What’s Vulnerable › Vulnerabilities

CVE-2021-33500

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.9% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2021-05-21 · updated 2026-06-17

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.

Affected products

• PuTTY: before 0.75 — fixed in 0.75

How to fix

Upgrade to a fixed version: PuTTY 0.75. Check the vendor advisory for mitigations.

References

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

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