CVE-2021-36367Network attack vector

Credential capture via trivial SSH authentication success

Published Jul 9, 2021 · Updated Aug 4, 2024

Authentication-prompt spoofing in PuTTY 0.75 and earlier allows remote attackers to capture credentials through a malicious SSH server. PuTTY accepts SSH_MSG_USERAUTH_SUCCESS without substantive client authentication, letting server-controlled session output imitate a later authentication prompt. The user must initiate the connection and submit a secret to the forged prompt; upstream disputes classifying this standards-permitted behavior as a vulnerability.

CVSS severity8.1
High
EPSS probability1.11%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Authentication-prompt spoofing in PuTTY 0.75 and earlier allows remote attackers to capture credentials through a malicious SSH server. PuTTY accepts SSH_MSG_USERAUTH_SUCCESS without substantive client authentication, letting server-controlled session output imitate a later authentication prompt. The user must initiate the connection and submit a secret to the forged prompt; upstream disputes classifying this standards-permitted behavior as a vulnerability.

The record

CVE
CVE-2021-36367
Published
Jul 9, 2021
Updated
Aug 4, 2024
Vendor
PuTTY
Product
PuTTY
Classifications
CWE-345, T1056
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 9, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=- <=0.75

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

PuTTY · PuTTYversion=- <=0.75

Published CVSS scores

5.8NVDAV:N/AC:M/Au:N/C:P/I:P/A:N
8.1CISA ADP VulnrichmentCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-345Insufficient Verification of Data Authenticity
T1056Input Capture

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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