CVE-2002-0640 in OpenSSH
Summary
by MITRE
Buffer overflow in sshd in OpenSSH 2.3.1 through 3.3 may allow remote attackers to execute arbitrary code via a large number of responses during challenge response authentication when OpenBSD is using PAM modules with interactive keyboard authentication (PAMAuthenticationViaKbdInt).
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Analysis
by VulDB Data Team • 09/14/2025
The vulnerability identified as CVE-2002-0640 represents a critical buffer overflow flaw within the sshd daemon of OpenSSH versions 2.3.1 through 3.3. This vulnerability specifically manifests during challenge-response authentication processes when OpenBSD systems utilize PAM modules with interactive keyboard authentication. The flaw occurs when the sshd service receives an excessive number of responses during the authentication phase, creating conditions where attacker-controlled input can overflow buffer structures and potentially execute arbitrary code on the target system. The vulnerability is classified under CWE-121 as a stack-based buffer overflow, which directly enables code execution through memory corruption techniques. This issue represents a significant security risk because it allows remote attackers to gain unauthorized access to systems without requiring authentication credentials, making it particularly dangerous in networked environments where ssh services are exposed to external networks.
The technical implementation of this vulnerability exploits the lack of proper input validation during PAM authentication processes. When PAMAuthenticationViaKbdInt is enabled, the sshd service processes keyboard interactive authentication requests through PAM modules, which may contain insufficient bounds checking on response data. The buffer overflow occurs when the number of authentication responses exceeds the allocated memory space, causing memory corruption that can be leveraged by attackers to overwrite critical program execution structures. This type of vulnerability falls under the ATT&CK technique T1078.004 which covers valid accounts and T1068 which covers exploit for privilege escalation, as successful exploitation can lead to full system compromise. The flaw demonstrates the dangerous intersection of authentication mechanisms and memory safety issues, where the legitimate requirement for interactive authentication becomes a vector for code execution.
The operational impact of CVE-2002-0640 extends beyond simple unauthorized access to encompass complete system compromise and potential lateral movement within network environments. Systems running vulnerable OpenSSH versions become vulnerable to remote code execution attacks that can result in data theft, system manipulation, and establishment of persistent backdoors. The vulnerability affects organizations that rely on ssh for remote administration and system management, creating a significant attack surface for threat actors. Network defenders must consider that this vulnerability can be exploited without requiring any prior authentication, making it particularly attractive to automated attack tools. The impact is amplified in environments where ssh services are publicly accessible or where systems are configured to use PAM authentication with keyboard interactive responses, as these configurations are common in enterprise and academic environments.
Mitigation strategies for CVE-2002-0640 primarily focus on immediate version upgrades to OpenSSH 3.4 or later, which contain patches addressing the buffer overflow conditions. System administrators should implement the principle of least privilege by disabling keyboard interactive authentication when not required and ensuring that PAM modules are properly configured to prevent excessive response handling. Network segmentation and firewall rules should restrict access to ssh services to trusted networks only, while monitoring systems should be configured to detect unusual authentication patterns that might indicate exploitation attempts. The vulnerability highlights the importance of input validation and bounds checking in authentication systems, aligning with security best practices outlined in NIST SP 800-53 and ISO 27001 standards. Additionally, organizations should implement regular security assessments and vulnerability scanning to identify and remediate similar issues in other network services and applications, as this vulnerability demonstrates the potential for buffer overflows to occur in authentication mechanisms where user input is processed without adequate sanitization.