CVE-2021-33500

PuTTY before 0.75 on Windows allows remote servers to cause a denial of service (Windows GUI hang...

7.5 High

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.

Attack profile

How easy it is to exploit

Attack vector: Network
Complexity: Low
Privileges needed: None
User interaction: None

What an attacker can affect

Confidentiality: None
Integrity: None
Availability: High

Exploit likelihood

1.91%Up 0.9 points since May 22, 2021

What each source reports

CVE List Score used 7.5

Record: CVE-2021-33500
CVSS 3.1: 7.5 HighCVSS:3.1/AC:L/AV:N/A:H/C:N/I:N/PR:N/S:U/UI:N
Updated: 2026-11-02

EUVDEUVD-2021-20201 7.5

Record: EUVD-2021-20201
CVSS 3.1: 7.5 HighCVSS:3.1/AC:L/AV:N/A:H/C:N/I:N/PR:N/S:U/UI:N
Updated: 2026-11-03

GitHub AdvisoriesGHSA-vmq2-m9cv-qwqx No score

PuTTY before 0.75 on Windows allows remote servers to cause a denial of service (Windows GUI hang...

Record: GHSA-vmq2-m9cv-qwqx
Updated: 2026-11-02

NVD 7.5

Record: CVE-2021-33500
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS 2.0: 5.0 MediumAV:N/AC:L/Au:N/C:N/I:N/A:P
Updated: 2026-11-02

OSVUBUNTU-CVE-2021-33500 7.5

Record: UBUNTU-CVE-2021-33500
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated: 2026-11-02

Affected products

putty

Affected packages

Affected packages
Package	Affected	Fixed
`putty` Ubuntu:16.04:LTS	All versions, plus listed versions 0.65-20.66-10.66-20.66-30.66-4 Show 4 moreShow less 0.67-10.67-20.67-3build0.16.04.10.67-3+deb9u1build0.16.04.1	None known
`putty` Ubuntu:18.04:LTS	All versions, plus listed versions 0.70-10.70-20.70-30.70-4	None known
`putty` Ubuntu:20.04:LTS	All versions, plus listed versions 0.72-10.73-10.73-2	None known
`putty` Ubuntu:22.04:LTS	All versions, plus listed versions 0.75-30.76-10.76-2	None known
`putty` Ubuntu:24.04:LTS	All versions, plus listed versions 0.78-30.79-10.80-10.80-1build20.80-1build30.81-1	None known
`putty` Ubuntu:25.10	All versions, plus listed versions 0.83-20.83-3	None known
`putty` Ubuntu:26.10	All versions, plus listed versions 0.83-3build10.84-1	None known
`putty` Ubuntu:26.04:LTS	All versions, plus listed versions 0.83-30.83-3build1	None known

History

History
Recorded	Changed	Sources
Nov 3, 2026	aliases	EUVD
Nov 2, 2026	first indexed	CVE List, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2021-33500 is a high vulnerability (CVSS 7.5) in Putty, published May 21, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 1.91%, the 79th percentile. No source names a fixed version.