GHSA-fjrm-76x2-c4q4: jwcrypto (CVE-2026-39373)

Package	Ecosystem	Affected	Fixed in
jwcrypto	PyPI	<1.5.7	1.5.7

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

JWCrypto versions before 1.5.7 are vulnerable to a denial of service through crafted JWE tokens compressed with ZIP. The size check in the earlier fix (1.5.6) applies to the compressed input, not the decompressed output, so an unauthenticated attacker can send a token under 250KB that expands to around 100MB, exhausting memory on constrained systems. Upgrade to jwcrypto 1.5.7 or later.

Written 5 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

Score	Rating	CVSS	Vector	Source
5.3	Medium	3.1	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L	CNA (GitHub_M)

CISA KEV Not listed.
EPSS 0.0031, the 23rd percentile, on 6 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

31 Oct 2026, 14:34 UTC First seen by agentprime
31 Oct 2026, 14:34 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.