CVE-2016-4541 in PHP
Summary
by MITRE
The grapheme_strpos function in ext/intl/grapheme/grapheme_string.c in before 5.5.35, 5.6.x before 5.6.21, and 7.x before 7.0.6 allows remote attackers to cause a denial of service (out-of-bounds read) or possibly have unspecified other impact via a negative offset.
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Analysis
by VulDB Data Team • 08/22/2022
The CVE-2016-4541 vulnerability resides within the grapheme_strpos function of PHP's internationalization extension, specifically affecting versions prior to 5.5.35, 5.6.21, and 7.0.6. This flaw represents a classic out-of-bounds read condition that occurs when processing string operations involving grapheme clusters, which are sequences of characters that represent a single logical character in international text processing. The vulnerability stems from insufficient input validation within the grapheme_string.c file where negative offset values are not properly sanitized before being used in memory access operations.
The technical implementation of this vulnerability allows remote attackers to manipulate the grapheme_strpos function by providing negative offset parameters that exceed the bounds of allocated memory regions. When PHP processes international text with grapheme functions, it maintains internal data structures that track character positions and boundaries. A negative offset value can cause the function to attempt reading memory locations that are outside the valid range of the string buffer, resulting in an out-of-bounds read condition. This type of vulnerability falls under CWE-129, which specifically addresses insufficient validation of length of buffers, and more broadly aligns with CWE-125, representing out-of-bounds read conditions that can lead to information disclosure or system instability.
The operational impact of this vulnerability extends beyond simple denial of service to potentially enable more severe security consequences. While the primary effect manifests as denial of service through out-of-bounds memory reads, the vulnerability could theoretically be exploited to disclose sensitive memory contents or even facilitate further attacks depending on the execution environment. Attackers could leverage this flaw in web applications that process user-supplied text through grapheme functions, particularly those handling internationalized content where grapheme operations are common. The vulnerability is particularly concerning in environments where PHP applications process untrusted input through international text processing functions, as it could be triggered through various input vectors including form submissions, API calls, or file uploads containing specially crafted text data.
Mitigation strategies for CVE-2016-4541 primarily involve immediate patching of affected PHP versions to the recommended secure releases. Organizations should prioritize updating their PHP installations to versions 5.5.35, 5.6.21, or 7.0.6, depending on their current version, as these releases contain the necessary code modifications to properly validate offset parameters. Additionally, implementing input validation at the application level can provide defense-in-depth measures, where developers should sanitize all user inputs before passing them to grapheme functions. The vulnerability's exploitation aligns with ATT&CK technique T1059.007, which covers scripting languages, as attackers may leverage PHP's text processing capabilities to execute malicious payloads. Network-level protections such as web application firewalls can also help detect and block malicious requests that attempt to trigger this vulnerability by monitoring for unusual offset parameter values in text processing operations.