CVE-2006-0805 in php-Nuke
Summary
by MITRE
The CAPTCHA functionality in php-Nuke 6.0 through 7.9 uses fixed challenge/response pairs that only vary once per day based on the User Agent (HTTP_USER_AGENT), which allows remote attackers to bypass CAPTCHA controls by fixing the User Agent, performing a valid challenge/response, then replaying that pair in the random_num and gfx_check parameters.
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Analysis
by VulDB Data Team • 07/19/2018
The vulnerability identified as CVE-2006-0805 affects the CAPTCHA implementation within php-Nuke versions 6.0 through 7.9, representing a critical security flaw that undermines the intended protection mechanisms designed to prevent automated abuse of web applications. This weakness stems from a fundamental design flaw in how the CAPTCHA system generates and validates challenge-response pairs, creating a predictable and exploitable pattern that adversaries can readily manipulate to bypass security controls. The vulnerability specifically targets the authentication and registration processes that rely on CAPTCHA validation to distinguish between human users and automated bots, thereby compromising the integrity of user access controls and potentially enabling unauthorized account creation or other malicious activities.
The technical implementation of this vulnerability lies in the deterministic nature of the CAPTCHA challenge generation algorithm, which uses the HTTP_USER_AGENT header as the primary variable for generating new challenge-response pairs. This approach fundamentally weakens the security model because it creates a limited set of predictable responses that repeat on a daily cycle based solely on the user agent string. Attackers can exploit this by first identifying a valid challenge-response pair through legitimate interaction with the system, then maintaining a consistent User Agent string throughout their attack sessions. The vulnerability operates through the manipulation of two specific parameters: random_num and gfx_check, which are used to transmit the CAPTCHA challenge and the user's response respectively. This allows attackers to replay previously obtained valid challenge-response pairs multiple times, effectively circumventing the CAPTCHA validation mechanism entirely.
The operational impact of this vulnerability extends beyond simple bypass of CAPTCHA controls, as it represents a significant weakening of the overall security posture of php-Nuke installations that rely on this protection mechanism. Attackers can leverage this flaw to perform automated registration attacks, spam submissions, or other malicious activities that would normally be blocked by proper CAPTCHA validation. The vulnerability's exploitation requires minimal technical skill and can be automated, making it particularly dangerous for web applications that depend on CAPTCHA for user authentication and access control. From a cybersecurity perspective, this vulnerability demonstrates a classic example of insufficient entropy in security mechanisms, where predictable patterns in challenge generation create exploitable weaknesses that can be systematically overcome by determined attackers. The impact is further amplified because CAPTCHA systems are often deployed as the first line of defense against automated abuse, and their failure creates a cascading effect that can compromise other security controls within the application.
The security implications of this vulnerability align with several established threat models and attack patterns documented in cybersecurity frameworks, particularly those related to authentication bypass and automated attack methodologies. This weakness corresponds to CWE-330, which addresses the use of insufficiently random values in security contexts, and can be categorized under ATT&CK technique T1110 for credential access through brute force or automated attacks. Organizations utilizing affected php-Nuke versions face significant risk of account takeover, spam generation, and other automated abuse scenarios that can severely impact system integrity and user trust. The vulnerability also demonstrates poor security engineering practices that fail to consider the importance of proper entropy and randomness in security mechanisms, representing a failure to implement robust cryptographic principles in the design of authentication controls. Mitigation strategies should focus on implementing proper randomization mechanisms for CAPTCHA challenges, ensuring that challenge-response pairs are truly unpredictable and do not follow deterministic patterns based on user agent or other static variables. Additionally, organizations should consider implementing multi-factor authentication controls and additional layers of security validation to compensate for the weaknesses in the CAPTCHA implementation and reduce the attack surface available to potential adversaries.