CVE-2016-5768 in macOS
Summary
by MITRE
Double free vulnerability in the _php_mb_regex_ereg_replace_exec function in php_mbregex.c in the mbstring extension in PHP before 5.5.37, 5.6.x before 5.6.23, and 7.x before 7.0.8 allows remote attackers to execute arbitrary code or cause a denial of service (application crash) by leveraging a callback exception.
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Analysis
by VulDB Data Team • 09/20/2022
The CVE-2016-5768 vulnerability represents a critical double free error in PHP's mbstring extension that affects multiple version ranges including PHP 5.5.x before 5.5.37, 5.6.x before 5.6.23, and 7.x before 7.0.8. This vulnerability resides within the _php_mb_regex_ereg_replace_exec function located in the php_mbregex.c file, making it a fundamental flaw in how PHP handles multibyte string regular expression operations. The double free condition occurs when memory allocated for regex processing is freed twice during exception handling scenarios, creating a potential for memory corruption that can be exploited remotely.
The technical exploitation of this vulnerability leverages callback exception handling within the mbstring extension's regular expression processing. When a callback function is invoked during regex replacement operations and subsequently throws an exception, the memory management logic in _php_mb_regex_ereg_replace_exec fails to properly track allocated memory resources. This results in the same memory block being freed twice, which can lead to heap corruption and unpredictable behavior. The vulnerability is particularly dangerous because it can be triggered through user-supplied input that is processed through mbstring functions, making it a remote code execution risk when combined with appropriate exploitation techniques.
From an operational perspective, this vulnerability poses significant risks to web applications running vulnerable PHP versions, as it can be exploited through HTTP requests containing specially crafted multibyte character sequences. Attackers can leverage this flaw to either execute arbitrary code on the target system or cause denial of service conditions that crash the application. The impact extends beyond simple application instability to potentially full system compromise, especially when the vulnerable PHP instance has elevated privileges or is part of a larger attack chain. The vulnerability aligns with CWE-415 Double Free, which specifically addresses the issue of freeing the same memory block twice, and can be mapped to ATT&CK technique T1059.007 for remote code execution through web application vulnerabilities.
Mitigation strategies for CVE-2016-5768 primarily involve immediate patching of affected PHP installations to versions that contain the fix, specifically PHP 5.5.37, 5.6.23, and 7.0.8 or later. Organizations should also implement input validation measures to restrict potentially malicious multibyte character sequences in user-supplied data, particularly when using mbstring functions. Network-based mitigations such as web application firewalls can help detect and block exploitation attempts, while monitoring systems should be configured to alert on unusual application crashes or memory allocation patterns that might indicate exploitation attempts. Additionally, application developers should avoid using vulnerable mbstring functions when possible and implement proper error handling to minimize the risk of triggering memory corruption conditions during exception processing.