CVE-2020-14002 in PuTTY
Summary
by MITRE
PuTTY 0.68 through 0.73 has an Observable Discrepancy leading to an information leak in the algorithm negotiation. This allows man-in-the-middle attackers to target initial connection attempts (where no host key for the server has been cached by the client).
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Analysis
by VulDB Data Team • 05/04/2026
The vulnerability CVE-2020-14002 represents a significant security flaw in PuTTY versions 0.68 through 0.73 that manifests as an observable discrepancy during the SSH algorithm negotiation process. This information leak occurs specifically during initial connection attempts when the client has no cached host key for the target server, creating a window of opportunity for man-in-the-middle attackers to exploit the system. The flaw stems from the predictable timing differences in the protocol negotiation that reveal information about the cryptographic algorithms being offered by the server, which can be leveraged to infer server configuration details and potentially aid in more sophisticated attacks.
The technical implementation of this vulnerability involves the SSH protocol's algorithm negotiation phase where PuTTY's handling of algorithm selection creates measurable timing variations that can be observed by an attacker positioned in the network path. This timing discrepancy allows attackers to determine which cryptographic algorithms are supported by the server during the initial handshake process, potentially exposing sensitive information about the server's security configuration and capabilities. The vulnerability is particularly dangerous because it occurs before any host key verification takes place, meaning that even if the user eventually detects the attack through host key verification, the initial information leakage has already occurred.
From an operational perspective, this vulnerability creates a serious risk for organizations that rely on PuTTY for SSH connections, particularly in environments where network traffic is monitored or where attackers have the capability to perform timing analysis attacks. The impact is most severe during initial connection attempts where no host key has been cached, as this represents the earliest phase of the connection process where attackers can gain intelligence about the target system. This information leakage could potentially enable attackers to tailor more sophisticated attacks against the system, such as targeted algorithm selection attacks or protocol downgrade attempts that exploit known weaknesses in specific cryptographic implementations. The vulnerability aligns with CWE-203, which describes Observable Discrepancy, and represents a clear violation of the principle of least information disclosure in cryptographic protocols.
The recommended mitigations for this vulnerability include immediate upgrading to PuTTY version 0.74 or later, which contains the necessary fixes to eliminate the timing discrepancies during algorithm negotiation. Organizations should also implement network monitoring to detect potential timing-based attacks and consider deploying additional security controls such as SSH key management systems that can detect and prevent unauthorized host key modifications. From an ATT&CK framework perspective, this vulnerability relates to T1566.001, which covers credential access through phishing, and T1071.004, which involves application layer protocol usage. Additionally, organizations should ensure their security policies include regular updates of SSH clients and maintain awareness of timing-based information leakage vulnerabilities that could compromise their network security posture.