CVE-2007-0956 in Kerberos
Summary
by MITRE
The telnet daemon (telnetd) in MIT krb5 before 1.6.1 allows remote attackers to bypass authentication and gain system access via a username beginning with a - character, a similar issue to CVE-2007-0882.
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Analysis
by VulDB Data Team • 01/12/2025
The vulnerability identified as CVE-2007-0956 represents a critical authentication bypass flaw in the telnet daemon component of MIT krb5 versions prior to 1.6.1. This issue stems from improper handling of username input validation within the telnetd service, creating a pathway for remote attackers to circumvent the authentication mechanism entirely. The flaw specifically manifests when a malicious user provides a username that begins with a hyphen character, exploiting a parsing weakness in how the daemon processes command-line arguments.
The technical root cause of this vulnerability lies in the insecure parsing of user input within the telnetd service implementation. When a username commencing with a hyphen is submitted, the daemon interprets this character sequence as the beginning of a command-line option rather than a legitimate username component. This misinterpretation occurs because the service fails to properly sanitize or validate input before processing it through argument parsing functions. The vulnerability is particularly dangerous because it allows attackers to bypass authentication entirely, effectively granting them system access without proper credentials.
From an operational impact perspective, this vulnerability creates a severe security risk for systems running vulnerable versions of MIT krb5. Remote attackers can exploit this flaw to gain unauthorized access to systems without needing valid credentials, potentially leading to complete system compromise. The attack vector requires only network connectivity to the telnet service, making it particularly dangerous in environments where telnet services are exposed to untrusted networks. This vulnerability directly aligns with CWE-20, which describes improper input validation, and represents a classic example of command injection or argument parsing vulnerability.
The security implications extend beyond simple unauthorized access, as this vulnerability can enable attackers to establish persistent access to compromised systems. Once authenticated through this bypass, attackers can execute arbitrary commands, modify system files, or escalate privileges to gain root access. This type of vulnerability is particularly concerning in enterprise environments where telnet services may be used for legitimate administrative purposes but are exposed to broader network access. The flaw's similarity to CVE-2007-0882 demonstrates a pattern of insecure argument handling that affects multiple components within the krb5 suite, highlighting systemic weaknesses in input validation practices.
Mitigation strategies for this vulnerability require immediate patching of affected MIT krb5 installations to version 1.6.1 or later, which includes proper input sanitization for username handling. Organizations should also implement network segmentation to limit access to telnet services, disable telnet services where possible in favor of more secure alternatives like SSH, and conduct comprehensive audits of all telnet daemon configurations. Security monitoring should be enhanced to detect unusual authentication patterns or attempts to exploit this specific vulnerability. Additionally, system administrators should consider implementing mandatory access controls and privilege separation mechanisms to limit the potential impact of successful exploitation attempts. The vulnerability demonstrates the critical importance of proper input validation and parameter sanitization in network services, aligning with ATT&CK technique T1190 for exploitation of remote services and T1078 for valid accounts usage.