rfc3161-client has insufficient verification for timestamp response signatures

CRITICAL 9.3CVE-2025-52556 · GHSA-6qhv-4h7r-2g9m · PYSEC-2026-525 · Published 21/06/2025 · updated 17/06/2026

In short

CVE-2025-52556 is a CRITICAL (CVSS 9.3) vulnerability affecting rfc3161-client.
Chance of exploitation in the next 30 days (EPSS): 0.17%, higher than 6.2% of all vulnerabilities.
Fix: update to 1.0.3 or later.

Package	Affected versions	Patched versions
rfc3161-client PyPI	`< 1.0.3`	`1.0.3`

Description

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.

NVD description

rfc3161-client is a Python library implementing the Time-Stamp Protocol (TSP) described in RFC 3161. Prior to version 1.0.3, there is a flaw in the timestamp response signature verification logic. In particular, chain verification is performed 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. This issue has been patched in version 1.0.3. There is no workaround for this issue.