CVE-2025-6816

Executive Summary

CVE-2025-6816 is a heap-based buffer overflow vulnerability in HDF5 version 1.14.6, specifically in the H5O__fsinfo_encode function within /src/H5Ofsinfo.c. This flaw allows a local attacker to trigger a heap overflow, potentially leading to a denial of service (DoS) or memory corruption. The vulnerability has a CVSS score of 3.3 (Low), indicating limited impact, but the public disclosure of an exploit increases the risk for systems running the affected version. No patch is currently available, and the EPSS score is 0.0%, suggesting low exploitation activity in the wild.

Supply Chain & Package Security

HDF5 is a widely used data model and file format for storing and managing large amounts of data, commonly integrated into scientific computing, machine learning, and data analysis pipelines. This vulnerability affects the core library, potentially impacting downstream packages that depend on HDF5. Users should monitor for updates from the HDF Group and consider temporary mitigations.

CVSS Breakdown & Threat Level

• CVSS Score: 3.3 (Low)
• Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• 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: None (UI:N) – no user action required.
• Scope: Unchanged (S:U) – impact is confined to the vulnerable component.
• Confidentiality Impact: None (C:N) – no data exposure.
• Integrity Impact: None (I:N) – no data modification.
• Availability Impact: Low (A:L) – potential for denial of service.

Threat Level: Low. The vulnerability requires local access and primarily affects availability. However, the public exploit disclosure could lead to targeted attacks.

Deep Dive & Mechanics

The vulnerability resides in the H5O__fsinfo_encode function, which is responsible for encoding file space information (fsinfo) into a buffer. The function fails to properly validate the size of the output buffer, leading to a heap-based buffer overflow when writing data.

Root Cause: The function calculates the required buffer size incorrectly or does not check the available space before writing. When processing crafted HDF5 files, the overflow can corrupt adjacent heap memory, potentially causing a crash or arbitrary code execution.

Attack Vector: An attacker with local access can craft a malicious HDF5 file that, when opened by an application using the vulnerable HDF5 library, triggers the overflow. The exploit is publicly available, making it easier for attackers to weaponize.

Impact: Successful exploitation can lead to denial of service (application crash) or, in more severe cases, memory corruption that could be leveraged for privilege escalation or code execution, though the CVSS score suggests limited impact.

Vulnerable vs. Patched Code

Vulnerable Code (HDF5 1.14.6):

// In /src/H5Ofsinfo.c
herr_t H5O__fsinfo_encode(H5F_t *f, H5O_t *oh, unsigned char *buf, size_t *buf_size) {
    // ... (missing size check)
    // Directly writes to buf without verifying buf_size
    *buf++ = some_data;
    // ... (more writes)
}

Patched Code (Hypothetical):

// In /src/H5Ofsinfo.c
herr_t H5O__fsinfo_encode(H5F_t *f, H5O_t *oh, unsigned char *buf, size_t *buf_size) {
    size_t needed_size = calculate_needed_size();
    if (needed_size > *buf_size) {
        // Handle error: buffer too small
        return FAIL;
    }
    // Safe writes with bounds checking
    // ...
}

Note: No official patch is available yet. The above is a hypothetical fix.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Local access to a system running HDF5 1.14.6.
• Ability to create or modify HDF5 files.

Exploit Steps:

1. Craft a malicious HDF5 file with specially crafted file space information that triggers the overflow in H5O__fsinfo_encode.
2. Deliver the file to the target system (e.g., via email, USB, or network share).
3. When the victim opens the file with an application linked against the vulnerable HDF5 library, the overflow occurs.

Example Exploit (Conceptual):

# exploit.py - Craft malicious HDF5 file
import h5py
import numpy as np

# Create a malicious HDF5 file with oversized fsinfo
with h5py.File('malicious.h5', 'w') as f:
    # Trigger the vulnerable code path
    f.attrs['fsinfo'] = np.zeros(10000, dtype=np.uint8)  # Oversized data

Note: The actual exploit code is publicly available but not included here for ethical reasons.

Blue Team (Detection & Defense)

Detection:

• Monitor for crashes in applications using HDF5 (e.g., segmentation faults).
• Use file integrity monitoring to detect creation of suspicious HDF5 files.
• Deploy endpoint detection and response (EDR) rules for heap overflow patterns.

Defense:

• Restrict local access to trusted users only.
• Use application whitelisting to prevent execution of untrusted HDF5 files.
• Apply memory protection mechanisms (ASLR, DEP) to mitigate exploitation.

Indicators of Compromise (IOCs)

• File Hashes (Malicious HDF5 files):
  • SHA256: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (example)
• Process Behavior:
  • Application crashes with heap corruption errors.
  • Unexpected memory access violations in H5O__fsinfo_encode.
• Network Indicators:
  • No network indicators (local vulnerability).

Detection Engineering

YARA Rule

rule CVE_2025_6816_HDF5_HeapOverflow {
    meta:
        description = "Detects malicious HDF5 files exploiting CVE-2025-6816"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-6816"
    strings:
        $hdf5_magic = { 89 48 44 46 0D 0A 1A 0A }  // HDF5 magic bytes
        $fsinfo_pattern = { 66 73 69 6E 66 6F }  // "fsinfo" string
    condition:
        $hdf5_magic at 0 and $fsinfo_pattern
}

Sigma Rule

title: CVE-2025-6816 HDF5 Heap Overflow Detection
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects potential exploitation of CVE-2025-6816 via heap overflow in HDF5
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\hdf5.dll'
        CommandLine|contains: 'H5O__fsinfo_encode'
    condition: selection
falsepositives:
    - Legitimate HDF5 file processing
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-6816 HDF5 Heap Overflow Attempt"; flow:to_server,established; content:"|89 48 44 46 0D 0A 1A 0A|"; depth:8; content:"fsinfo"; distance:0; within:100; classtype:attempted-dos; 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 exploitation.
3. Monitor for exploits: Use the provided detection rules to identify attacks.

Long-term Fix:

• Apply patch: Once available, update to the patched version of HDF5.
• Upgrade: Consider upgrading to a newer version of HDF5 (e.g., 1.14.7 or later) if a fix is released.
• Workaround: If patching is not possible, avoid opening untrusted HDF5 files.

Vendor Contact: The HDF Group (https://www.hdfgroup.org/) – no official response yet.

Valters IT
DevSecOps & Cyber Threat Intelligence