In 2022, a critical heap buffer overflow vulnerability, tracked as CVE-2022-27404, was found in the FreeType project—an open source font rendering engine used by Chrome, Firefox, and many others. The root cause was a flaw in the sfnt_init_face function, introduced by commit 1e2eb650. In this post, we’ll break down what happened, look at code, explain how an attacker could use it, and link you to the original resources.

What Is FreeType and Why Care?

FreeType renders fonts for many libraries and browsers. When you open a PDF or visit a website, your device may use FreeType to draw the text. Because FreeType deals with (sometimes untrusted) font files, a bug here can let attackers attack users by simply tricking them into opening a webpage or document.

The Commit and Vulnerability Details

The risky code appeared in commit 1e2eb650:

diff --git a/src/sfnt/sfobjs.c b/src/sfnt/sfobjs.c
@@ -1116,7 +1116,13 @@
     ...
-    face->ttc_header.version = FT_GET_LONG();
-    face->ttc_header.count   = FT_GET_LONG();
-    ...
+    face->ttc_header.version = FT_GET_LONG();
+    face->ttc_header.count   = FT_GET_LONG();
+    if ( face->ttc_header.count > xFFFF )
+      return FT_THROW( Invalid_Table );
+    if ( FT_NEW_ARRAY( face->ttc_header.offsets, face->ttc_header.count ) )
+      goto Exit;

Here, there was a dangerous mishandling of font header fields: user-supplied values were read from the font file without proper validation, and then used to allocate memory and load offsets. If an attacker supplies a very large count, the memory allocation can go wrong.

Buffer Overflow

Why is this dangerous? If a malicious font sets a huge count (say, x100000), the resulting allocation might overflow or misbehave, and later code that writes to those offset slots will smash the heap.

Simplified vulnerable code pattern

uint32_t count = read_from_font_file();
Offset *offsets = malloc(count * sizeof(Offset));
// No check if count * sizeof(Offset) wrapped!
for (uint32_t i = ; i < count; ++i) {
    offsets[i] = read_offset_from_file();
    // Heap buffer overflow if offsets is too small!
}

Exploiting The Vulnerability

1. Craft a Malicious Font: The attacker creates a specially crafted TrueType Collection (TTC) font file, where the collection count is set to a huge number to force the heap buffer allocation to misbehave.

2. Heap Corruption: When the vulnerable function (sfnt_init_face) is run, it tries to allocate a huge buffer, possibly causing integer overflow, allocating too little, and then writing data, overrunning the buffer.

3. Code Execution: With heap corruption, attackers can often manipulate program memory, potentially hijacking code execution if the environment isn't hardened (like browsers without sandboxing).

Example Exploit Snippet

Below is a proof of concept (POC) font header that could trigger the bug (in a real attack, the attacker would build a valid-looking but malicious TTC file):

# PoC TTF header with huge count (oversized TTC index)
ttc_header = b'ttcf'               # TTC magic
ttc_version = b'\x00\x01\x00\x00'  # version 1.
ttc_count = b'\x00\x10\x00\x00'    # x100000 collections (16,777,216 in hex)
# Followed by offsets (empty for simplicity)
malicious_ttc = ttc_header + ttc_version + ttc_count
open('crash.ttc', 'wb').write(malicious_ttc)

Loading this file with a vulnerable FreeType build can crash or exploit the host.

The Fix

The FreeType team limited the count field size and added error handling. Always update to latest FreeType.

Resources & Further Reading

- FreeType GitHub commit: 1e2eb65048f75c64b68708efed6ce904c31f3b2f
- NVD CVE-2022-27404 entry
- oss-security mailing list for CVE-2022-27404
- FreeType changelog about the bug
- How Heap buffer overflows work

Always run untrusted content in sandboxes or containers when possible.

If you run software relying on FreeType, upgrade as soon as possible. Even a simple font file can open the door to a heap buffer overflow—and, potentially, much worse.

Timeline

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