CVE-2024-26540

Executive Summary

CVE-2024-26540 is a heap-based buffer overflow vulnerability in the Clmg library (versions prior to 3.3.3). The flaw resides in the cimg_library::CImg<unsigned char>::_load_analyze function, which is responsible for parsing Analyze 7.5 format medical image files. An attacker can trigger this vulnerability by providing a specially crafted file, leading to memory corruption. Successful exploitation could allow arbitrary code execution or denial of service. No public proof-of-concept (PoC) or exploit code is currently available, and the CVSS score is listed as 0.0 due to incomplete NVD data. The vulnerability is classified under MITRE ATT&CK tactic T1203 (Exploitation for Client Execution).

CVSS Breakdown & Threat Level

The NVD has not assigned a CVSS score or vector for this CVE. Based on the nature of the vulnerability (heap-based buffer overflow in a file parsing library), the following is an estimated assessment:

• Attack Vector (AV): Local (L) – Requires the victim to open a malicious file.
• Attack Complexity (AC): Low (L) – No special conditions beyond file opening.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): Required (R) – Victim must open the crafted file.
• Scope (S): Unchanged (U) – Exploitation does not cross trust boundaries.
• Confidentiality (C): High (H) – Potential for arbitrary code execution.
• Integrity (I): High (H) – Potential for arbitrary code execution.
• Availability (A): High (H) – Potential for denial of service.

Estimated CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H Estimated Base Score: 7.8 (High)

Threat Level: High. The vulnerability is in a widely used image processing library, and while no public exploit exists, the potential for remote code execution makes it a significant risk.

Deep Dive & Mechanics

The vulnerability is a heap-based buffer overflow in the _load_analyze function of the Clmg library. The Analyze 7.5 file format is a medical imaging format that includes a header with dimensions (e.g., width, height, depth) and a data block containing pixel values.

The root cause is a lack of proper bounds checking when reading the pixel data into a heap-allocated buffer. The function likely calculates the buffer size based on the header dimensions but fails to validate that the actual data in the file does not exceed this allocation. An attacker can craft a file with manipulated header fields (e.g., inflated dimensions) to cause the function to write more data into the buffer than allocated, resulting in a heap overflow.

Technical Flow:

1. The _load_analyze function reads the Analyze 7.5 header to obtain image dimensions (e.g., dim[0], dim[1], dim[2]).
2. It allocates a heap buffer of size width * height * depth * bytes_per_pixel.
3. It then reads the pixel data from the file into this buffer.
4. If the file contains more data than expected (due to manipulated header values), the read operation overflows the heap buffer.

This can corrupt adjacent heap metadata or other objects, potentially leading to code execution.

Vulnerable vs. Patched Code

Vulnerable Code (Clmg < 3.3.3): The vulnerable code likely resides in CImg.h within the _load_analyze function. The exact vulnerable snippet is not publicly available, but the issue is a missing bounds check during data reading.

// Hypothetical vulnerable code
template<typename T>
bool CImg<T>::_load_analyze(std::FILE *file) {
    // Read header
    analyze_header header;
    std::fread(&header, sizeof(header), 1, file);
    unsigned int width = header.dim[0];
    unsigned int height = header.dim[1];
    unsigned int depth = header.dim[2];
    unsigned int data_size = width * height * depth * sizeof(T);
    T *buffer = new T[data_size];
    // No bounds check - reads data_size bytes from file
    std::fread(buffer, sizeof(T), data_size, file);
    // ... process buffer
}

Patched Code (Clmg >= 3.3.3): The patch adds validation to ensure the data read does not exceed the allocated buffer size.

// Hypothetical patched code
template<typename T>
bool CImg<T>::_load_analyze(std::FILE *file) {
    analyze_header header;
    std::fread(&header, sizeof(header), 1, file);
    unsigned int width = header.dim[0];
    unsigned int height = header.dim[1];
    unsigned int depth = header.dim[2];
    unsigned int data_size = width * height * depth * sizeof(T);
    T *buffer = new T[data_size];
    // Bounds check added
    unsigned int file_size = get_file_size(file);
    if (data_size > file_size) {
        // Handle error
        delete[] buffer;
        return false;
    }
    std::fread(buffer, sizeof(T), data_size, file);
    // ... process buffer
}

Exploitation & Lab Setup

Red Team (Exploitation): No public exploit code is available. To develop a proof-of-concept, an attacker would need to:

1. Create a valid Analyze 7.5 file with a header specifying large dimensions (e.g., width=10000, height=10000).
2. Fill the data block with shellcode or a ROP chain.
3. Ensure the file size exceeds the calculated buffer size to trigger the overflow.
4. Use a tool like pwntools to craft the file and test against a vulnerable Clmg application.

Blue Team (Defense):

• Detection: Monitor for crashes or abnormal behavior in applications using Clmg (e.g., image viewers, medical software).
• Prevention: Use address space layout randomization (ASLR) and data execution prevention (DEP) to mitigate exploitation.
• Hunting: Search for processes that open Analyze 7.5 files and exhibit heap corruption.

Lab Setup:

• Victim: Ubuntu 22.04 with Clmg 3.3.2 installed.
• Attacker: Kali Linux with Python and pwntools.
• Steps:
  1. Install vulnerable Clmg: sudo apt install libcimg-dev=3.3.2
  2. Compile a test application that loads an Analyze file.
  3. Generate a malicious file using a Python script.
  4. Run the test application with the malicious file to observe the crash.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (No known malicious files yet)
• Process Names: Applications using Clmg (e.g., image_viewer, medical_app)
• Network Signatures: None (local file-based vulnerability)
• Registry Keys: Not applicable (Linux library)
• YARA Rule: See Detection Engineering section.

Detection Engineering

YARA Rule:

rule CVE_2024_26540_Analyze_Overflow {
    meta:
        description = "Detects malicious Analyze 7.5 files exploiting CVE-2024-26540"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2024-26540"
    strings:
        $analyze_header = { 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 } // Simplified header pattern
        $large_dim = { [0-9]{4} } // Large dimension values
    condition:
        $analyze_header at 0 and #large_dim > 2
}

Sigma Rule:

title: CVE-2024-26540 Heap Overflow in Clmg
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects crashes or heap corruption in applications using Clmg when processing Analyze 7.5 files.
author: Valters IT
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '.img' or '.hdr'
        CommandLine|contains: 'analyze'
    condition: selection
falsepositives:
    - Legitimate medical imaging software
level: high

Suricata Rule:

alert tcp any any -> any any (msg:"CVE-2024-26540 Potential Analyze File Transfer"; flow:to_client; fileext:"img"; content:"|00 00 00 00|"; offset:0; depth:4; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Clmg: Update to version 3.3.3 or later.
2. Input Validation: If upgrading is not possible, implement file validation to reject Analyze files with suspicious dimensions (e.g., width > 4096).
3. Sandboxing: Run applications that process Analyze files in a sandboxed environment.

Long-Term Fix:

• The vendor has patched the issue in version 3.3.3. Ensure all dependencies are updated.

Workarounds:

• Disable support for Analyze 7.5 format if not needed.
• Use a different image processing library.

Valters IT Signature:

Report generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.