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CVE-2026-33753 — Improper Certificate Validation in rfc3161-client

Affected: trailofbits rfc3161-client

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.

GHSA-3xxc-pwj6-jgrj — rfc3161-client Has Improper Certificate Validation

Affected: PyPI: rfc3161-client

### Summary

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.

### Details

The root cause lies in `rfc3161_client.verify.Verifier._verify_leaf_certs()`. The library attempts to locate the leaf certificate within the parsed TimeStampResponse PKCS#7 `SignedData` bag using a naive algorithm:

```python leaf_certificate_found = None for cert in certs: if not [c for c in certs if c.issuer == cert.subject]: leaf_certificate_found = cert break ```

This loop erroneously assumes that the valid leaf certificate is simply the first certificate in the bag that does not issue any other certificate. It does **not** rely on checking the `ESSCertID` or `ESSCertIDv2` cryptographic bindings specified in RFC 3161 (which binds the signature securely to the exact signer certificate).

An attacker can exploit this by: