CVE-2017-9047 in libxml2
Summary
by MITRE
A buffer overflow was discovered in libxml2 20904-GITv2.9.4-16-g0741801. The function xmlSnprintfElementContent in valid.c is supposed to recursively dump the element content definition into a char buffer 'buf' of size 'size'. The variable len is assigned strlen(buf). If the content->type is XML_ELEMENT_CONTENT_ELEMENT, then (i) the content->prefix is appended to buf (if it actually fits) whereupon (ii) content->name is written to the buffer. However, the check for whether the content->name actually fits also uses 'len' rather than the updated buffer length strlen(buf). This allows us to write about "size" many bytes beyond the allocated memory. This vulnerability causes programs that use libxml2, such as PHP, to crash.
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Analysis
by VulDB Data Team • 12/24/2020
The buffer overflow vulnerability CVE-2017-9047 resides within the libxml2 library version 2.9.4, specifically in the xmlSnprintfElementContent function located in the valid.c source file. This flaw represents a classic buffer overrun condition that occurs during the recursive dumping of element content definitions into a character buffer. The vulnerability stems from improper length calculation during string concatenation operations, creating a scenario where memory beyond the allocated buffer boundaries can be overwritten. The affected function processes XML element content definitions and employs a char buffer named 'buf' with a predetermined size 'size' for storing the output. When handling XML_ELEMENT_CONTENT_ELEMENT type content, the function first appends the content->prefix to the buffer if sufficient space exists, followed by writing content->name to the buffer. The critical flaw emerges from the subsequent validation check that incorrectly utilizes the original 'len' variable instead of the updated buffer length calculated by strlen(buf) after the prefix insertion. This error creates a condition where the function calculates available space based on outdated buffer state, allowing the content->name field to be written beyond the allocated memory boundaries. The vulnerability directly impacts programs that rely on libxml2 for XML processing, including PHP applications, potentially causing application crashes and system instability. This type of vulnerability falls under CWE-121, which describes stack-based buffer overflow conditions, and aligns with ATT&CK technique T1190 for exploitation of memory corruption vulnerabilities. The flaw demonstrates a common programming error in buffer management where developers fail to account for the dynamic nature of string operations and their impact on available memory space. When exploited, the buffer overflow can lead to denial of service conditions, application crashes, and potentially more severe consequences depending on the execution environment and memory layout. The vulnerability affects the XML parsing functionality of numerous applications that utilize libxml2 as their underlying XML processing library, making it a significant concern for systems handling XML data. The crash occurs during normal XML parsing operations when the parser encounters specific element content structures that trigger the flawed buffer handling logic. This vulnerability type represents a critical security concern because it can be exploited by malicious actors to disrupt services or potentially execute arbitrary code depending on the target system's memory protection mechanisms. The flaw highlights the importance of proper buffer size validation and the need for careful consideration of string manipulation operations in security-critical code. Organizations using applications that depend on libxml2 should prioritize patching this vulnerability to prevent potential exploitation and maintain system stability. The impact extends beyond simple crashes to encompass broader security implications, particularly in environments where XML processing is a core component of application functionality. This vulnerability exemplifies why regular security audits and code reviews are essential for identifying and mitigating memory corruption issues in widely-used library components. The issue demonstrates how seemingly minor programming errors in fundamental libraries can have widespread consequences across numerous applications and systems that depend on their stable operation.