CVE-2021-33500
CVE-2021-33500 is a high-severity vulnerability in Putty with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low.
Key facts
- Severity: High (CVSS 3.x base score 7.5)
- CVSS v2: 5.0
- EPSS exploit prediction: 2% (78th percentile)
- Actively exploited: Not listed in CISA KEV
- Affected product: Putty
- Published:
- Last modified:
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.
Frequently asked questions
- What is 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 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.
- How severe is CVE-2021-33500?
- CVE-2021-33500 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
- Is CVE-2021-33500 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (78th percentile), an estimate of the probability of exploitation in the next 30 days.
- What products are affected by CVE-2021-33500?
- CVE-2021-33500 affects Putty. See the affected-products list for the exact vulnerable versions.
- How do I fix CVE-2021-33500?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
- When was CVE-2021-33500 published?
- CVE-2021-33500 was published on 2021-05-21 and last updated on 2026-06-17.
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
Affected products (1)
- cpe:2.3:a:putty:putty:*:*:*:*:*:*:*:*
More vulnerabilities in Putty
- CVE-2004-1008 — Critical (CVSS 10.0): Integer signedness error in the ssh2_rdpkt function in PuTTY before 0.56 allows remote attackers to execute arbitrary…
- CVE-2002-1357 — Critical (CVSS 10.0): Multiple SSH2 servers and clients do not properly handle packets or data elements with incorrect length specifiers,…
- CVE-2002-1358 — Critical (CVSS 10.0): Multiple SSH2 servers and clients do not properly handle lists with empty elements or strings, which may allow remote…
- CVE-2002-1360 — Critical (CVSS 10.0): Multiple SSH2 servers and clients do not properly handle strings with null characters in them when the string length is…
- CVE-2002-1359 — Critical (CVSS 10.0): Multiple SSH2 servers and clients do not properly handle large packets or large fields, which may allow remote…
- CVE-2019-17067 — Critical (CVSS 9.8): PuTTY before 0.73 on Windows improperly opens port-forwarding listening sockets, which allows attackers to listen on…