A newly discovered vulnerability, identified as CVE-2021-46310, has been reported in DjVuLibre 3.5.28, an open-source implementation of the DjVu file format used for distributing and displaying documents, images, and metadata. The vulnerability is particularly concerning due to its ability to allow remote attackers to trigger a denial-of-service (DoS) attack through a divide-by-zero issue in the IW44Image.cpp file.

This blog post aims to provide an overview of the vulnerability, including the affected code snippet, references to the original report, and details regarding the exploitation process. It is essential for developers and administrators using DjVuLibre 3.5.28 to be aware of this issue and take the necessary precautions to prevent potential exploitation.

Code Snippet

The reported vulnerability is found in the IW44Image.cpp file of DjVuLibre 3.5.28. The problematic code section responsible for the divide-by-zero issue is shown below:

...
virtual void Encode::code_slice(ByteStream &gbs, ByteStream &mbs, const int * data)
{
  [...]
  const int max_coeff = rowmaxtab[maxidx];
  const int prethresh = (max_coeff + thresh_denom - 1) / thresh_denom;
...

In this code snippet, the prethresh variable is susceptible to a divide-by-zero situation. The thresh_denom variable, if it takes a value of zero, will lead to an undefined behavior resulting in a program crash.

Original References

The vulnerability has been publicly disclosed by the responsible security researcher and can be found in the following references:

- CVE Information: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-46310
- National Vulnerability Database (NVD): https://nvd.nist.gov/vuln/detail/CVE-2021-46310

Exploit Details

Exploiting this vulnerability requires an attacker to craft a malicious DjVu file containing an image section with specific parameters that trigger the divide-by-zero condition in IW44Image.cpp. By compelling a victim to open the malicious DjVu file using an application that utilizes DjVuLibre 3.5.28, the attacker can remotely implement a denial-of-service attack, causing the application to crash.

Mitigation

There are no available patches or updates for this vulnerability at the time of writing. However, developers and administrators are advised to apply the following best practices to minimize the risk of exploitation:

Avoid opening untrusted DjVu files from unknown sources.

- Implement proper input validation and error handling in applications using DjVuLibre to mitigate potential crashes.
- Keep monitoring the DjVuLibre's official website and other sources for updates and patches related to this vulnerability.

Conclusion

The CVE-2021-46310 vulnerability in DjVuLibre 3.5.28 presents a significant risk to users being targeted by remote denial-of-service attacks. The code snippet provided in this post highlights the divide-by-zero issue, while the references offer details about the vulnerability disclosure. Developers and administrators must remain vigilant in applying protection measures and updating their systems once a fix or patch is available.

Timeline

Published on: 08/22/2023 19:16:00 UTC
Last modified on: 08/28/2023 17:46:00 UTC