CVE-2025-6856

Executive Summary

CVE-2025-6856 is a use-after-free vulnerability in HDF5 version 1.14.6, specifically within the H5FL__reg_gc_list function in src/H5FL.c. This flaw allows a local attacker to trigger a use-after-free condition, potentially leading to a denial of service or other unspecified impacts. The vulnerability has a CVSS score of 3.3 (Low), and a proof of concept has been publicly disclosed. No official patch is currently available.

Supply Chain & Package Security

This vulnerability affects the HDF5 library, which is a widely used data model and file format for storing and managing large amounts of data. It is commonly integrated into scientific computing, data analysis, and simulation software. Organizations using HDF5 in their software supply chain should assess their exposure and apply mitigations until a patch is released.

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)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: Low (A:L)

The low severity is due to the requirement for local access and the limited impact on availability. However, the public availability of an exploit increases the risk for targeted attacks.

Deep Dive & Mechanics

The vulnerability resides in the H5FL__reg_gc_list function within the HDF5 library's free list management system. The function is responsible for garbage collecting memory blocks from a free list. A use-after-free occurs when the function attempts to access a memory block that has already been freed, leading to undefined behavior. This can be triggered by a crafted sequence of memory allocation and deallocation operations, likely through a malicious HDF5 file or a specially crafted API call.

The root cause is a race condition or improper handling of memory pointers during the garbage collection process. When a block is freed and then later accessed by the garbage collector, the memory may have been reallocated for another purpose, leading to a crash or potential code execution.

Vulnerable vs. Patched Code

The vulnerable code is in src/H5FL.c within the H5FL__reg_gc_list function. The exact vulnerable and patched code snippets are not publicly available. However, the fix would likely involve adding proper checks to ensure that a memory block is still valid before accessing it, or by using a safe memory management pattern such as reference counting or a mark-and-sweep approach.

Vulnerable Code (Conceptual):

void H5FL__reg_gc_list(H5FL_reg_head_t *head) {
    H5FL_reg_node_t *node = head->free_list;
    while (node) {
        // ... process node ...
        // Node is freed here
        H5FL__reg_gc_free(node);
        // Use-after-free: node is accessed after being freed
        node = node->next;
    }
}

Patched Code (Conceptual):

void H5FL__reg_gc_list(H5FL_reg_head_t *head) {
    H5FL_reg_node_t *node = head->free_list;
    while (node) {
        H5FL_reg_node_t *next = node->next; // Save next pointer before freeing
        // ... process node ...
        H5FL__reg_gc_free(node);
        node = next; // Use saved pointer
    }
}

Exploitation & Lab Setup

Red Team

A proof of concept (PoC) for this vulnerability has been publicly disclosed. The exploit likely involves creating a specially crafted HDF5 file or making a series of API calls that trigger the use-after-free condition. The following is a conceptual example of how the exploit might be structured:

#include "hdf5.h"

int main() {
    // Create a file and dataset to trigger memory operations
    hid_t file_id = H5Fcreate("exploit.h5", H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
    hid_t dataspace_id = H5Screate_simple(1, (hsize_t[]){100}, NULL);
    hid_t dataset_id = H5Dcreate2(file_id, "data", H5T_NATIVE_INT, dataspace_id, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);

    // Perform operations that trigger the garbage collector
    // ... (specific sequence to trigger use-after-free)

    H5Dclose(dataset_id);
    H5Sclose(dataspace_id);
    H5Fclose(file_id);
    return 0;
}

Lab Setup:

1. Install HDF5 version 1.14.6 on a Linux virtual machine.
2. Compile the PoC with gcc -o exploit exploit.c -lhdf5.
3. Run the exploit: ./exploit.
4. Observe the crash or denial of service.

Blue Team

To detect exploitation attempts, monitor for abnormal crashes or memory corruption in applications using HDF5. Use the following detection rules.

Indicators of Compromise (IOCs)

• File Hashes: (SHA256 of known PoC files - not available yet)
• Process Names: Applications using libhdf5.so (e.g., h5dump, h5ls, custom applications)
• Log Entries: Segmentation faults or memory corruption errors in system logs (e.g., /var/log/syslog)
• Network Traffic: None (local vulnerability)
• File Artifacts: Malicious HDF5 files (.h5, .hdf5) that trigger the vulnerability

Detection Engineering

YARA Rule

rule CVE_2025_6856_HDF5_UseAfterFree {
    meta:
        description = "Detects HDF5 files or binaries potentially exploiting CVE-2025-6856"
        author = "Valters IT"
        date = "2025-01-01"
        reference = "CVE-2025-6856"
    strings:
        $hdf5_magic = { 89 48 44 46 0D 0A 1A 0A } // HDF5 magic bytes
        $vuln_func = "H5FL__reg_gc_list"
    condition:
        $hdf5_magic at 0 and $vuln_func
}

Sigma Rule

title: CVE-2025-6856 HDF5 Use-After-Free Exploitation Attempt
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects potential exploitation of CVE-2025-6856 via crash or memory corruption in HDF5 applications
author: Valters IT
date: 2025-01-01
logsource:
    product: linux
    service: syslog
detection:
    selection:
        - "segfault"
        - "SIGSEGV"
        - "memory corruption"
        - "H5FL__reg_gc_list"
    condition: selection
falsepositives:
    - Legitimate crashes due to other bugs
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-6856 HDF5 Use-After-Free Exploit Attempt"; flow:to_server; content:"|89 48 44 46 0D 0A 1A 0A|"; depth:8; content:"H5FL__reg_gc_list"; distance:0; within:100; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

1. Patch: No official patch is available. Monitor the HDF5 project for updates and apply the patch as soon as it is released.
2. Workaround: Limit local access to systems running HDF5 applications. Use sandboxing or containerization to isolate HDF5 processes.
3. Compile with Mitigations: Recompile HDF5 with address sanitizer (ASan) or other memory safety tools to detect and prevent use-after-free vulnerabilities.
4. Input Validation: Validate HDF5 files from untrusted sources before processing.
5. Monitoring: Implement the detection rules above to identify exploitation attempts.

Valters IT
Securing Digital Frontiers