medium CVSS 6.2 EPSS <0.1%

CVE-2026-33753·Python vulnerability

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.

Severity
medium
Software
Python
Fixed in
1.0.6
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — in the bottom 1% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Affected versions

Until: 1.0.6

Fixed in: 1.0.6

How to fix this CVE

Update the rfc3161-client library to version 1.0.6 or later to address a certificate validation bypass that allows attackers to impersonate trusted Time-Stamping Authorities. This vulnerability affects applications using the library for timestamp verification, so patching should be prioritized in production environments. Verify the update is applied by checking the installed package version against 1.0.6 and restarting dependent services.

sudo dnf update python3-rfc3161-client

Defensia detects this vulnerability

How to check if you are affected

  1. Run 'pip show rfc3161-client' or 'python3 -m pip show rfc3161-client' to check the installed version of the rfc3161-client library.
  2. Search your codebase for imports of 'rfc3161_client' or direct usage of TSP/timestamp verification functions to determine if the vulnerable component is in use.
  3. Query application logs for successful timestamp verifications followed by unusual certificate chains or mismatched issuer names in audit trails.
  4. After updating, re-run 'pip show rfc3161-client' to confirm version 1.0.6 or later is installed, then test timestamp validation with a known trusted TSA endpoint.

FAQ

What is CVE-2026-33753?

This is a certificate validation flaw in the rfc3161-client Python library that allows an attacker to forge a fake Time-Stamping Authority certificate by exploiting how the library selects certificates from a PKCS#7 bundle, bypassing the pinning of trusted TSAs.

Is CVE-2026-33753 being actively exploited?

No, there is currently no evidence of active exploitation in the wild, and no public exploit code is available for this vulnerability.

What versions of Python are affected by CVE-2026-33753?

All versions of rfc3161-client prior to 1.0.6 are affected; Python version itself is not directly impacted, only applications using the vulnerable library.

How do I check if my server is vulnerable to CVE-2026-33753?

Run 'pip show rfc3161-client' and compare the version number to 1.0.6; if your installed version is lower, your environment is vulnerable.

Does Defensia detect CVE-2026-33753?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If rfc3161-client is installed on a monitored server, CVE-2026-33753 will appear in your dashboard with remediation steps.

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References

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