high CVSS 7.5

CVE-2025-65015·Python vulnerability

joserfc is a Python library that provides an implementation of several JSON Object Signing and Encryption (JOSE) standards. In versions from 1.3.3 to before 1.3.5 and from 1.4.0 to before 1.4.2, the ExceededSizeError exception messages are embedded with non-decoded JWT token parts and may cause Python logging to record an arbitrarily large, forged JWT payload. In situations where a misconfigured — or entirely absent — production-grade web server sits in front of a Python web application, an attacker may be able to send arbitrarily large bearer tokens in the HTTP request headers. When this occurs, Python logging or diagnostic tools (e.g., Sentry) may end up processing extremely large log messages containing the full JWT header during the joserfc.jwt.decode() operation. The same behavior also appears when validating claims and signature payload sizes, as the library raises joserfc.errors.ExceededSizeError() with the full payload embedded in the exception message. Since the payload is already fully loaded into memory at this stage, the library cannot prevent or reject it. This issue has been patched in versions 1.3.5 and 1.4.2.

Severity
high
Software
Python
Fixed in
1.4.2
Published
2025-11-18

Affected versions

From: 1.4.0

Until: 1.4.2

Fixed in: 1.4.2

How to fix this CVE

Update the joserfc library to version 1.4.2 or later to resolve a logging denial-of-service vulnerability where oversized JWT tokens cause exception messages to embed non-decoded token parts. Organizations running joserfc 1.4.0–1.4.1 should prioritize this update, especially if their logging infrastructure (e.g., Sentry) processes application errors. Verify the patched version is active and restart affected Python services.

pip install --upgrade joserfc>=1.4.2 && systemctl restart <your-python-app>

Defensia detects this vulnerability

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement request header size limits to reject HTTP requests with Bearer token headers exceeding 8KB. Configure your web application firewall (WAF) or reverse proxy (e.g., nginx, Apache) to drop requests where the Authorization header payload exceeds reasonable JWT token sizes.

How to check if you are affected

  1. Check joserfc version: python3 -c "import joserfc; print(joserfc.__version__)"
  2. Identify if joserfc is imported in your application: grep -r "from joserfc\|import joserfc" /path/to/app --include="*.py"
  3. Search application logs for ExceededSizeError exceptions: grep -i "ExceededSizeError" /var/log/application/*.log
  4. Verify patched version is running: python3 -c "import joserfc; assert joserfc.__version__ >= '1.4.2', 'Vulnerable version detected'"

FAQ

What is CVE-2025-65015?

CVE-2025-65015 is a denial-of-service vulnerability in joserfc where maliciously crafted oversized JWT tokens cause the library to embed non-decoded token data into exception messages, potentially flooding logs and diagnostic systems with arbitrarily large payloads.

Is CVE-2025-65015 being actively exploited?

No, this vulnerability is not listed on CISA's Known Exploited Vulnerabilities (KEV) catalog and no public exploits are currently available.

What versions of joserfc are affected by CVE-2025-65015?

Versions 1.4.0 through 1.4.1 are affected. Versions 1.3.3–1.3.4 are also vulnerable but are addressed by upgrading to 1.3.5 or later.

How do I check if my server is vulnerable to CVE-2025-65015?

Run 'python3 -c "import joserfc; print(joserfc.__version__)"' and verify the result is 1.4.2 or later. If it is 1.4.0 or 1.4.1, the system is vulnerable.

Does Defensia detect CVE-2025-65015?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If joserfc is installed on a monitored server, CVE-2025-65015 will appear in your dashboard with remediation steps.

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References

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