JWCrypto: JWE ZIP decompression bomb

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-39373: 0.29% estimated exploitation activity in the next 30 days; percentile 22.13 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The fix for GHSA-j857-7rvv-vj97 in v1.5.6 is weak in that it does not allow to fully control the amount of plaintext the receiver is willing to deal with and provides just a weak upper bound. The patch limits input token size to 250KB but does not validate the decompressed output size. An unauthenticated attacker can craft a JWE token under the 250KB input limit that decompresses to very large data that may exceed small devices memory availability, causing Denial of Service via memory exhaustion. Although this is technically not unbounded I do recognize that it may be too much for devices and is something that could be surprising to developers, and we can do better than that. NOTE: the original report was sloppy (probably AI slop) and claimed arbitrary memory consumption, but simple testing showed that while 100MB could be decompressed a 1GB output was denied because the token exceeded the 250K compressed serialization. NOTE WELL: The proposed solution was also sloppy,…

Published by the source

8 Apr 2026, 00:16 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:42 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-fjrm-76x2-c4q4

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-fjrm-76x2-c4q4

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# Patch Next advisory brief: CVE-2026-39373

Advisory: JWCrypto: JWE ZIP decompression bomb
IDs: CVE-2026-39373, GHSA-fjrm-76x2-c4q4
Record: https://patchnext.com/advisories/cve-2026-39373 (revision 2, source checked 26 Oct 2026, 13:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: jwcrypto 1.5.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI jwcrypto: affected <= 1.5.6; fixed in 1.5.7

## Suggested inspection (read-only)
- `python -m pip show jwcrypto`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on jwcrypto.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-fjrm-76x2-c4q4 (OSV record GHSA-fjrm-76x2-c4q4 lists affected ranges: PyPI jwcrypto >=0 <1.5.7 (fixed))
- Verified: https://github.com/advisories/GHSA-fjrm-76x2-c4q4 (GitHub-reviewed advisory GHSA-fjrm-76x2-c4q4 lists affected ranges: PyPI jwcrypto >=0 <=1.5.6)

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