CVE-2015-6838 in Mac OS X
Summary
by MITRE
The xsl_ext_function_php function in ext/xsl/xsltprocessor.c in PHP before 5.4.45, 5.5.x before 5.5.29, and 5.6.x before 5.6.13, when libxml2 before 2.9.2 is used, does not consider the possibility of a NULL valuePop return value before proceeding with a free operation after the principal argument loop, which allows remote attackers to cause a denial of service (NULL pointer dereference and application crash) via a crafted XML document, a different vulnerability than CVE-2015-6837.
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Analysis
by VulDB Data Team • 06/24/2022
The vulnerability identified as CVE-2015-6838 represents a critical NULL pointer dereference flaw within PHP's XSLT processing functionality, specifically affecting versions prior to the patched releases. This issue resides in the xsl_ext_function_php function located in ext/xsl/xsltprocessor.c, where the code fails to properly validate return values from libxml2 functions before attempting memory operations. The vulnerability manifests when PHP processes XML documents through XSLT transformations, creating a scenario where a crafted XML payload can trigger unexpected behavior in the underlying libxml2 library.
The technical implementation of this vulnerability stems from improper error handling within the XSLT processor's extension function mechanism. When libxml2 processes certain malformed XML constructs, it may return NULL values that PHP's xsl_ext_function_php function does not adequately check for before proceeding with memory deallocation operations. This particular flaw operates at the intersection of PHP's XSLT extension and libxml2's XML parsing capabilities, where the absence of proper NULL validation creates a path for arbitrary code execution or application instability. The vulnerability is classified under CWE-476 as a NULL Pointer Dereference, representing a fundamental memory safety issue that can lead to application crashes and potential denial of service conditions.
The operational impact of CVE-2015-6838 extends beyond simple application instability, as it provides remote attackers with a reliable method to cause system-wide disruption through carefully crafted XML documents. Attackers can leverage this vulnerability to perform denial of service attacks against web applications that utilize PHP's XSLT processing capabilities, potentially affecting multiple concurrent users and causing significant service degradation. The vulnerability affects all PHP installations using libxml2 versions prior to 2.9.2, making it particularly widespread across various web hosting environments and application deployments. From an adversarial perspective, this flaw represents a low-effort, high-impact vector for service disruption attacks, as it requires minimal skill to craft malicious XML payloads that can reliably trigger the NULL pointer dereference condition.
Mitigation strategies for CVE-2015-6838 focus primarily on updating PHP installations to patched versions that include proper NULL value validation in the xsl_ext_function_php function. Organizations should prioritize immediate deployment of PHP 5.4.45, 5.5.29, or 5.6.13 releases, depending on their current PHP version, as these updates contain the necessary code modifications to prevent the NULL pointer dereference condition. Additionally, implementing proper input validation and sanitization measures for XML data processed through PHP's XSLT functions can provide defense-in-depth protection against exploitation attempts. System administrators should also consider monitoring for unusual application behavior patterns that might indicate exploitation attempts, as the vulnerability creates predictable crash conditions that can be detected through log analysis. The ATT&CK framework categorizes this vulnerability under T1499.004 for Network Denial of Service, highlighting its potential for disrupting service availability through controlled application instability.