CVE-2002-0639 in OpenSSH
Summary
by MITRE
Integer overflow in sshd in OpenSSH 2.9.9 through 3.3 allows remote attackers to execute arbitrary code during challenge response authentication (ChallengeResponseAuthentication) when OpenSSH is using SKEY or BSD_AUTH authentication.
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Analysis
by VulDB Data Team • 09/14/2025
The vulnerability described in CVE-2002-0639 represents a critical integer overflow flaw within the sshd daemon of OpenSSH versions 2.9.9 through 3.3. This vulnerability specifically impacts the challenge response authentication mechanism, which is a security feature designed to provide additional authentication layers beyond traditional password methods. The flaw occurs when OpenSSH employs either SKEY or BSD_AUTH authentication methods, both of which rely on challenge-response protocols to verify user identity through time-based tokens or system authentication modules. The integer overflow condition creates a scenario where an attacker can manipulate input data to cause the system to process incorrect values, potentially leading to memory corruption and arbitrary code execution. This vulnerability operates at the core of the authentication subsystem, making it particularly dangerous as it can be exploited during legitimate authentication attempts.
The technical implementation of this vulnerability stems from improper handling of integer values within the challenge response authentication code path. When SKEY or BSD_AUTH authentication is configured and active, the sshd process processes authentication challenges that involve calculations or comparisons of integer values. The overflow occurs when the system attempts to perform arithmetic operations or memory allocations based on user-provided input data that exceeds the maximum value that can be represented by the integer data type. This overflow can result in buffer overflows, memory corruption, or other exploitable conditions that allow attackers to inject and execute malicious code within the context of the sshd process. The vulnerability is classified under CWE-190 as an integer overflow condition, specifically involving signed integer overflow that can lead to memory corruption. The exploitability of this vulnerability is enhanced by the fact that it occurs during legitimate authentication operations, making it difficult to detect and potentially allowing for stealthy exploitation.
The operational impact of CVE-2002-0639 extends beyond simple privilege escalation to include full system compromise when successfully exploited. An attacker who can successfully exploit this vulnerability gains the ability to execute arbitrary code with the privileges of the sshd process, which typically runs with elevated permissions to manage authentication and access control. The vulnerability is particularly concerning because it can be triggered during normal authentication flows, meaning that legitimate users attempting to authenticate could inadvertently provide the malicious input required to exploit the vulnerability. This creates a scenario where the authentication process itself becomes a vector for compromise, potentially allowing attackers to maintain persistent access to systems. The attack surface is further expanded by the fact that this vulnerability affects multiple versions of OpenSSH, making it widespread across many deployed systems. Organizations using OpenSSH with challenge response authentication enabled are particularly at risk, as the authentication mechanisms are actively utilized and the vulnerability can be exploited without requiring special privileges or prior access to the system.
Mitigation strategies for CVE-2002-0639 primarily focus on immediate version upgrades and authentication configuration changes. The most effective immediate solution involves upgrading to OpenSSH versions 3.4 or later, where the integer overflow has been addressed through proper input validation and boundary checking. Organizations should also consider disabling challenge response authentication methods when they are not required, specifically turning off SKEY and BSD_AUTH authentication options in the sshd configuration. The sshd_config file should be modified to set ChallengeResponseAuthentication to no if challenge response authentication is not necessary for the environment. Additionally, implementing network segmentation and access controls can limit the potential impact of successful exploitation attempts. Security monitoring should be enhanced to detect unusual authentication patterns or failed authentication attempts that might indicate exploitation attempts. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and execution through authentication mechanisms, specifically targeting the authentication process to gain unauthorized access. The vulnerability also relates to defense evasion techniques, as the exploitation occurs during normal authentication operations, making detection more challenging. Organizations should also consider implementing intrusion detection systems that can monitor for anomalous behavior in authentication protocols and ensure that all systems are regularly patched and updated to prevent exploitation of known vulnerabilities.