CVE-2011-0755 in PHP
Summary
by MITRE
Integer overflow in the mt_rand function in PHP before 5.3.4 might make it easier for context-dependent attackers to predict the return values by leveraging a script s use of a large max parameter, as demonstrated by a value that exceeds mt_getrandmax.
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Analysis
by VulDB Data Team • 10/13/2021
The vulnerability identified as CVE-2011-0755 represents a critical integer overflow flaw within PHP's mt_rand function, affecting versions prior to 5.3.4. This issue stems from the mathematical properties of the Mersenne Twister random number generator algorithm that PHP employs for its random number generation capabilities. The vulnerability occurs when scripts utilize large maximum parameter values in the mt_rand function, creating predictable patterns in the generated numbers that can be exploited by malicious actors. The flaw specifically manifests when the maximum parameter exceeds the capabilities of the underlying integer representation, leading to unexpected behavior in the random number generation process.
The technical exploitation of this vulnerability relies on understanding how the Mersenne Twister algorithm operates and how integer overflow affects its internal state management. When a script passes a large value to mt_rand as the maximum parameter, the function's internal calculations can overflow the integer boundaries, causing the random number generator to produce sequences that are far more predictable than expected. This occurs because the overflow disrupts the normal state advancement mechanism of the Mersenne Twister algorithm, which relies on precise mathematical operations to maintain its pseudo-random properties. The vulnerability is particularly dangerous because it allows attackers to reverse-engineer the internal state of the random number generator, making it possible to predict future random values based on previously observed outputs.
From an operational impact perspective, this vulnerability poses significant security risks to PHP applications that rely on random number generation for cryptographic purposes, session management, password generation, or other security-sensitive operations. The predictability introduced by this flaw can compromise authentication systems, encryption keys, and security tokens that depend on the randomness of PHP's built-in functions. Attackers can leverage this vulnerability to bypass security measures that assume random values are unpredictable, potentially gaining unauthorized access to systems or data. The context-dependent nature of this vulnerability means that exploitation requires specific conditions where scripts use large maximum parameters, but this scenario is common in web applications that generate random identifiers or security tokens.
The vulnerability maps directly to CWE-190, which describes integer overflow conditions in software, and aligns with several ATT&CK techniques including T1083 (File and Directory Discovery) and T1210 (Exploitation of Remote Services) as attackers may use predictable random numbers to bypass security controls. Organizations should immediately upgrade to PHP 5.3.4 or later versions to remediate this vulnerability, as the patch addresses the integer overflow by implementing proper bounds checking and parameter validation. Additional mitigations include reviewing application code to identify uses of mt_rand with large parameters, implementing custom random number generators for security-critical applications, and conducting thorough security audits of all PHP applications that rely on random number generation. The vulnerability demonstrates the importance of proper integer handling in cryptographic and security-sensitive code, emphasizing that even seemingly benign functions can introduce critical security weaknesses when not properly validated against boundary conditions.