CVE-2016-6515 in OpenSSH
Summary
by MITRE
The auth_password function in auth-passwd.c in sshd in OpenSSH before 7.3 does not limit password lengths for password authentication, which allows remote attackers to cause a denial of service (crypt CPU consumption) via a long string.
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Analysis
by VulDB Data Team • 10/24/2024
The vulnerability identified as CVE-2016-6515 represents a critical denial of service flaw in the OpenSSH implementation that affects versions prior to 7.3. This vulnerability specifically targets the authentication subsystem where the sshd daemon processes password authentication requests. The flaw exists in the auth-passwd.c source file within the OpenSSH server implementation, where the auth_password function fails to enforce reasonable limits on password length during the authentication process. This oversight creates a scenario where malicious actors can exploit the system by submitting excessively long password strings during authentication attempts, leading to resource exhaustion and system instability.
The technical mechanism behind this vulnerability stems from the cryptographic functions used during password authentication. When OpenSSH processes password authentication, it invokes the crypt() function to hash and verify passwords against stored credentials. The crypt() function, particularly in older implementations, exhibits exponential CPU consumption when processing very long input strings. This behavior occurs because the cryptographic hashing algorithm must process each character of the input string, and when the string length approaches or exceeds certain thresholds, the computational complexity grows dramatically. The vulnerability is classified under CWE-770, which addresses the allocation of resources without proper limits or checks, and it directly maps to the ATT&CK technique T1499.004 for resource exhaustion attacks.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire authentication infrastructure of systems relying on OpenSSH. Remote attackers can exploit this flaw without requiring prior authentication credentials, making it particularly dangerous in networked environments where sshd services are exposed to external traffic. The DoS attack vector allows an attacker to consume significant CPU resources on the target system, potentially causing legitimate authentication requests to fail or resulting in complete service unavailability. This vulnerability affects systems where OpenSSH is configured for password-based authentication, particularly when password authentication is enabled and not properly restricted through other security measures.
Mitigation strategies for CVE-2016-6515 primarily focus on upgrading to OpenSSH version 7.3 or later, which includes the necessary patches to limit password length during authentication. System administrators should also implement additional security controls such as rate limiting for authentication attempts, connection throttling, and monitoring for unusual authentication patterns. Network-level protections including firewall rules that restrict sshd access to trusted networks and intrusion detection systems that can identify and block malformed authentication requests provide additional defense layers. The vulnerability highlights the importance of implementing proper input validation and resource limitation mechanisms in cryptographic implementations, aligning with security best practices that emphasize defense in depth and the principle of least privilege in authentication systems. Organizations should conduct comprehensive vulnerability assessments to identify all systems running vulnerable OpenSSH versions and implement remediation procedures across their infrastructure to prevent exploitation of this denial of service vulnerability.