rfc3161-client has insufficient verification for timestamp response signatures

Exploitation

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

CVSS 4.0

9.3 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2025-52556: 0.17% estimated exploitation activity in the next 30 days; percentile 6.28 (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 rfc3161-client 1.0.2 and earlier contain a flaw in their timestamp response signature verification logic. In particular, it performs chain verification against the TSR's embedded certificates up to the trusted root(s), but fails to verify the TSR's own signature against the timestamping leaf certificates. Consequently, vulnerable versions perform insufficient signature validation to properly consider a TSR verified, as the attacker can introduce any TSR signature so long as the embedded leaf chains up to some root TSA. Patches Users should immediately upgrade to rfc3161-client 1.0.3 or later. Workarounds There is no workaround possible. Users should immediately upgrade to a fixed version.

Published by the source

20 Jun 2025, 18:08 UTC

Last modified by the source

29 Jun 2026, 12:26 UTC

Source last checked

26 Oct 2026, 13:32 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-6qhv-4h7r-2g9m

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-6qhv-4h7r-2g9m

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# Patch Next advisory brief: CVE-2025-52556

Advisory: rfc3161-client has insufficient verification for timestamp response signatures
IDs: CVE-2025-52556, GHSA-6qhv-4h7r-2g9m
Record: https://patchnext.com/advisories/cve-2025-52556 (revision 3, source checked 26 Oct 2026, 13:32 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rfc3161-client 1.0.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI rfc3161-client: affected <= 1.0.2; fixed in 1.0.3

## Suggested inspection (read-only)
- `python -m pip show rfc3161-client`: prints the installed version. Run with the environment that serves production active.

## 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 rfc3161-client.
- 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://osv.dev/vulnerability/GHSA-6qhv-4h7r-2g9m (OSV record GHSA-6qhv-4h7r-2g9m lists affected ranges: PyPI rfc3161-client >=0 <1.0.3 (fixed))
- Verified: https://github.com/advisories/GHSA-6qhv-4h7r-2g9m (GitHub-reviewed advisory GHSA-6qhv-4h7r-2g9m lists affected ranges: PyPI rfc3161-client >=0 <=1.0.2)

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