CVE-2022-27404 - Heap Buffer Overflow in FreeType's sfnt_init_face Function: A Deep Dive into the Exploit and Analysis

A newly discovered vulnerability, CVE-2022-27404, has been found in the FreeType library in the commit 1e2eb65048f75c64b68708efed6ce904c31f3b2f. The vulnerability is characterized as a heap buffer overflow and occurs within the sfnt_init_face function. In this post, we will take a detailed look at the exploit, analyze the code snippet responsible, and provide links to original references for further analysis.

The Vulnerability

CVE-2022-27404 is a heap buffer overflow vulnerability present in FreeType's sfnt_init_face function. Heap buffer overflows occur when a program writes more data to a block of memory than it was allocated, which may result in corruption of adjacent memory blocks, crashes, or potentially the execution of arbitrary code.

To better understand this vulnerability, let's first take a look at the function where the bug was found. The sfnt_init_face function is part of FreeType's sfnt module, which is responsible for handling fonts with the SFNT structure.

The Code Snippet

The relevant code snippet in the function sfnt_init_face that contains the bug can be found in the following GitHub repository: FreeType commit 1e2eb65048f75c64b68708efed6ce904c31f3b2f.

Below is a snippet of the code that contains the vulnerability

  static FT_Error
  sfnt_init_face( ... )
  {
    ...
    FT_ULong             advance;
    ...
    for ( gindex = ; gindex < maxstrike;
          gindex++, sbit = (FT_Byte*)sbit + 4 + advance )
    {
      err = face->goto_table( face, TTAG_EBLC,
                              face->sbit_table_offset,  );
      ...
      advance = FT_NEXT_ULONG( sbit );
      ...
      FT_ULong  next_offset = FT_NEXT_ULONG( sbit );
      ...
      scipy_GlyphMap[i].size_  = next_offset - gOffset_;
      ...
    }
  }

The Exploit

The exploit involves manipulating the advance value, which is read from the sbit input buffer. By providing a carefully crafted value for advance, an attacker can cause the sbit pointer to point outside the range of the intended memory region. This will cause a buffer overflow when the scipy_GlyphMap data structure is updated with the value of next_offset - gOffset_.

This vulnerability could be exploited by an attacker to compromise the security of a system using the FreeType library to process untrusted font files.

Original References

For in-depth technical details, additional context, and the complete analysis, we recommend the following resources:

1. FreeType commit 1e2eb65048f75c64b68708efed6ce904c31f3b2f
2. CVE-2022-27404 on NVD
3. FreeType Project Homepage

Conclusion

In this post, we have explored the heap buffer overflow vulnerability CVE-2022-27404, which affects a widely used library, FreeType. This vulnerability highlights the importance of being cautious when handling untrusted input and ensuring that software dependencies are kept up to date with the latest security patches. We hope that this analysis will serve as a useful resource for developers and security professionals alike in understanding and mitigating this type of vulnerability.

Timeline

Published on: 04/22/2022 14:15:00 UTC
Last modified on: 07/27/2022 13:44:00 UTC