CVE-2026-53720 in pymonocypher
Summary
by MITRE • 09/03/2026
pymonocypher uses cython to wrap the Monocypher C library. Prior to version 4.0.2.8, the argon2i_32 implementation does not check the nb_blocks size. If the caller does not provide a sufficiently large buffer based on the API contract, then argon2i_32 will write past the end of the buffer and possibly corrupt the heap. This issue has been patched in version 4.0.2.8.
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Analysis
by VulDB Data Team • 09/03/2026
The vulnerability identified in pymonocypher versions prior to 4.0.2.8 represents a critical memory safety flaw rooted in insufficient input validation within the argon2i_32 implementation. This Python library serves as a wrapper around the Monocypher C cryptographic library, utilizing Cython to bridge the gap between high-level Python code and low-level C execution. The core issue arises from a failure to validate the nb_blocks parameter before proceeding with memory allocation or usage operations. In secure software design, parameters that dictate buffer sizes must be rigorously checked against expected constraints to prevent out-of-bounds access. Here, the absence of such checks allows an attacker who controls the input arguments to specify values that exceed the allocated heap space, leading directly to a heap-based buffer overflow.
From a technical perspective, this flaw constitutes a classic case of improper validation of array or memory bounds. When the nb_blocks size is not verified against the actual capacity of the provided buffer, the argon2i_32 function proceeds to write data beyond the allocated limits. This behavior aligns with CWE-120, which describes Buffer Copy without Checking Size of Input Classic Buffer Overflow, and more specifically CWE-787, Out-of-bounds Write. The consequence is severe memory corruption on the heap, where adjacent memory structures are overwritten with unintended data. Such corruption can destabilize the application's runtime environment, potentially leading to crashes or denial of service conditions as the program attempts to access invalid memory regions during subsequent operations.
The operational impact extends beyond simple instability, as heap overflows in cryptographic libraries often present opportunities for arbitrary code execution. An attacker could craft a malicious input that exploits this overflow to overwrite function pointers or other critical control data within the heap structure. This technique is consistent with ATT&CK tactic T1203, Exploitation for Client Execution, particularly when combined with social engineering or web application attacks where user-supplied data flows into cryptographic functions without sanitization. By manipulating the nb_blocks parameter, an adversary might achieve remote code execution on systems running vulnerable versions of pymonocypher, compromising confidentiality, integrity, and availability of sensitive information processed by these applications.
Mitigation strategies primarily involve upgrading to version 4.0.2.8 or later, where this validation gap has been closed through rigorous input checking mechanisms. Developers integrating pymonocypher should ensure that their build pipelines enforce dependency updates automatically to prevent exposure to known vulnerabilities in third-party cryptographic components. Additionally, implementing defense-in-depth measures such as Address Space Layout Randomization and heap protection features can mitigate the exploitability of memory corruption bugs even if they exist within dependencies. Regular security audits focusing on boundary checks for all parameters influencing memory allocation are essential to maintain robustness against similar flaws in future updates or related libraries.