CVE-2021-36367 in PuTTY
Summary
by MITRE • 07/10/2021
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).
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Analysis
by VulDB Data Team • 07/15/2021
The vulnerability identified as CVE-2021-36367 affects PuTTY versions through 0.75 and represents a significant authentication bypass flaw in the SSH client implementation. This weakness stems from the client's failure to properly validate authentication responses before proceeding with session establishment, creating a window of opportunity for malicious SSH servers to exploit the trust relationship between client and server. The flaw operates at the core of the SSH protocol's authentication mechanism where PuTTY's client implementation does not adequately verify that authentication challenges are genuinely part of the legitimate session establishment process.
The technical nature of this vulnerability can be categorized under CWE-287 which addresses improper authentication issues in software systems. When an SSH server controlled by an attacker establishes a connection with a PuTTY client, the malicious server can present spoofed authentication prompts after the initial connection has been accepted. This occurs because PuTTY's authentication flow does not enforce proper sequence validation, allowing the client to accept authentication requests that should not be processed until after successful initial authentication has occurred. The vulnerability essentially allows for a form of credential harvesting where attackers can capture user credentials through deceptive authentication prompts that appear legitimate to the user.
The operational impact of this vulnerability extends beyond simple credential theft, as it enables a range of malicious activities that can compromise user security and system integrity. Attackers can leverage this flaw to conduct credential harvesting attacks where they capture usernames and passwords through spoofed authentication prompts, potentially gaining unauthorized access to systems that users believe are secure. The vulnerability also creates opportunities for man-in-the-middle attacks where attackers can intercept and manipulate authentication flows, potentially leading to privilege escalation or unauthorized system access. This makes the vulnerability particularly dangerous in environments where users connect to untrusted or potentially compromised SSH servers.
Mitigation strategies for CVE-2021-36367 should focus on immediate software updates to PuTTY versions that address the authentication flow validation issue. Organizations should ensure all PuTTY clients are updated to version 0.76 or later, which includes proper authentication sequence validation. Additionally, users should implement network-level controls such as SSH known hosts verification and key pinning to reduce the risk of connecting to malicious servers. The vulnerability also highlights the importance of following secure coding practices and proper input validation in cryptographic software implementations. Security teams should monitor for potential exploitation attempts and implement network detection measures to identify suspicious authentication patterns that may indicate exploitation of this vulnerability. This issue aligns with ATT&CK technique T1566 which covers credential harvesting through social engineering and man-in-the-middle attacks, emphasizing the need for robust authentication validation in client software implementations.