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CVE-2021-36367

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a...

8.1 HighFix available

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
Required

What an attacker can affect

Confidentiality
High
Integrity
High
Availability
None

Exploit likelihood

1.11%Up 0.5 points since Jul 10, 2021
90%50%10%1%

What each source reports

NVD Score used 8.1

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Record
CVE-2021-36367
CVSS 3.1
8.1 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
CVSS 2.0
5.8 MediumAV:N/AC:M/Au:N/C:P/I:P/A:N
Weakness
CWE-345
Updated
2026-11-02
CVE List 8.1

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Record
CVE-2021-36367
CVSS 3.1
8.1 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Weakness
CWE-345
Updated
2026-11-02
EUVDEUVD-2021-22987 8.1

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Record
EUVD-2021-22987
CVSS 3.1
8.1 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Updated
2026-11-03
GitHub AdvisoriesGHSA-wccm-6vx2-7p8c 8.1

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a...

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Record
GHSA-wccm-6vx2-7p8c
CVSS 3.1
8.1 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Weakness
CWE-345
Updated
2026-11-02
OSVALPINE-CVE-2021-36367 8.1

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Record
ALPINE-CVE-2021-36367
CVSS 3.1
8.1 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Updated
2026-11-03
OSV 8.1

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Record
CVE-2021-36367
CVSS 3.1
8.1 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Updated
2026-11-02
OSVDEBIAN-CVE-2021-36367 8.1

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Record
DEBIAN-CVE-2021-36367
CVSS 3.1
8.1 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Updated
2026-11-02
OSVUBUNTU-CVE-2021-36367 8.1

PuTTY through 0.75 proceeds with establishing an SSH session even if it has never sent a substantive authentication response. This makes it easier for an attacker-controlled SSH server to present a later spoofed authentication prompt (that the attacker can use to capture credential data, and use that data for purposes that are undesired by the client user).

Record
UBUNTU-CVE-2021-36367
CVSS 3.1
8.1 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
putty
Alpine:v3.11
before 0.76-r0
0.76-r0
putty
Alpine:v3.12
before 0.76-r0
0.76-r0
putty
Alpine:v3.13
before 0.76-r0
0.76-r0
putty
Alpine:v3.14
before 0.76-r0
0.76-r0
putty
Alpine:v3.15
before 0.76-r0
0.76-r0
putty
Alpine:v3.16
before 0.76-r0
0.76-r0
putty
Alpine:v3.17
before 0.76-r0
0.76-r0
putty
Alpine:v3.18
before 0.76-r0
0.76-r0
putty
Alpine:v3.19
before 0.76-r0
0.76-r0
putty
Alpine:v3.20
before 0.76-r0
0.76-r0
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Affected packages
PackageAffectedFixed
putty
Alpine:v3.21
before 0.76-r0
0.76-r0
putty
Alpine:v3.22
before 0.76-r0
0.76-r0
putty
Alpine:v3.23
before 0.76-r0
0.76-r0
putty
Alpine:v3.24
before 0.76-r0
0.76-r0
putty
Debian:12
before 0.75-3
0.75-3
putty
Debian:13
before 0.75-3
0.75-3
putty
Debian:14
before 0.75-3
0.75-3
putty
Debian:10
before 0.74-1+deb11u1~deb10u1, plus listed versions
0.70-60.71-10.72-10.72-20.73-10.73-20.73-30.74-1
0.74-1+deb11u1~deb10u1
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

History

History
RecordedChangedSources
affectedOSV
aliasesEUVD
first indexedCVE List, GitHub Advisories, NVD, OSV

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

In short

CVE-2021-36367 is a high vulnerability (CVSS 8.1) in Putty, published July 9, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 1.11%, the 65th percentile. Fixed in putty 0.76-r0.

References

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