CVE-2017-9049 in libxml2
Summary
by MITRE
libxml2 20904-GITv2.9.4-16-g0741801 is vulnerable to a heap-based buffer over-read in the xmlDictComputeFastKey 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 libxml2 Bug 759398.
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Analysis
by VulDB Data Team • 12/18/2025
The vulnerability identified as CVE-2017-9049 affects libxml2 version 2.9.4 and represents a heap-based buffer over-read condition within the xmlDictComputeFastKey function located in the dict.c source file. This flaw manifests when processing XML documents through applications that utilize libxml2 as their underlying XML parsing library, including but not limited to PHP web applications. The vulnerability stems from an incomplete remediation of a previously identified issue documented as Bug 759398, indicating that the original fix for similar buffer over-read conditions was insufficient to address all potential attack vectors within the dictionary computation logic.
The technical implementation of this vulnerability exploits memory access patterns within the xmlDictComputeFastKey function where the library attempts to compute fast dictionary keys for XML element names and attribute values. When malformed or specially crafted XML input is processed, the function reads beyond the allocated heap buffer boundaries, potentially accessing uninitialized memory or memory belonging to other data structures. This over-read condition occurs during the dictionary lookup process where the library maintains hash tables for efficient XML element name resolution. The flaw specifically impacts the memory management operations that occur when the library processes large or complex XML structures, particularly those involving repeated element names or attribute processing.
The operational impact of CVE-2017-9049 extends beyond simple application crashes to potentially enable more sophisticated attack vectors. While the immediate effect is program termination through segmentation faults or access violations, the underlying memory corruption could theoretically be exploited to achieve arbitrary code execution depending on the target environment and memory layout. Applications using libxml2 are particularly vulnerable when processing untrusted XML input from web forms, file uploads, or external XML services, making web applications the primary attack surface. The vulnerability affects systems where PHP or other applications relying on libxml2 for XML processing are deployed, creating widespread exposure across web server environments and enterprise applications that handle XML data.
Security mitigations for this vulnerability involve immediate patching of libxml2 to version 2.9.5 or later, which contains the complete fix for the buffer over-read condition. System administrators should prioritize updating all affected applications and libraries that depend on libxml2, particularly web applications running PHP or other scripting languages that utilize XML parsing capabilities. Additionally, input validation and sanitization measures should be implemented at application layers to filter malformed XML content before it reaches the libxml2 parsing functions. Organizations should also consider implementing memory protection mechanisms such as address space layout randomization and stack canaries to reduce the exploitability of potential memory corruption issues. The vulnerability aligns with CWE-125, which describes out-of-bounds read conditions, and may map to ATT&CK technique T1059 for command and scripting interpreter usage in exploitation scenarios. Regular security auditing of third-party libraries and maintaining updated security patches remains crucial for preventing exploitation of similar buffer over-read vulnerabilities in XML processing components.