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 <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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0.10.100.110.120.12.10.130.140.14.10.14.20.14.3+42 more1.6.5Exploitability
AV:NAC:LPR:NUI:NScope
S:UImpact
C:NI:NA:HCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H