Summary
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
Attack vector: unauthenticated network attacker submits a malicious JWS/JWT.
Effect: base64 decode + JSON/crypto processing of huge buffers pegs CPU and allocates large amounts of RAM; a single request can exhaust service capacity.
Observed behaviour: on a test host, the legacy code verified a 500 MB header, consuming ~4 GB RSS and ~9 s CPU before failing.
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
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
Apply the upstream patch that introduces decoded size limits:
MAX_HEADER_SEGMENT_BYTES = 256 KB
MAX_SIGNATURE_SEGMENT_BYTES = 256 KB
Enforce Limits in authlib/jose/util.extract_segment and _extract_signature.
Deploy the patched release immediately.
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)
Enforce input size limits before handing tokens to Authlib.
Use application-level throttling to reduce amplification risk.
Resources
Demo script: jws_segment_dos_demo.py
Tests: authlib/tests/dos/test_jose_dos.py
OWASP JWT Cheat Sheet (DoS guidance)
Impact
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2025-61920 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.6.5); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2025-61920? CVE-2025-61920 is a high-severity improper input validation vulnerability in authlib (pip), affecting versions < 1.6.5. It is fixed in 1.6.5. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- How severe is CVE-2025-61920? CVE-2025-61920 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of authlib are affected by CVE-2025-61920? authlib (pip) versions < 1.6.5 is affected.
- Is there a fix for CVE-2025-61920? Yes. CVE-2025-61920 is fixed in 1.6.5. Upgrade to this version or later.
- Is CVE-2025-61920 exploitable, and should I be worried? Whether CVE-2025-61920 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-61920 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-61920? Upgrade
authlibto 1.6.5 or later.