Crash due to malformed relay protocol message

Exploitation

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

CVSS

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

EPSS

• CVE-2021-21404: 1.97% estimated exploitation activity in the next 30 days; percentile 80.44 (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.

Impact 1. syncthing can be caused to crash and exit if sent a malformed relay protocol message message with a negative length field. 2. The relay server strelaysrv can be caused to crash and exit if sent a malformed relay protocol message with a negative length field. At no point is sensitive data exposed or liable to be altered due to this issue. Sensitive data is never exposed to relay operators. Syncthing itself would need to be lured to connect to a malicious relay server in order to exploit the issue. Patches Fixed in version 1.15.0. Workarounds 1. No known workaround for strelaysrv. 2. syncthing can be configured to not use relays, or to only use specific, trusted relays. If Syncthing is used in a closed environment or with relaying disabled, i.e., it does not communicate with unknown relays, Syncthing is not vulnerable. For more information If you have any questions or comments about this advisory, please discuss it on the forum. Thanks to Wojciech Paciorek for discovering and…

Published by the source

21 May 2021, 16:23 UTC

Last modified by the source

1 Feb 2023, 05:05 UTC

Source last checked

26 Oct 2026, 14:54 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-x462-89pf-6r5h

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# Patch Next advisory brief: CVE-2021-21404

Advisory: Crash due to malformed relay protocol message
IDs: CVE-2021-21404, GHSA-x462-89pf-6r5h
Record: https://patchnext.com/advisories/cve-2021-21404 (revision 1, source checked 26 Oct 2026, 14:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/syncthing/syncthing 1.15.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/syncthing/syncthing: affected < 1.15.0; fixed in 1.15.0

## Suggested inspection (read-only)
- Go github.com/syncthing/syncthing: 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/syncthing/syncthing.
- 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-x462-89pf-6r5h (GitHub-reviewed advisory GHSA-x462-89pf-6r5h lists affected ranges: Go github.com/syncthing/syncthing >=0 <1.15.0)

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