6.9 Medium

joserfc: Compact JWS size guard allocates attacker-controlled segments before rejection

CVE-2026-119284

Summary joserfc 1.7.4 defines compact-JWS header, payload, and signature limits, but calls value.split(b".") before validating them. Python bytes.split() copies the attacker-controlled segments, so the current guard allocates roughly one complete payload before rejecting it. Reproduction bash .venv-research/bin/python 06-practical-testing/preauth-memory-profile.py The input is resident before a fork-isolated verifier runs, and every signature is invalid. | Input | Peak RSS delta | Amplification | Result | |---:|---:|---:|---| | 1 MiB | 1.11 MiB | 1.11× | size rejected | | 4 MiB | 4.04 MiB | 1.01× | size rejected | | 8 MiB | 8.04 MiB | 1.01× | size rejected | | 16 MiB | 16.04 MiB | 1.00× | size rejected | | 32 MiB | 31.74 MiB | 0.99× | size rejected | | 64 MiB | 63.80 MiB | 1.00× | size rejected | The unconstrained profile is the primary evidence: joserfc allocates an additional attacker-sized object before producing its intended ExceededSizeError. A separate baseline-plus-headroom stress test raises MemoryError when that copy cannot fit, but this report does not characterize an absolute RLIMITAS value as a realistic worker memory budget. Security Argument The issue is not that…

Assessment

CVSS: 6.9 (v4.0)
Severity: Medium
EPSS: 0.00192% chance of exploitation in the next 30 days
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 30 October 2026
Last updated: 30 October 2026
Affected: pip · joserfc
Weakness: CWE-400

CVSS vectorCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

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