The pymonocypher library contains a heap buffer overflow vulnerability in its argon2i_32 implementation. The issue arises because the function does not properly check the size of the nb_blocks buffer before writing to it. If the buffer is smaller than expected, this can lead to out-of-bounds writes and heap corruption. This vulnerability is identified as CVE-2026-53720 and is addressed in pymonocypher version 4.0.2.8 by adding buffer size verification. Users of versions prior to 4.0.2.8 are affected.

Exploitation of this vulnerability could result in heap corruption, which may lead to application crashes or potentially enable further exploitation depending on the context. The CVSS 4.0 vector indicates local attack vector with low complexity and no privileges or user interaction required, but with limited impact on confidentiality and availability. No known exploits are reported in the wild.

A fix is available in pymonocypher version 4.0.2.8. Users should upgrade to version 4.0.2.8 or later to remediate this vulnerability. Alternatively, ensuring that the nb_blocks buffer is correctly sized before use can mitigate the risk. No additional vendor advisories indicate other mitigation steps.