CVE-2016-4537 in PHP
Summary
by MITRE
The bcpowmod function in ext/bcmath/bcmath.c in PHP before 5.5.35, 5.6.x before 5.6.21, and 7.x before 7.0.6 accepts a negative integer for the scale argument, which allows remote attackers to cause a denial of service or possibly have unspecified other impact via a crafted call.
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Analysis
by VulDB Data Team • 08/21/2022
The vulnerability identified as CVE-2016-4537 represents a critical flaw in PHP's bcmath extension that affects multiple version lines including 5.5.x, 5.6.x, and 7.x series. This issue stems from the bcpowmod function implementation within ext/bcmath/bcmath.c file where the function fails to properly validate the scale argument. The scale parameter in bcmath operations typically controls the number of decimal places in the result, but in this case the function accepts negative integer values that should logically be restricted to non-negative integers. When a negative scale value is passed, it creates an abnormal execution path that can lead to memory corruption or other undefined behaviors.
The technical nature of this vulnerability places it under CWE-129, which specifically addresses insufficient validation of length or count parameters, and CWE-191, which deals with integer underflow conditions. Attackers can exploit this weakness by crafting malicious input that passes a negative integer to the scale parameter of the bcpowmod function. This particular attack vector falls under the ATT&CK technique T1499.004 for network denial of service, as it can be leveraged to crash PHP processes or cause system instability. The vulnerability manifests as a denial of service condition where the application becomes unresponsive or crashes, but there is also potential for unspecified other impacts that could include information disclosure or arbitrary code execution depending on the system configuration.
The operational impact of this vulnerability is significant for web applications that rely on PHP's bcmath extension for cryptographic operations, financial calculations, or any scenario involving large number arithmetic. When exploited, the vulnerability can cause complete service disruption for applications that process user input through the bcpowmod function, potentially affecting thousands of users depending on the application's scale. The risk is particularly elevated in environments where PHP applications handle untrusted input from web forms, API calls, or other external data sources. Organizations running affected PHP versions face potential downtime, service degradation, and possible data loss if the vulnerability is exploited to cause memory corruption or process crashes.
Mitigation strategies for CVE-2016-4537 primarily involve immediate patching of PHP installations to versions that have addressed the validation issue. PHP 5.5.35, 5.6.21, and 7.0.6 contain the necessary fixes that validate the scale parameter to prevent negative values from being accepted. Administrators should also implement input validation at the application level to ensure that any user-provided scale parameters are validated before being passed to bcmath functions. Additionally, network segmentation and monitoring should be employed to detect unusual patterns of bcmath function calls that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper parameter validation in cryptographic and mathematical functions, as highlighted in security best practices for preventing buffer overflows and integer underflows that are commonly exploited in application-level attacks.