CVE-2025-6818

Executive Summary

CVE-2025-6818 is a locally exploitable heap-based buffer overflow vulnerability in HDF5 version 1.14.6. The flaw resides in the H5O__chunk_protect function within /src/H5Ochunk.c. Successful exploitation requires local access and user interaction, leading to a denial of service (application crash) due to memory corruption. The CVSS score is 3.3 (Low), and no patch is currently available. The exploit has been publicly disclosed, increasing the risk for systems using this specific version.

CVSS Breakdown & Threat Level

• CVSS Score: 3.3 (Low)
• Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
• Attack Vector (AV): Local – The attacker must have local access to the system.
• Attack Complexity (AC): Low – No special conditions are required.
• Privileges Required (PR): None – No authentication needed.
• User Interaction (UI): Required – The victim must open a malicious HDF5 file.
• Scope (S): Unchanged – The vulnerability does not affect other components.
• Confidentiality (C): None – No data exposure.
• Integrity (I): None – No data modification.
• Availability (A): Low – The application may crash, causing a denial of service.

Threat Level: Low. The requirement for local access and user interaction significantly limits the attack surface. However, the public availability of the exploit code means that targeted attacks against specific users or systems are possible.

Deep Dive & Mechanics

The vulnerability is a classic heap-based buffer overflow in the H5O__chunk_protect function. This function is responsible for protecting object header chunks during concurrent access. The flaw occurs when the function fails to properly validate the size of a chunk before copying data into a heap-allocated buffer. By crafting a malicious HDF5 file with an oversized chunk, an attacker can cause the function to write beyond the allocated buffer, corrupting adjacent heap memory.

Technical Details:

• Vulnerable Function: H5O__chunk_protect in /src/H5Ochunk.c
• Root Cause: Insufficient bounds checking on chunk size before a memcpy or similar operation.
• Trigger: Opening a specially crafted HDF5 file.
• Impact: Heap corruption leading to application crash (denial of service). In some cases, this could potentially be leveraged for arbitrary code execution, though the CVSS score suggests a lower impact.

Vulnerable vs. Patched Code

Since no patch is available, a direct comparison is not possible. However, the vulnerable code likely resembles:

// Vulnerable code (hypothetical)
void H5O__chunk_protect(H5O_chunk_t *chunk) {
    char *buffer = (char *)malloc(chunk->size);
    // No size validation
    memcpy(buffer, chunk->data, chunk->size); // Potential overflow if chunk->size is larger than allocated
}

A patched version would include a size check:

// Patched code (hypothetical)
void H5O__chunk_protect(H5O_chunk_t *chunk) {
    size_t max_size = 1024; // Example limit
    if (chunk->size > max_size) {
        // Handle error
        return;
    }
    char *buffer = (char *)malloc(chunk->size);
    memcpy(buffer, chunk->data, chunk->size);
}

Exploitation & Lab Setup

Red Team (Attack Simulation)

1. Setup: Install HDF5 1.14.6 on a test system.
2. Craft Payload: Create a malicious HDF5 file with an oversized chunk. This can be done using a hex editor or a custom script.
3. Deliver: Send the file to the victim via email, USB, or other local means.
4. Trigger: The victim opens the file using an application that relies on HDF5 (e.g., a scientific data viewer).
5. Result: The application crashes due to heap corruption.

Example Exploit (Conceptual):

# exploit.py - Craft a malicious HDF5 file
import struct

def create_malicious_hdf5(filename):
    # Minimal HDF5 file structure with oversized chunk
    # This is a simplified example; real exploit would require detailed HDF5 format knowledge
    with open(filename, 'wb') as f:
        # Superblock
        f.write(b'\x89HDF\r\n\x1a\n')
        # ... (other headers)
        # Object header with oversized chunk
        chunk_size = 0xFFFFFFFF  # Overflow value
        f.write(struct.pack('<I', chunk_size))
        f.write(b'A' * 1000)  # Padding

create_malicious_hdf5('malicious.h5')

Blue Team (Defense & Detection)

1. Prevention: Restrict execution of untrusted HDF5 files. Use application sandboxing.
2. Detection: Monitor for crashes in HDF5-based applications. Use the YARA and Sigma rules below.
3. Response: If a crash is detected, isolate the system and analyze the file for malicious content.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (Example – replace with actual if available)
  • malicious.h5: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
• Process Behavior: Application crash with heap corruption error.
• Network: No network indicators (local vulnerability).
• Registry/File System: Presence of HDF5 files with unusual chunk sizes.

Detection Engineering

YARA Rule

rule CVE_2025_6818_HDF5_HeapOverflow {
    meta:
        description = "Detects malicious HDF5 files exploiting CVE-2025-6818"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-6818"
    strings:
        $hdf5_magic = { 89 48 44 46 0D 0A 1A 0A }
        $large_chunk = { FF FF FF FF }  // 4-byte chunk size of 0xFFFFFFFF
    condition:
        $hdf5_magic at 0 and $large_chunk
}

Sigma Rule

title: CVE-2025-6818 HDF5 Heap Overflow Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects crashes or abnormal behavior in HDF5 applications
author: Valters IT
date: 2025-04-10
logsource:
    category: application
    product: hdf5
detection:
    selection:
        EventID: 1000  # Application Error (Windows)
        Application: "hdf5*"
        Description: "*heap*"
    condition: selection
falsepositives:
    - Legitimate large files
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-6818 HDF5 Heap Overflow Attempt"; flow:to_server; content:"|89 48 44 46 0D 0A 1A 0A|"; offset:0; depth:8; content:"|FF FF FF FF|"; within:100; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Avoid opening HDF5 files from untrusted sources. Use file validation tools to check for anomalies.
• Long-term: Monitor the HDF5 project for a patch. Once available, upgrade to the patched version.
• Workaround: If possible, use an alternative library or version (e.g., HDF5 1.14.5 or earlier) until a fix is released.
• Compensating Controls: Implement application whitelisting and sandboxing to limit the impact of exploitation.

Valters IT – DevSecOps & Cyber Threat Intelligence