CVE-2025-61920 is a high-severity (CVSS 7.5) Improper Input Validation vulnerability in authlib. A fix is available for authlib — see the affected versions and patch details below.
Authlib is vulnerable to Denial of Service via Oversized JOSE Segments
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-61920.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
authlibReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Summary Authlib’s JOSE implementation accepts unbounded JWS/JWT header and signature segments. A remote attacker can craft a token whose base64url‑encoded header or signature spans hundreds of megabytes. During verification, Authlib decodes and parses the full input before it is rejected, driving CPU and memory consumption to hostile levels and enabling denial of service.
Impact
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Attack vector: unauthenticated network attacker submits a malicious JWS/JWT.
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Effect: base64 decode + JSON/crypto processing of huge buffers pegs CPU and allocates large amounts of RAM; a single request can exhaust service capacity.
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Observed behaviour: on a test host, the legacy code verified a 500 MB header, consuming ~4 GB RSS and ~9 s CPU before failing.
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Severity: High. CVSS v3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (7.5).
Affected Versions Authlib ≤ 1.6.3 (and earlier) when verifying JWS/JWT tokens. Later snapshots with 256 KB header/signature limits are not affected.
Proof of concept
Local demo (do not run against third-party systems): Download jws_segment_dos_demo.py the PoC in direcotry authlib/ Run following Command
python3 jws_segment_dos_demo.py --variant both --sizes "500MB" --fork-per-case
Environment: Python 3.13.6, Authlib 1.6.4, Linux x86_64, CPUs=8 Sample output: Refined <img width="1295" height="306" alt="image" src="https://github.com/user-attachments/assets/6dd8410f-bc36-4717-8cee-649bac9bf291" />
The compilation script prints separate “[ATTACKER]” (token construction) and “[SERVER]” (Authlib verification) RSS deltas so defenders can distinguish client-side preparation from server-side amplification. Regression tests authlib/tests/dos/test_jose_dos.py further capture the issue; the saved original_util.py/original_jws.py reproductions still accept the malicious payload.
Remediation
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Apply the upstream patch that introduces decoded size limits:
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MAX_HEADER_SEGMENT_BYTES = 256 KB
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MAX_SIGNATURE_SEGMENT_BYTES = 256 KB
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Enforce Limits in authlib/jose/util.extract_segment and _extract_signature.
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Deploy the patched release immediately.
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For additional defence in depth, reject JWS/JWT inputs above a few kilobytes at the proxy or WAF layer, and rate-limit verification endpoints.
Workarounds (temporary)
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Enforce input size limits before handing tokens to Authlib.
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Use application-level throttling to reduce amplification risk.
Resources
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Demo script: jws_segment_dos_demo.py
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Tests: authlib/tests/dos/test_jose_dos.py
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OWASP JWT Cheat Sheet (DoS guidance)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | authlib | all versions | 1.6.5pip install --upgrade 'authlib==1.6.5' |
Affected Products
authlibauthlibDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for authlib, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update authlib to 1.6.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-61920 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Users unable to upgrade may manually enforce input size limits before handing tokens to Authlib and/or use application-level throttling to reduce amplification risk.Source: Red Hat security advisory for CVE-2025-61920 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Quay 3.10 | quay/quay-rhel8:3.10.17-1764168428 | RHSA-2025:22182 |
| Red Hat Quay 3.10 | quay/quay-rhel8:3.10.17-1765384144 | RHSA-2025:23064 |
| Red Hat Quay 3.12 | quay/quay-rhel8:3.12-1765361593 | RHSA-2025:23028 |
| Red Hat Quay 3.12 | quay/quay-rhel8:3.12-1765381502 | RHSA-2025:23059 |
| Red Hat Quay 3.12 | quay/quay-rhel8:3.12-1765382602 | RHSA-2025:23060 |
| Red Hat Quay 3.13 | quay/quay-rhel8:v3.13.10-1765782391 | RHSA-2025:23176 |
| Red Hat Quay 3.14 | quay/quay-rhel8:1773097621 | RHSA-2026:4215 |
| Red Hat Quay 3.9 | quay/quay-rhel8:3.9-1764254756 | RHSA-2025:22287 |
Frequently Asked Questions
Is CVE-2025-61920 in your dependencies?
Find it across PyPI, including transitive dependencies.