CVE-2016-4538 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 modifies certain data structures without considering whether they are copies of the _zero_, _one_, or _two_ global variable, which allows remote attackers to cause a denial of service or possibly have unspecified other impact via a crafted call.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 08/21/2022
The vulnerability identified as CVE-2016-4538 represents a critical flaw in the PHP bcmath extension that affects multiple PHP versions including 5.5.35, 5.6.21, and 7.0.6. This issue resides within the bcpowmod function located in ext/bcmath/bcmath.c, where improper handling of global variable references creates a potential for both denial of service and unspecified security impacts. The vulnerability stems from the function's failure to properly account for whether data structures are copies of the global zero, one, or two variables, which are fundamental constants used in arbitrary precision arithmetic operations.
The technical root cause of this vulnerability lies in the improper management of memory references within the bcmath extension's bcpowmod implementation. When the function processes mathematical operations involving these global constants, it fails to verify whether the data structures being manipulated are actual references to the global variables or copies thereof. This oversight creates a scenario where modifications to what appears to be a local copy can inadvertently affect the global state of these critical mathematical constants. The issue is particularly concerning because zero, one, and two are foundational elements in mathematical computations, and their corruption can lead to unpredictable behavior in the entire bcmath extension.
From an operational perspective, this vulnerability presents significant risks to PHP applications that rely on the bcmath extension for precise mathematical calculations. Remote attackers can exploit this flaw by crafting specific input parameters that trigger the vulnerable code path, potentially leading to denial of service conditions where the web server or application becomes unresponsive. The unspecified other impacts mentioned in the vulnerability description suggest that beyond simple service disruption, attackers might be able to achieve more severe consequences including arbitrary code execution or data corruption. The vulnerability's exploitation requires careful crafting of input data that forces the bcmath extension to operate on these specific global variables in an unsafe manner, making it particularly insidious as it can be triggered through normal user input processing.
The impact of this vulnerability extends beyond immediate service disruption to encompass broader security implications within PHP environments. According to CWE classification, this represents a weakness in the design or implementation of data structure handling, specifically related to improper handling of global variables and memory management. The ATT&CK framework would categorize this under privilege escalation and denial of service techniques, as attackers can leverage this vulnerability to compromise system availability and potentially gain elevated privileges. Organizations running affected PHP versions face significant exposure risks, particularly those with web applications that process user input through mathematical calculations or cryptographic operations that depend on the bcmath extension.
Mitigation strategies for CVE-2016-4538 primarily focus on immediate patching of affected PHP installations to the latest secure versions. System administrators should prioritize upgrading to PHP 5.5.35, 5.6.21, or 7.0.6 respectively, depending on their current version. Additionally, implementing input validation and sanitization measures can help reduce the attack surface by limiting the ability of malicious actors to craft specific payloads that trigger the vulnerable code path. Network monitoring should be enhanced to detect unusual patterns in mathematical computation requests that might indicate exploitation attempts. Organizations should also consider implementing application firewalls or web application firewalls that can detect and block suspicious mathematical operation parameters. The vulnerability highlights the importance of proper memory management practices in cryptographic and mathematical extensions, emphasizing the need for thorough testing of global variable handling in critical system components.