Vulnerability · Published Apr 8, 2026 · Assigned by GitHub_M
CVE-2026-33753Improper Certificate Validation in rfc3161-client
Medium 6.2
In plain English
A medium-severity improper certificate validation in rfc3161-client from trailofbits. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 9% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
rfc3161-client is a Python library implementing the Time-Stamp Protocol (TSP) described in RFC 3161. Prior to 1.0.6, an Authorization Bypass vulnerability in rfc3161-client's signature verification allows any attacker to impersonate a trusted TimeStamping Authority (TSA). By exploiting a logic flaw in how the library extracts the leaf certificate from an unordered PKCS#7 bag of certificates, an attacker can append a spoofed certificate matching the target common_name and Extended Key Usage (EKU) requirements. This tricks the library into verifying these authorization rules against the forged certificate while validating the cryptographic signature against an actual trusted TSA (such as FreeTSA), thereby bypassing the intended TSA authorization pinning entirely. This vulnerability is fixed in 1.0.6.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| PyPI | rfc3161-client | 1.0.6 | GHSA-3xxc-pwj6-jgrj |
| PyPI | rfc3161-client | 1.0.6 | PYSEC-2026-3051 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| trailofbits | rfc3161-client | < 1.0.6 |
Weakness: CWE-295 · Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N