CVE-2017-9050 in libxml2
Summary
by MITRE
libxml2 20904-GITv2.9.4-16-g0741801 is vulnerable to a heap-based buffer over-read in the xmlDictAddString function in dict.c. This vulnerability causes programs that use libxml2, such as PHP, to crash. This vulnerability exists because of an incomplete fix for CVE-2016-1839.
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Analysis
by VulDB Data Team • 12/18/2025
The vulnerability identified as CVE-2017-9050 affects libxml2 version 2.9.4, specifically within the xmlDictAddString function located in the dict.c source file. This heap-based buffer over-read represents a critical security flaw that can lead to application crashes and potential system instability. The vulnerability manifests when programs utilizing libxml2, including PHP applications, process malformed XML data structures that trigger the flawed memory handling logic. The issue stems from an incomplete remediation of a previous vulnerability, CVE-2016-1839, which demonstrates how remediation efforts can sometimes introduce new attack surfaces if not thoroughly validated. The buffer over-read occurs during dictionary string addition operations where the software attempts to read memory beyond the allocated buffer boundaries, potentially exposing sensitive information or causing unauthorized code execution.
The technical implementation of this vulnerability involves the xmlDictAddString function failing to properly validate input parameters before performing memory operations. When processing XML content, the function attempts to add strings to an internal dictionary structure without adequate bounds checking, leading to memory access violations that result in program termination. This flaw operates at the intersection of memory management and input validation, creating a scenario where malformed XML data can trigger undefined behavior. The vulnerability aligns with CWE-125, which describes out-of-bounds read conditions, and represents a classic example of how improper input validation can lead to memory corruption vulnerabilities. The incomplete fix for CVE-2016-1839 suggests that developers may have addressed the primary vector of attack while leaving secondary or edge case scenarios unaddressed, creating a false sense of security.
The operational impact of CVE-2017-9050 extends beyond simple application crashes to potentially enable more sophisticated attacks. Systems running affected versions of libxml2, particularly those processing untrusted XML data through PHP or similar applications, become vulnerable to denial of service attacks that can disrupt service availability. Attackers could exploit this vulnerability by crafting malicious XML payloads that trigger the buffer over-read condition, causing the target application to crash and potentially allowing for further exploitation. The vulnerability's presence in widely-used libraries like libxml2 means that numerous applications across different platforms and technologies could be affected, creating a broad attack surface. Organizations using PHP web applications, XML processing services, or any system relying on libxml2 for XML parsing operations face significant risk exposure, as these systems often handle data from untrusted sources.
Mitigation strategies for CVE-2017-9050 should prioritize immediate patching of affected libxml2 installations to the latest stable versions that contain complete fixes for both the current vulnerability and its predecessor CVE-2016-1839. System administrators should implement comprehensive monitoring of XML processing activities to detect potential exploitation attempts and establish network-level controls to filter malformed XML traffic. Additional defensive measures include deploying input validation controls at multiple layers, implementing application firewalls, and utilizing sandboxing techniques for XML processing operations. Organizations should also conduct thorough vulnerability assessments to identify all systems utilizing vulnerable versions of libxml2 and ensure proper patch management procedures are in place. The remediation process must include verification that the applied patches fully address both the primary vulnerability and the underlying issues that allowed the incomplete fix for CVE-2016-1839 to persist. Security teams should consider implementing the ATT&CK framework's mitigation strategies for memory corruption vulnerabilities, focusing on process isolation, input sanitization, and runtime protection mechanisms to reduce the overall risk exposure.