CVE-2022-40303 - Analyzing the Remote Code Execution Vulnerability in libxml2 (version < 2.9.) with Exploit Details, Code Snippets, and Original References

Recently, a critical vulnerability (CVE-2022-40303) was identified in libxml2, a widely used library for parsing XML documents. This vulnerability can cause a segmentation fault, which can potentially lead to remote code execution. In this blog post, we will analyze and discuss the root cause of the vulnerability, the impact, the code segment responsible for the vulnerability, and the mitigation steps to prevent exploitation.

Vulnerability Details

An issue was discovered in libxml2 before version 2.10.3, particularly when parsing multi-gigabyte XML documents with the XML_PARSE_HUGE parser option enabled. Several integer counters can overflow, which results in an attempt to access an array at a negative 2GB offset. This, in turn, typically leads to a segmentation fault that has significant security implications.

The CVE number associated with this vulnerability is CVE-2022-40303 with a CVSS score of 7.5 (HIGH).

Exploit Details

To exploit this vulnerability, an attacker can create a specially crafted XML document with a size of over 2GB and cause the application using libxml2 to parse this XML document with the XML_PARSE_HUGE option enabled. This malformed XML document will trigger an integer overflow, causing the parser to read from an out-of-bounds memory address. Consequently, the application will crash, leading to a potential Denial of Service (DoS) attack.

Code Snippet

The vulnerable code segment is located in the parser.c file in the libxml2 source code. When the XML_PARSE_HUGE option is enabled, the code does not properly handle the integer overflow, as demonstrated here:

int parsehuge = (ctxt->options & XML_PARSE_HUGE) != ;
if (parsehuge) {
    if (ctxt->inputNr - ctxt->inputTabAlloc->size > ) {
        xmlParserInput **newTab;

        ctxt->inputTabAlloc->size *= 2;
        newTab = (xmlParserInput **) xmlRealloc((void *) ctxt->inputTab, ctxt->inputTabAlloc->size * sizeof(xmlParserInputPtr));
        if (newTab == NULL) {
            // Error handling skipped for brevity
        }
        ctxt->inputTab = newTab;
    }
}

In the above code, the variable ctxt->inputTabAlloc->size can overflow when multiplying by 2, causing an out-of-bounds memory allocation. This results in the segmentation fault when attempting to access the array at a negative 2GB offset.

Mitigation

The best way to mitigate this vulnerability is to update the libxml2 library to version 2.10.3 or later. If updating is not possible, it is recommended not to parse large XML documents (more than 2GB) with the XML_PARSE_HUGE option enabled.

Moreover, developers should consider applying input validation to restrict the size of the XML document being parsed, thus reducing the risk of a potential attack.

Original References

For more information about this vulnerability, please refer to the official libxml2 repository and security advisory:

- libxml2 repository on GitLab
- libxml2 security advisory

Conclusion

In summary, CVE-2022-40303 poses a serious risk to any application that uses the libxml2 library for parsing large XML documents with the XML_PARSE_HUGE option enabled. By updating to a newer version and implementing mitigation measures, developers can protect their applications from potential attacks related to this vulnerability.

Timeline

Published on: 11/23/2022 00:15:00 UTC
Last modified on: 01/11/2023 17:29:00 UTC