# 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.

- **Severity:** Medium 6.2 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 8 Apr 2026
- **Updated:** 8 Apr 2026
- **Also known as:** GHSA-3xxc-pwj6-jgrj, PYSEC-2026-3051, openSUSE-SU-2026:10546-1

## 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 (Improper Certificate Validation)

## Which versions are affected by CVE-2026-33753?

3 open source packages ship an affected version, including rfc3161-client. It affects 1 product.


| Package | Ecosystem | 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 |
| `python-rfc3161-client` | openSUSE:Tumbleweed | All versions | 1.0.6-1.1 |

Products: Trailofbits Rfc3161-Client.

## How do I fix CVE-2026-33753?

Upgrade rfc3161-client to 1.0.6 or later.

## Is CVE-2026-33753 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33753 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33753 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33753 exists.

Affected packages and fixed versions:
- rfc3161-client (PyPI): affected all versions; fixed in 1.0.6
- rfc3161-client (PyPI): affected all versions; fixed in 1.0.6
- rfc3161-client (PyPI): affected all versions; fixed in 1.0.6

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-33753/ (Markdown: https://insights.sbomify.com/CVE-2026-33753/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33753
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33753
- OSV: https://osv.dev/vulnerability/CVE-2026-33753

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-33753/
Summaries are generated and may contain mistakes; the official description above is the record.