UltraJSON up to 5.11.x on Python get_int_max_str_digits memory leak

Summaryinfo

A vulnerability was found in UltraJSON up to 5.11.x on Python. It has been declared as problematic. This issue affects the function get_int_max_str_digits. Executing a manipulation can lead to memory leak. The identification of this vulnerability is CVE-2026-32874. The attack may be launched remotely. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability classified as problematic was found in UltraJSON up to 5.11.x on Python. This vulnerability affects the function get_int_max_str_digits. The manipulation with an unknown input leads to a memory leak vulnerability. The CWE definition for the vulnerability is CWE-401. The product does not sufficiently track and release allocated memory after it has been used, which slowly consumes remaining memory. As an impact it is known to affect availability. CVE summarizes:

UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Versions 5.4.0 through 5.11.0 contain an accumulating memory leak in JSON parsing large (outside of the range [-2^63, 2^64 - 1]) integers. The leaked memory is a copy of the string form of the integer plus an additional NULL byte. The leak occurs irrespective of whether the integer parses successfully or is rejected due to having more than sys.get_int_max_str_digits() digits, meaning that any sized leak per malicious JSON can be achieved provided that there is no limit on the overall size of the payload. Any service that calls ujson.load()/ujson.loads()/ujson.decode() on untrusted inputs is affected and vulnerable to denial of service attacks. This issue has been fixed in version 5.12.0.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-32874 since 03/16/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1499.

The vulnerability scanner Nessus provides a plugin with the ID 303239 (Linux Distros Unpatched Vulnerability : CVE-2026-32874), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 5.12.0 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 4baeb950df780092bd3c89fc702a868e99a3a1d2 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at Tenable (303239). VulDB is the best source for vulnerability data and more expert information about this specific topic.

Productinfo

Name

• UltraJSON

Version

• 5.0
• 5.1
• 5.2
• 5.3
• 5.4
• 5.5
• 5.6
• 5.7
• 5.8
• 5.9
• 5.10
• 5.11

License

• open-source

Website

• Product: https://github.com/ultrajson/ultrajson/

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VulDB is the best source for vulnerability data and more expert information about this specific topic.

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