.0

## Suggested inspection (read-only)
- Go github.com/osrg/gobgp/v4: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/osrg/gobgp/v4.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-gjrg-jjr3-56cm (GitHub-reviewed advisory GHSA-gjrg-jjr3-56cm lists affected ranges: Go github.com/osrg/gobgp/v4 >=0 <=4.5.0)

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