5.3

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-39373

CVE-2026-39373: JWCrypto: JWE ZIP decompression bomb

JWCrypto implements JWK, JWS, and JWE specifications using python-cryptography. Prior to 1.5.7, an unauthenticated attacker can exhaust server memory by sending crafted JWE tokens with ZIP compression. The existing patch for CVE-2024-28102 limits input token size to 250KB but does not validate the decompressed output size. An unauthenticated attacker can cause memory exhaustion on memory-constrained systems. A token under the 250KB input limit can decompress to approximately 100MB. This vulnerability is fixed in 1.5.7.

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2026-39373

CVE-2026-39373:

5.3

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
jwcrypto	pip	<= 1.5.6

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability, identified as CVE-2026-39373, is a decompression bomb attack in the jwcrypto library. The root cause is the lack of validation on the size of the decompressed data when processing a JWE token with zip compression. The previous mitigation only checked the size of the compressed data, which is insufficient to prevent an attacker from crafting a small, highly-compressed payload that exhausts system memory upon decompression.

The analysis of the patch between versions 1.5.6 and 1.5.7 reveals the exact location of the vulnerability. The fixing commit 25db861d8b29434838669a94a843af03d29ea6ed modifies the jwcrypto/jwe.py file.

The core of the vulnerability lies within the JWE._decrypt method. In the vulnerable version, this method used zlib.decompress to decompress the payload in a single operation, without any limits on the resulting data size. The patch replaces this with zlib.decompressobj, which allows for controlled, incremental decompression and enforces a maximum size for the plaintext output (max_plaintext).

The public-facing JWE.decrypt method was also modified to accept the max_plaintext parameter, allowing developers to specify a safe limit for their applications. This parameter is then passed down to the _decrypt method. During exploitation, a call to JWE.deserialize with a malicious token and a key would trigger JWE.decrypt and subsequently the vulnerable JWE._decrypt method, leading to excessive memory allocation.

Vulnerable functions

JWE._decrypt

jwcrypto/jwe.py

The `_decrypt` method is responsible for decompressing the JWE payload. The vulnerable version of this function used `zlib.decompress` without any limit on the output size, making it susceptible to a decompression bomb attack where a small compressed input can expand to a very large output, causing memory exhaustion.

JWE.decrypt

jwcrypto/jwe.py

This public method initiates the decryption process and calls the vulnerable `_decrypt` method. It was vulnerable as it lacked a mechanism to enforce a limit on the size of the decompressed plaintext. The patch introduces a `max_plaintext` parameter to allow applications to set a safe limit, which is then passed to the `_decrypt` method.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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