HarfBuzz, an open-source text shaping engine, is widely used in numerous applications and operating systems for rendering text. It handles complex text layout, shaping, and conversion of text characters to glyphs. However, a recent vulnerability, identified as CVE-2024-56732, has been discovered in HarfBuzz, specifically in versions 8.5. through 10..1. This vulnerability, a heap-based buffer overflow, exists in the hb_cairo_glyphs_from_buffer function, which could lead to potential exploitation and system compromise.

In this article, we'll dive deep into understanding this vulnerability, examining its root cause, providing code snippets for better clarity, and discussing possible exploitation methods.

Heap-Based Buffer Overflow in hb_cairo_glyphs_from_buffer

The buffer overflow vulnerability resides in the hb_cairo_glyphs_from_buffer() function in HarfBuzz. This function processes input text and generates the corresponding glyph indices and positions for rendering text using the Cairo graphics library. The vulnerability stems from improper handling of malformed input data, leading to a buffer overflow.

Here's a sample code snippet that demonstrates the issue

// File: hb_cairo_glyphs_from_buffer.c
#include "hb_cairo.h"

void hb_cairo_glyphs_from_buffer (hb_buffer_t *buffer,
                                  cairo_glyph_t *glyphs,
                                  unsigned int num_glyphs)
{
  // ... (omitted for brevity)

  for (unsigned int i = ; i < num_glyphs; ++i)
  {
    cairo_glyph_t *glyph = &glyphs[i];
    // ... (omitted for brevity)

    // Vulnerable Operation - overwrite space past allocated memory
    glyph->index = buffer->info[i].codepoint;
    glyph->x = (hb_position_t) buffer->pos[i].x;
    glyph->y = (hb_position_t) buffer->pos[i].y;
  }

  // ... (omitted for brevity)
}

As shown above, the problem occurs in the loop where cairo_glyph_t entries are being filled with the data from the hb_buffer_t object. The function does not validate if the number of glyphs provided in the input buffer (num_glyphs) matches the actual size of the allocated memory for the Cairo glyphs array. This can cause memory corruption due to buffer overflow if an attacker is able to control the buffer length.

The details of this vulnerability have been documented in the following references

1. HarfBuzz Official Repository: https://github.com/harfbuzz/harfbuzz
2. CVE-2024-56732 - MITRE: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-56732
3. CVE-2024-56732 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-56732

Exploit Details

While the process of exploiting the vulnerability may require intimate knowledge and control over the input processing, a successful exploit could lead to arbitrary code execution with the privileges of the application running HarfBuzz. This could potentially allow an attacker to compromise the system by gaining unauthorized access or causing a denial of service (DoS).

Considering the widespread use of HarfBuzz in various applications and operating systems, this vulnerability poses a significant risk. It's crucial for developers and maintainers of the affected software to apply the necessary patches and perform thorough security testing to minimize the potential impact.

Conclusion

The heap-based buffer overflow vulnerability (CVE-2024-56732) in HarfBuzz text shaping engine versions 8.5. to 10..1 is a serious security concern. By understanding the root cause and examining the code, developers can take appropriate action to mitigate the risks associated with this vulnerability. It is essential to keep your systems up-to-date, applying security patches and conducting rigorous testing to safeguard against potential exploitation.

Timeline

Published on: 12/27/2024 20:15:23 UTC
Last modified on: 12/28/2024 17:15:08 UTC