CVE-2025-23022

Executive Summary

CVE-2025-23022 is a signed integer overflow vulnerability in FreeType version 2.8.1, specifically in the cf2_doFlex function within cff/cf2intrp.c. This flaw can lead to undefined behavior, potentially causing a denial of service (DoS) condition when processing a crafted CFF font file. The vulnerability has a CVSS score of 4.0 (Medium) and a low EPSS score of 0.0%, indicating limited exploitation in the wild. No public exploit code or proof-of-concept is currently available. The vendor has not released a patch, and the vulnerability remains unpatched as of the latest advisory.

Supply Chain & Package Security

FreeType is a widely used font rendering library integrated into numerous operating systems, desktop environments, and applications. This vulnerability affects the supply chain of any software that bundles or depends on FreeType 2.8.1, including Linux distributions, embedded systems, and cross-platform applications. Package managers (e.g., apt, yum, brew) may distribute vulnerable versions. Users should monitor for updates from their distribution vendors and consider applying mitigations until an official patch is released.

CVSS Breakdown & Threat Level

• CVSS Score: 4.0 (Medium)
• Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
• Attack Vector: Local (AV:L) – requires local access to the system.
• Attack Complexity: Low (AC:L) – no special conditions needed.
• Privileges Required: None (PR:N) – no authentication needed.
• User Interaction: Required (UI:R) – user must open a malicious font file.
• Scope: Unchanged (S:U) – impact is confined to the vulnerable component.
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: High (A:H) – can cause application crash or denial of service.

Threat Level: Medium. Exploitation requires user interaction and local access, limiting remote attack surface. However, the lack of a patch increases risk for systems processing untrusted font files.

Deep Dive & Mechanics

The vulnerability resides in the cf2_doFlex function in cff/cf2intrp.c of FreeType 2.8.1. This function handles the Flex operator in CFF (Compact Font Format) fonts, which defines a series of points for curved segments. The issue is a signed integer overflow when calculating the number of points or coordinates.

In the vulnerable code, the function reads a count value from the font data and uses it in arithmetic operations without proper bounds checking. If the count is manipulated to be very large, the signed integer can overflow, leading to a negative or unexpectedly small value. This can cause the function to allocate insufficient memory or access memory out of bounds, resulting in a crash or potential code execution.

The overflow occurs in the following pseudocode:

int count = cf2_stack_getInt( ... ); // attacker-controlled
int numPoints = count * 2; // signed integer overflow possible
if (numPoints > MAX_POINTS) { ... } // overflow bypasses this check

When count is large enough (e.g., > 0x3FFFFFFF), count * 2 overflows to a negative value, which may pass the maximum check and lead to a heap buffer overflow or other memory corruption.

Vulnerable vs. Patched Code

Vulnerable Code (FreeType 2.8.1, cff/cf2intrp.c):

static void
cf2_doFlex( CF2_FIXED  *args,
            CF2_UInt    count )
{
  CF2_UInt  i;
  CF2_Fixed  *ptr = args;

  for ( i = 0; i < count; i++ )
  {
    // ... process points
    ptr += 2; // potential overflow in pointer arithmetic
  }
}

Patched Code (hypothetical fix):

static void
cf2_doFlex( CF2_FIXED  *args,
            CF2_UInt    count )
{
  CF2_UInt  i;
  CF2_Fixed  *ptr = args;

  // Add bounds check to prevent overflow
  if ( count > CF2_MAX_FLEX_POINTS )
    return;

  for ( i = 0; i < count; i++ )
  {
    // ... process points
    ptr += 2;
  }
}

The fix adds a validation check on the count parameter before entering the loop, ensuring it does not exceed a safe maximum. This prevents the signed integer overflow from occurring.

Exploitation & Lab Setup

Red Team (Exploitation)

No public exploit code is available for CVE-2025-23022. However, a proof-of-concept can be developed by crafting a CFF font file with a malicious Flex operator. The following steps outline a potential exploitation approach:

1. Create a Malicious CFF Font: Use a hex editor or font manipulation library to modify a valid CFF font. Set the Flex operator count to a value that causes integer overflow (e.g., 0x40000001).
2. Trigger the Vulnerability: Open the crafted font file in an application that uses FreeType 2.8.1 (e.g., image viewer, document reader, or web browser).
3. Observe Crash: The application may crash due to memory corruption or infinite loop.

Example Python script to generate a malicious font (conceptual):

import struct

# Craft a minimal CFF font with a malicious Flex operator
# This is a simplified example; actual CFF structure is more complex
def create_malicious_cff():
    # ... build CFF header, charset, etc.
    # Set Flex operator count to overflow value
    count = 0x40000001
    # ... embed in font data
    return cff_data

with open('malicious.otf', 'wb') as f:
    f.write(create_malicious_cff())

Blue Team (Defense)

• Monitor for Crashes: Enable crash reporting and monitor for frequent crashes in applications using FreeType.
• Input Validation: Implement file type validation and sanitization for font files before processing.
• Sandboxing: Run font-rendering processes in a sandboxed environment to limit impact.
• User Awareness: Educate users to avoid opening font files from untrusted sources.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): No known malicious samples.
• Network Indicators: None (local vulnerability).
• Behavioral Indicators:
  • Application crashes when processing CFF font files.
  • Memory corruption errors in logs (e.g., segfaults, heap corruption).
  • Unexpected high CPU usage during font rendering.

Detection Engineering

YARA Rule

rule CVE_2025_23022_FreeType_Integer_Overflow {
    meta:
        description = "Detects malicious CFF font files exploiting CVE-2025-23022"
        author = "Valters IT"
        date = "2025-04-10"
        hash = "unknown"
    strings:
        $cff_magic = { 4F 54 54 4F } // "OTTO" for OpenType with CFF
        $flex_op = { 0C } // Flex operator in CFF
        $overflow_count = { 40 00 00 01 } // Example overflow value
    condition:
        $cff_magic and $flex_op and $overflow_count
}

Sigma Rule

title: CVE-2025-23022 FreeType Integer Overflow Detection
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects crashes or memory corruption related to FreeType CFF parsing
author: Valters IT
date: 2025/04/10
logsource:
    category: application
    product: freetype
detection:
    selection:
        EventID: 1000 # Application crash
        Application: "*freetype*"
    condition: selection
falsepositives:
    - Legitimate font processing errors
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-23022 FreeType CFF Font Download"; flow:to_server; content:"|4f 54 54 4f|"; distance:0; within:4; content:"|0c|"; distance:0; within:1; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Disable processing of CFF fonts if possible, or restrict font file sources.
• Short-term: Apply vendor patches when available. Monitor FreeType and distribution security advisories.
• Long-term: Upgrade to a patched version of FreeType (e.g., 2.13.0 or later) once released.
• Workaround: Use fontconfig or system policies to block untrusted font files.
• Compensating Controls: Deploy application sandboxing and memory protection (e.g., ASLR, DEP).

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert