CVE-2001-0361 in SSH
Summary
by MITRE
Implementations of SSH version 1.5, including (1) OpenSSH up to version 2.3.0, (2) AppGate, and (3) ssh-1 up to version 1.2.31, in certain configurations, allow a remote attacker to decrypt and/or alter traffic via a "Bleichenbacher attack" on PKCS#1 version 1.5.
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Analysis
by VulDB Data Team • 10/06/2025
The vulnerability described in CVE-2001-0361 represents a critical cryptographic flaw affecting SSH version 1.5 implementations including OpenSSH versions up to 2.3.0, AppGate, and ssh-1 up to version 1.2.31. This vulnerability stems from the improper implementation of PKCS#1 version 1.5 padding in the RSA encryption scheme used within SSH protocol versions. The flaw specifically manifests when these implementations fail to properly validate the padding structure of RSA signatures, creating an exploitable condition that allows remote attackers to perform cryptographic attacks against the SSH communication channel.
The technical exploitation of this vulnerability relies on the Bleichenbacher attack, a well-documented cryptographic attack targeting RSA implementations that use PKCS#1 version 1.5 padding. This attack exploits the deterministic nature of PKCS#1 v1.5 padding and the way implementations handle padding validation errors. When an attacker sends specially crafted RSA encrypted data to a vulnerable SSH implementation, they can observe the responses to determine whether the padding is valid. Through repeated attempts and statistical analysis, the attacker can gradually reconstruct the plaintext, effectively decrypting communications or modifying traffic in transit. This attack is particularly dangerous because it operates at the cryptographic protocol level rather than exploiting application-level flaws, making it more difficult to detect and prevent.
The operational impact of CVE-2001-0361 is severe and multifaceted, affecting the fundamental security guarantees of SSH communications. Remote attackers can not only decrypt sensitive data transmitted through SSH connections but also potentially alter traffic, leading to man-in-the-middle attacks and data integrity compromises. The vulnerability affects critical infrastructure components that rely on SSH for secure remote access, including network devices, servers, and administrative systems. Organizations using affected SSH implementations face significant risk of unauthorized access, data exfiltration, and system compromise, particularly in environments where SSH is used for privileged access management. The attack does not require authentication credentials, making it particularly dangerous as it can be exploited against any system running vulnerable SSH implementations.
Mitigation strategies for this vulnerability focus on both immediate remediation and long-term architectural improvements. The primary recommendation is to upgrade to SSH implementations that properly implement PKCS#1 v2.0 padding or use more secure padding schemes such as OAEP. For OpenSSH specifically, upgrading to version 2.3.1 or later resolves the vulnerability. Organizations should also consider implementing additional security controls such as using SSH key-based authentication with strong key lengths, enabling host key verification, and implementing network segmentation to limit exposure. From a compliance perspective, this vulnerability aligns with CWE-327 which addresses the use of weak cryptographic algorithms and improper implementation of cryptographic protocols. The attack pattern described corresponds to techniques documented in ATT&CK framework under credential access and defense evasion categories, particularly leveraging cryptographic weaknesses for unauthorized access and data manipulation. Organizations should also implement network monitoring to detect anomalous traffic patterns that might indicate exploitation attempts, and establish robust patch management processes to ensure timely updates to cryptographic implementations.