CVE-2025-53367: An exploitable out-of-bounds write in DjVuLibre

DjVuLibre version 3.5.29 was released today. It fixes CVE-2025-53367 (GHSL-2025-055), an out-of-bounds (OOB) write in the MMRDecoder::scanruns method. The vulnerability could be exploited to gain code execution on a Linux Desktop system when the user tries to open a crafted document.

DjVu is a document file format that can be used for similar purposes to PDF. It is supported by Evince and Papers, the default document viewers on many Linux distributions. In fact, even when a DjVu file is given a filename with a .pdf extension, Evince/Papers will automatically detect that it is a DjVu document and run DjVuLibre to decode it.

Antonio found this vulnerability while researching the Evince document reader. He found the bug with fuzzing.

Kev has developed a proof of concept exploit for the vulnerability, as demoed in this video.

The POC works on a fully up-to-date Ubuntu 25.04 (x86_64) with all the standard security protections enabled. To explain what’s happening in the video:

1. Kev clicks on a malicious DjVu document in his ~/Downloads directory.
2. The file is named poc.pdf, but it’s actually in DjVu format.
3. The default document viewer (/usr/bin/papers) loads the document, detects that it’s in DjVu format, and uses DjVuLibre to decode it.
4. The file exploits the OOB write vulnerability and triggers a call to system("google-chrome https://www.youtube.com/…").
5. Rick Astley appears.

Although the POC is able to bypass ASLR, it’s somewhat unreliable: it’ll work 10 times in a row and then suddenly stop working for several minutes. But this is only a first version, and we believe it’s possible to create an exploit that’s significantly more reliable.

You may be wondering: why Astley, and not a calculator? That’s because /usr/bin/papers runs under an AppArmor profile. The profile prohibits you from starting an arbitrary process but makes an exception for google-chrome. So it was easier to play a YouTube video than pop a calc. But the AppArmor profile is not particularly restrictive. For example, it lets you write arbitrary files to the user’s home directory, except for the really obvious one like ~/.bashrc. So it wouldn’t prevent a determined attacker from gaining code execution.

The MMRDecoder::scanruns method is affected by an OOB-write vulnerability, because it doesn’t check that the xr pointer stays within the bounds of the allocated buffer.

During the decoding process, run-length encoded data is written into two buffers: lineruns and prevruns:

//libdjvu/MMRDecoder.h
class DJVUAPI MMRDecoder : public GPEnabled
{
...
public:

  unsigned short *lineruns;
...
  unsigned short *prevruns;
...
}

The variables named pr and xr point to the current locations in those buffers. 

scanruns does not check that those pointers remain within the bounds of the allocated buffers.

//libdjvu/MMRDecoder.cpp
const unsigned short *
MMRDecoder::scanruns(const unsigned short **endptr)
{
...
  // Swap run buffers
  unsigned short *pr = lineruns;
  unsigned short *xr = prevruns;
  prevruns = pr;
  lineruns = xr;
...
  for(a0=0,rle=0,b1=*pr++;a0 < width;)
    {
     ...
            *xr = rle; xr++; rle = 0;
     ...
            *xr = rle; xr++; rle = 0;
 ...
          *xr = inc+rle-a0;
          xr++;
}

This can lead to writes beyond the allocated memory, resulting in a heap corruption condition. An out-of-bounds read with pr is also possible for the same reason.

We will publish the source code of our proof of concept exploit in a couple of weeks’ time in the GitHub Security Lab repository.

We would like to thank Léon Bottou and Bill Riemers for responding incredibly quickly and releasing a fix less than two days after we first contacted them!

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