A recent vulnerability, dubbed CVE-2023-39615, has been identified in the popular Xmlsoft Libxml2 library, specifically version 2.11.. This vulnerability allows attackers to cause a Denial of Service (DoS) by supplying a maliciously crafted XML file. In this blog post, we will be taking a deep dive into the vulnerability, exploring the code snippet where the issue resides, as well as the exploit details and original references on how you can safeguard your systems.

Vulnerability Details

The root of the CVE-2023-39615 vulnerability lies in an out-of-bounds read, which is occurring within the xmlSAX2StartElement() function located at /libxml2/SAX2.c. The out-of-bounds read allows attackers to feed in carefully designed XML files to the affected library and trigger a crash, leading to a DoS attack.

It is important to note, though, that the vendor of Libxml2 disputes this vulnerability claim, stating that the product does not support the legacy SAX1 interface with custom callbacks. According to the vendor, a crash occurs even without any crafted input. However, given the vulnerability's potential impact on systems, it's critical to review the exploit and possible mitigations.

Exploit Details

The out-of-bounds read is occurring within the xmlSAX2StartElement() function in the /libxml2/SAX2.c file. A simplified code snippet showcasing the issue would look something like this:

void xmlSAX2StartElement(xmlParserCtxtPtr ctxt, const xmlChar * fullname, const xmlChar ** atts) {
    // ... (remaining code)

    if (atts != NULL) {
        for (cur = atts; *cur != NULL; cur += 2) {
            if (ctxt->attsDefault != NULL) {
                // Out-of-bounds read occurs here
                ctxt->attsDefault[*cur] = cur[1];
            }
        }
    }
}

The issue lies in the manipulation of the

attsDefault

array, which is being accessed in an out-of-bounds manner due to the lack of proper bounds checking for the

*cur

index.

For further details about this vulnerability, consider looking into the following sources

- The National Vulnerability Database entry for CVE-2023-39615 provides a comprehensive overview of the issue's technical details, including CVSS scores and affected products.
- The official GitHub repository for Xmlsoft Libxml2 contains information on the library, its latest updates, as well as issue tracking.
- The xmlSAX2StartElement() function's source code can be viewed here.

Mitigation Strategies

Safeguarding your systems against this potential vulnerability is key to maintaining a secure environment. Implement the following mitigation strategies to help mitigate the CVE-2023-39615 vulnerability:

1. Update Libxml2: Make sure you are using the latest version of the library, as it might include fixes for previously identified security issues. Keep an eye on the official Xmlsoft Libxml2 repository for any updates or patches.

2. Input Validation: Always validate user-supplied input, including XML files, to ensure that only legitimate content is being processed.

3. Segmentation and Monitoring: Segregate critical systems from non-critical systems to prevent the potential spread of a successful attack. Regularly monitor access to systems and employ real-time alerting to notify you of any unauthorized access or suspicious behavior.

Although the vendor disputes the CVE-2023-39615 vulnerability as not being applicable to their product, following these mitigation steps will help you keep your systems secure, regardless of the vulnerability's status.

Timeline

Published on: 08/29/2023 17:15:00 UTC
Last modified on: 09/06/2023 17:15:00 UTC