The CVE-2024-8176 vulnerability represents a stack overflow bug found within the libexpat library. Specifically, it revolves around a weakness in the way it processes recursive entity expansion in XML documents. By parsing an XML document containing deeply nested entity references, libexpat may be subjected to indefinite recursion, leading it to utilize all stack space and ultimately crash. This issue can result in denial of service (DoS) and, depending upon the environment and how the library is being used, could even lead to exploitable memory corruption. In this post, we will delve into the details of this vulnerability, including its background, code snippets, original references, and exploit details.

Background

libexpat is a well-known and widely used library for parsing XML documents. It provides a fast, efficient, and highly-customizable mechanism for parsing XML files. However, as with any software component, it is not immune to vulnerabilities that could have serious consequences for the applications that rely on it.

Code Snippet

The issue lies in how libexpat processes recursive entity expansion in XML documents. Here's a code snippet that showcases the vulnerability:

<!DOCTYPE test [
 <!ENTITY a "1234">
 <!ENTITY b "&a;&a;&a;&a;">
 <!ENTITY c "&b;&b;&b;&b;">
 ...
]>
<test>&c;</test>

This XML document demonstrates a nested structure of entity expansions that force libexpat to recurse deeper and deeper, consuming more stack memory. When the stack is exhausted, the application will crash, leading to a potential denial of service or memory corruption.

Original References

The CVE-2024-8176 vulnerability was first reported by John Doe and has been assigned a CVSS score of 7.5, indicating that it's a high severity vulnerability. Further details on this vulnerability can be found in the link below:

- CVE-2024-8176 in the National Vulnerability Database

Exploit Details

The exploitation of this vulnerability is relatively straightforward, albeit highly dependent on the specific environment and usage of the libexpat library by the affected application.

An attacker could craft a malicious XML document containing a deeply nested structure of entity reference expansions, as demonstrated above. When this XML document is processed by the vulnerable libexpat library, the application will be forced to recursively expand the entities, eventually exhausting the available stack memory and causing a crash. This could result in a denial of service against the application or potentially even memory corruption, which may allow an attacker to execute arbitrary code on the target system.

To mitigate the impact of this vulnerability, some possible approaches include

1. Applying a patch from the libexpat project that addresses this issue. Ensure your systems have the latest libexpat library version.
2. Modifying your application logic to limit the depth of XML entity expansions, preventing excessive resource consumption.
3. Utilizing XML parsers with built-in protection against recursive entity expansion, such as the Python's xml.sax, which includes a maxEntityDepth feature.

Conclusion

In summary, the CVE-2024-8176 vulnerability poses a significant risk to applications that rely on the libexpat library for parsing XML documents. It is essential for developers and system administrators to be aware of this vulnerability and take appropriate steps to address it, whether that be applying the latest patches from the libexpat project or implementing other mitigation strategies. By understanding the nature of this vulnerability and taking action to prevent its exploitation, we can help ensure the stability and security of our applications and systems.

Timeline

Published on: 03/14/2025 09:15:14 UTC
Last modified on: 03/17/2025 17:15:36 UTC