CVE-2026-33753 Improper Certificate Validation in rfc3161-client
Attackers can bypass TimeStamping Authority authorization checks by spoofing certificates, allowing impersonation of trusted TSAs in rfc3161-client.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-33753?
CVE-2026-33753 is a medium-severity authentication bypass vulnerability in trailofbits rfc3161-client, published on 8 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.
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.
Weakness: CWE-295
Which versions are affected by CVE-2026-33753?
3 open source packages ship an affected version, including rfc3161-client. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
rfc3161-client PyPI | All versions | 1.0.6 |
rfc3161-client PyPI | All versions | 1.0.6 |
rfc3161-client PyPI | All versions | 1.0.6 |
Linux distribution packages (1)
python-rfc3161-client openSUSE:Tumbleweed | All versions | 1.0.6-1.1 |
Products include Trailofbits Rfc3161-Client.
How do I fix CVE-2026-33753?
Upgrade rfc3161-client to 1.0.6 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-33753 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33753 exists.
Sources
Also known as GHSA-3xxc-pwj6-jgrj, PYSEC-2026-3051, openSUSE-SU-2026:10546-1.