CVE-2025-27363 is a high-severity security vulnerability discovered in FreeType, a widely used open-source font rendering engine. If you run Linux, Android, or software that displays custom fonts (including many browsers and graphics editors), you may be indirectly at risk. This vulnerability lurks in all FreeType versions up to 2.13.. The issue is already patched in *newer versions*, but older releases remain *dangerously exploitable*.

Let's walk through what this bug is, how it works under the hood, example code, indications of live exploitation, and—most importantly—what you should do.

What Is CVE-2025-27363?

At its core, CVE-2025-27363 is an *out-of-bounds write* bug—a kind of memory corruption error. In vulnerable FreeType versions, a specific pathway for parsing "subglyph" glyph structures in TrueType GX and variable font files mishandles data types.

How?
A signed short value, coming from the font file, gets assigned to an *unsigned long* variable. When the engine adds a static offset to the new unsigned variable—without proper checks—it can cause an *integer wraparound*. This leads to allocating a heap buffer that's too small, but the code will later write up to six long values beyond the actual heap allocation. In practice, this could let a crafted font file overwrite heap memory, alter program flow, and run arbitrary code—a classic path to remote exploitation.

The Vulnerable Code: What Went Wrong?

Here’s a simplified version of the kind of logic that led to the bug (extracted and clarified for readability):

// input: num_subglyphs is from the font file, may be negative (signed short)
short num_subglyphs;
// ... read num_subglyphs from file ...

// BAD: assign to unsigned long (automatic wrap-around if negative)
unsigned long buffer_size = num_subglyphs + 6;  // 6 extra slots for later use

// Allocate memory for buffer_size slots - but if num_subglyphs is negative,
// buffer_size could wrap around to a huge number or be too small!
long* subglyph_loader = calloc(buffer_size, sizeof(long));

// Later, code writes up to 6 longs beyond original num_subglyphs location
for (int i = ; i < num_subglyphs + 6; i++) {
    subglyph_loader[i] = some_data[i];
}

If num_subglyphs is negative—let's say -6—the wrap-around causes buffer_size to be zero, but the for loop can still write six values, leading to *heap corruption*.

References

- FreeType Bugzilla report (example - not the real link)
- FreeType Commit Fixing the Bug (example)
- Official FreeType security advisories

Proof-of-Concept: Exploit Details

How could an attacker use this bug?
By crafting a malicious TrueType or variable font file with a negative subglyph count, an attacker could trigger the out-of-bounds write when the font gets loaded by any vulnerable FreeType-using application. This might be as simple as sending a font to a document viewer, a PDF renderer, or opening a malicious website in a browser that relies on host-side FreeType.

Minimal Exploit Snippet (Pseudocode)

# Generation of a malicious font (pseudocode - actual exploitation requires
# deeper TrueType format knowledge):

font_file = make_ttfont()
font_file.set_subglyph_count(-6)  # This gets stored as xFFF...FA in the file

save_font_to_disk(font_file, 'evil.ttf')

# Victim opens 'evil.ttf' in vulnerable app -> crash or code execution

Exploitation in the Wild

While there are unconfirmed reports, security researchers have raised alarms that similar bugs in FreeType often see -day exploitation—especially in browsers and PDF viewers on Linux and Android. It's strongly encouraged to assume this bug has been, or will be, exploited until you update.

Who Is Affected?

- Apps: Any that directly use FreeType ≤ 2.13., including many Linux desktop environments (KDE, GNOME), some image editors, and terminal emulators.

Check your version with

freetype-config --version

or

dpkg -l | grep freetype

Upgrade FreeType to 2.13.1 or newer

Download FreeType latest

(Use your platform’s package manager)

3. Rebuild/upgrade apps statically linked against FreeType

Conclusion

*CVE-2025-27363* is a classic example of how font parsing bugs can be just as dangerous as browser or image vulnerabilities—and why keeping low-level libraries up to date is critical. This out-of-bounds write bug is easy to trigger if you have the right crafted font file, so update FreeType and any applications that use it today.

References for Further Reading

- FreeType official security page
- Exploit-DB - FreeType vulnerabilities
- CVE Details - FreeType


*Written exclusively for you—please share responsibly! If you have more questions, drop them below or check the official advisories.*

Timeline

Published on: 03/11/2025 14:15:25 UTC
Last modified on: 03/13/2025 23:15:36 UTC