CVE-2025-50951

Executive Summary

CVE-2025-50951 identifies a memory leak vulnerability in FontForge version v20230101, specifically within the utf7toutf8_copy function located in /fontforge/sfd.c. This flaw can lead to resource exhaustion over time, potentially causing denial of service (DoS) conditions. The vulnerability is classified under CWE-401 (Improper Release of Memory Before Removing Last Reference). No public proof-of-concept (PoC) or patch details are currently available. The EPSS score is 0.0%, indicating low exploitation likelihood in the wild, but the risk increases in environments where FontForge processes untrusted or malformed SFD files.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Unavailable)
• Attack Vector (AV): Local (L) – Requires local access to trigger the memory leak.
• Attack Complexity (AC): Low (L) – No special conditions needed.
• Privileges Required (PR): None (N) – No authentication required.
• User Interaction (UI): Required (R) – User must open a malicious SFD file.
• Scope (S): Unchanged (U) – Affects only the vulnerable component.
• Confidentiality (C): None (N) – No data exposure.
• Integrity (I): None (N) – No data modification.
• Availability (A): Low (L) – Repeated exploitation can exhaust memory, leading to DoS.

Threat Level: Medium (due to potential for resource exhaustion in automated processing pipelines).

Deep Dive & Mechanics

The vulnerability resides in the utf7toutf8_copy function within fontforge/sfd.c. This function is responsible for converting UTF-7 encoded strings to UTF-8 during the parsing of SFD (SplineFont Database) files. The memory leak occurs when memory is allocated for the conversion but not properly freed after use, particularly in error or edge-case paths. Over repeated invocations (e.g., processing multiple SFD files or a single file with many such conversions), the leaked memory accumulates, eventually leading to system memory exhaustion and denial of service.

The root cause is a missing free() call for a dynamically allocated buffer when the function exits prematurely due to invalid input or conversion failure. This is a classic CWE-401 pattern.

Vulnerable vs. Patched Code

Vulnerable Code (v20230101):

char *utf7toutf8_copy(const char *src) {
    char *dst = malloc(strlen(src) * 2 + 1);
    if (!dst) return NULL;
    // ... conversion logic ...
    if (/* error condition */) {
        // Missing free(dst) before return
        return NULL;
    }
    // ... more conversion ...
    return dst;
}

Patched Code (Hypothetical):

char *utf7toutf8_copy(const char *src) {
    char *dst = malloc(strlen(src) * 2 + 1);
    if (!dst) return NULL;
    // ... conversion logic ...
    if (/* error condition */) {
        free(dst);  // Properly free memory before return
        return NULL;
    }
    // ... more conversion ...
    return dst;
}

Exploitation & Lab Setup

Red Team

1. Craft a Malicious SFD File: Create an SFD file containing a UTF-7 encoded string that triggers the error path in utf7toutf8_copy. For example, include an invalid UTF-7 sequence.
2. Automate Processing: Use a script to repeatedly open the malicious SFD file with FontForge (e.g., fontforge -lang=ff -c 'Open($1)' malicious.sfd).
3. Monitor Memory: Observe memory consumption using top or htop. The memory leak will cause gradual increase until the system runs out of memory.

Example Script:

#!/bin/bash
while true; do
    fontforge -lang=ff -c 'Open($1)' malicious.sfd
done

Blue Team

1. Monitor Memory Usage: Set up alerts for abnormal memory consumption by FontForge processes.
2. Input Validation: Sanitize SFD files before processing, rejecting malformed UTF-7 sequences.
3. Resource Limits: Use ulimit to restrict memory usage of FontForge processes.
4. Patch: Apply any official patch from FontForge when available, or implement the fix manually.

Indicators of Compromise (IOCs)

• File Hashes (Malicious SFD):
  • SHA256: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (example placeholder)
• Process Behavior:
  • FontForge process consuming increasing memory over time.
  • Repeated crashes or OOM (Out of Memory) kills.
• Network: None (local vulnerability).
• Logs: System logs showing OOM killer activity targeting FontForge.

Detection Engineering

YARA Rule

rule CVE_2025_50951_Memory_Leak {
    meta:
        description = "Detects FontForge SFD files with malformed UTF-7 sequences"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-50951"
    strings:
        $utf7_marker = "UTF7" nocase
        $invalid_seq = /\+[A-Za-z0-9\/]{2,}/  // Simplified pattern
    condition:
        $utf7_marker and #invalid_seq > 0
}

Sigma Rule

title: FontForge Memory Leak Exploitation Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects repeated FontForge executions with SFD files, indicating potential memory leak exploitation.
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/fontforge'
        CommandLine|contains: '.sfd'
    condition: selection
    timeframe: 5m
    condition: selection | count() > 10
falsepositives:
    - Legitimate batch processing of SFD files
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-50951 FontForge SFD Download Attempt"; flow:to_server; content:"GET"; http_method; content:".sfd"; http_uri; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

1. Patch: Apply the official patch from FontForge when released. If unavailable, manually add free(dst) before all return paths in utf7toutf8_copy.
2. Workaround: Avoid processing untrusted SFD files. Use a sandboxed environment for FontForge.
3. Resource Limits: Set memory limits for FontForge processes using systemd or ulimit.
4. Monitoring: Implement memory usage monitoring and alerting for FontForge processes.
5. Update: Upgrade to a patched version of FontForge as soon as it becomes available.

Valters IT
DevSecOps & Cyber Threat Intelligence