CVE-2026-32874 in UltraJSON
Summary
by MITRE • 03/20/2026
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.
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Analysis
by VulDB Data Team • 03/25/2026
The vulnerability identified as CVE-2026-32874 represents a critical memory leak flaw within the UltraJSON library, a high-performance JSON processing component widely utilized in Python applications. This issue manifests specifically when parsing large integers that exceed the standard integer range of [-2^63, 2^64 - 1], creating a persistent memory accumulation problem that can be exploited for denial of service attacks. The vulnerability affects all versions from 5.4.0 through 5.11.0, making it a significant concern for systems relying on this library for JSON processing operations.
The technical mechanism behind this flaw involves the library's handling of integer values that surpass the standard Python integer limits. When UltraJSON encounters such oversized integers during parsing, it creates memory copies of the string representation of these integers along with an additional NULL byte, resulting in a cumulative memory leak. This memory allocation occurs regardless of whether the parsing operation succeeds or fails due to exceeding the sys.get_int_max_str_digits() threshold, meaning the vulnerability is not limited by parsing validation but rather by the fundamental memory management approach during integer processing. The leak is particularly dangerous because it can be scaled indefinitely based on the size of the malicious input, allowing attackers to control the amount of memory consumed per malicious payload.
The operational impact of this vulnerability extends beyond simple resource exhaustion, as any service utilizing ujson.load(), ujson.loads(), or ujson.decode() functions with untrusted input becomes susceptible to denial of service conditions. Attackers can craft specially formatted JSON payloads containing arbitrarily large integers to systematically consume system memory, potentially leading to application crashes, service unavailability, or complete system resource exhaustion. This makes the vulnerability particularly attractive for attackers targeting applications that process external JSON data without proper input sanitization or size limiting mechanisms. The vulnerability's severity is amplified by its presence in widely-used Python libraries, meaning that numerous applications across different domains could be simultaneously affected.
Mitigation strategies for this vulnerability require immediate version upgrades to 5.12.0 or later, which contains the necessary fixes to prevent the memory leak behavior. Organizations should also implement input validation measures to limit JSON payload sizes and enforce reasonable integer value constraints when processing untrusted data. Additional defensive measures include monitoring memory consumption patterns in applications using UltraJSON and implementing rate limiting or input size restrictions at network boundaries. This vulnerability aligns with CWE-401, specifically addressing memory leak issues in software components, and represents a significant concern under the ATT&CK framework's resource exhaustion tactics, where adversaries exploit memory management flaws to disrupt system operations. The fix implemented in version 5.12.0 addresses the core memory allocation issue while maintaining backward compatibility with existing application functionality.