CVE-2026-39373: jwcrypto Memory Exhaustion Vulnerability Due to Unvalidated JWE Token Decompression

jwcrypto Updated 20 May 2026

Summary

07/04/2026

Disclosed on

CVE-2026-39373

CVE

Yes i

Fix available

Moderate

Severity

5.3

CVSS Score

CVE-2026-39373: JWCrypto implements JWK, JWS, and JWE specifications using python-cryptography. Affects jwcrypto versions < 1.5.7. Fixed in 1.5.7. How to fix ↓

Free scan

Check if this vulnerability affects your application

Upload a manifest or SBOM and we'll show whether you're affected, plus every other risky package in the same scan.

Check my app free

No signup required · Results in about 60 seconds

Upload your dependency manifest or SBOM

Drag & drop your file here or Browse

Java Javascript PHP Python Go Rust Ruby Dart C/C++ Elixir CycloneDX SPDX

Upload a supported dependency manifest or SBOM. No installation or source-code access required.

See supported files and SBOM formats

SBOM sbom.json · bom.json · bom.xml · *.spdx.json · *.cdx.json · *.cdx.xml

Java pom.xml · gradle.lockfile · buildscript-gradle.lockfile

Javascript package-lock.json · yarn.lock · pnpm-lock.yaml

PHP composer.lock

Python poetry.lock · requirements.txt · Pipfile.lock

Go go.mod

Rust Cargo.lock

Ruby Gemfile.lock

Dart pubspec.lock

C/C++ conan.lock

Elixir mix.lock

Enter your work email to receive your report link.

Verify your email to view the full report and start 30 days of continuous monitoring. No credit card required.

Background

jwcrypto is a Python library that implements JSON Web Key (JWK), JSON Web Signature (JWS), and JSON Web Encryption (JWE) specifications using the python-cryptography library. It is widely used for secure data transmission and authentication in web applications. However, prior to version 1.5.7, jwcrypto had a vulnerability that allowed unauthenticated attackers to exploit memory resources on servers, particularly those with limited memory capacity.

Vulnerability Detail

The vulnerability arises from the handling of crafted JWE tokens that utilize ZIP compression. Although a patch was introduced to limit the input token size to 250KB, it failed to validate the size of the decompressed output. This oversight allows an attacker to send a token that, while under the input limit, can decompress to approximately 100MB, leading to memory exhaustion on affected systems. This poses a significant risk, especially for memory-constrained environments, as it can result in denial of service.

How To Fix

Minimum fixed version

1.5.7

This is the first release that contains the fix.

pip install --upgrade "jwcrypto==1.5.7"

Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →

Workaround

As a temporary workaround, you can implement input validation on the server-side to limit the size of incoming JWE tokens before processing them. However, the best solution is to update to the patched version.

Conclusion

The CVE-2026-39373 vulnerability in the jwcrypto library poses a significant risk of memory exhaustion due to unvalidated decompression of JWE tokens. It is crucial for users of this library to update to version 1.5.7 or later to mitigate this risk effectively.

See every vulnerability recorded for jwcrypto →