# CVE-2026-39373: Latchset JWCrypto denial of service via memory exhaustion in JWE decompression

Source: https://junglewise.ai/threats/cve-2026-39373-latchset-jwcrypto-denial-of-service-via-memory-exhaustion-in-jwe  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-07. Updated: 2026-07-24  
License: CC BY 4.0 with a link back

CVE-2026-39373 · Severity: medium · CVSS 5.3 · Published 2026-04-07

JWCrypto is a Python library used to implement encrypted and signed web tokens (JWT/JWE). A vulnerability exists where the library fails to properly limit the size of data after it has been decompressed. An unauthenticated attacker can send a specially crafted, small encrypted token that expands significantly when processed, potentially crashing the application or server by exhausting its available memory.

Technologies: [jwcrypto (PyPI)](https://junglewise.ai/threats/technologies/jwcrypto). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-39373 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-39373 being exploited in the wild.

**How severe is CVE-2026-39373?** Medium severity, with a CVSS score of 5.3, published 7 April 2026.

**Which products and versions are affected?** jwcrypto (PyPI) <= 1.5.6, ECOSYSTEM:introduced=0,fixed=1.5.7.

**Is there a fix for CVE-2026-39373?** Yes. jwcrypto (PyPI): upgrade to 1.5.7 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-39373 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-39373?** Check whether you run jwcrypto (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- jwcrypto (PyPI): upgrade to 1.5.7 or later

  ```sh
  pip install "jwcrypto>=1.5.7"
  ```


## Technical details

JWCrypto prior to version 1.5.7 is vulnerable to a data amplification (ZIP bomb) attack in its JWE decryption/decompression logic. While a previous fix (CVE-2024-28102) limited the input token size to 250KB, the library still fails to validate the size of the output after decompression. An unauthenticated remote attacker can provide a crafted JWE token with the 'zip' header set to 'DEF' that remains under the input limit but expands to approximately 100MB in memory. This can lead to denial-of-service via memory exhaustion, particularly on memory-constrained systems. The issue is rooted in the use of zlib.decompress without an explicit limit on the resulting plaintext size. This is fixed in version 1.5.7.

## Affected products

- latchset jwcrypto < 1.5.7

## Identifiers

- CVE-2026-39373
- GHSA-fjrm-76x2-c4q4
- PYSEC-2026-70

## Timeline

- 2026-04-07: disclosed
- 2026-04-07: advisory
- 1.5.7: patched

## References

- [https://github.com/latchset/jwcrypto/security/advisories/GHSA-fjrm-76x2-c4q4](https://github.com/latchset/jwcrypto/security/advisories/GHSA-fjrm-76x2-c4q4)

## Related threats

- [PYSEC-2026-1484 - JWCrypto vulnerable to JWT bomb Attack in `deserialize` function](https://junglewise.ai/threats/cve-2024-28102-jwcrypto-vulnerable-to-jwt-bomb-attack-in-deserialize-function)
- [PYSEC-2026-827 - jwcrypto token substitution can lead to authentication bypass](https://junglewise.ai/threats/cve-2022-3102-jwcrypto-token-substitution-can-lead-to-authentication-bypass)
- [DoS with algorithms that use PBKDF2 due to unbounded PBES2 Count value](https://junglewise.ai/threats/cve-2023-6681-dos-with-algorithms-that-use-pbkdf2-due-to-unbounded-pbes2-count)
- [jwcrypto lacks the Random Filling protection mechanism](https://junglewise.ai/threats/cve-2016-6298-jwcrypto-lacks-the-random-filling-protection-mechanism)