CVE-2026-10231

Executive Summary

CVE-2026-10231 is a heap-based buffer overflow vulnerability discovered in the Assimp library, specifically within the Half-Life 1 MDL Loader component. The flaw resides in the HL1MDLLoader::extract_anim_value function within the HL1MDLLoader.cpp file. By manipulating the num.total argument, an attacker can trigger a heap-based buffer overflow, potentially leading to application crashes or arbitrary code execution. The vulnerability affects Assimp versions up to and including 6.0.4. The attack vector is local, requiring user interaction to load a maliciously crafted Half-Life 1 MDL file. The exploit has been publicly released, increasing the risk of exploitation. The project has acknowledged the issue as a bug, but no official patch has been released at this time.

Supply Chain & Package Security

Assimp (Open Asset Import Library) is a widely used library for importing various 3D model formats into applications. It is integrated into numerous software projects, including game engines, 3D modeling tools, and rendering applications. This vulnerability poses a significant supply chain risk, as any application that uses the vulnerable version of Assimp to load Half-Life 1 MDL files could be exploited. Developers and organizations using Assimp in their software stack should immediately assess their exposure and apply mitigations until an official patch is available.

CVSS Breakdown & Threat Level

• CVSS Score: 5.3 (Medium)
• Attack Vector (AV): Local (L) - The attacker must have local access to the system or trick a user into loading a malicious file.
• Attack Complexity (AC): Low (L) - The exploit is straightforward and does not require special conditions.
• Privileges Required (PR): None (N) - No privileges are needed to trigger the vulnerability.
• User Interaction (UI): Required (R) - The victim must load a malicious MDL file.
• Scope (S): Unchanged (U) - The vulnerability does not affect other components.
• Confidentiality (C): None (N) - No data confidentiality impact.
• Integrity (I): None (N) - No data integrity impact.
• Availability (A): High (H) - Successful exploitation can cause a denial of service or potentially allow code execution, impacting availability.

Threat Level: Medium. While the CVSS score is moderate, the public availability of exploit code and the widespread use of Assimp elevate the risk. The vulnerability is particularly dangerous in environments where users load untrusted 3D model files.

Deep Dive & Mechanics

The vulnerability is a classic heap-based buffer overflow. The HL1MDLLoader::extract_anim_value function is responsible for extracting animation values from a Half-Life 1 MDL file. The function reads a num.total value from the file, which is used to allocate a buffer on the heap. However, the code does not properly validate this value against the actual size of the input data. By providing a crafted MDL file with an excessively large num.total value, an attacker can cause the function to allocate a smaller buffer than needed. When the function then writes animation data into this buffer, it overflows into adjacent heap memory, corrupting heap metadata or other data structures.

The overflow can lead to:

• Denial of Service (DoS): Corrupting heap metadata can cause the application to crash.
• Arbitrary Code Execution (ACE): With careful heap manipulation, an attacker may be able to overwrite function pointers or other critical data to gain control of the execution flow.

The attack is initiated locally, meaning the attacker must either have local access to the system or convince a user to open a malicious MDL file (e.g., via email, download, or a compromised website).

Vulnerable vs. Patched Code

The vulnerable code is in the HL1MDLLoader::extract_anim_value function. The issue is the lack of bounds checking on the num.total value before using it for memory allocation and subsequent writes.

Vulnerable Code (Conceptual):

void HL1MDLLoader::extract_anim_value(const unsigned char* data, size_t data_size) {
    // Read num.total from the data
    int num_total = *(int*)(data + offset);
    // Allocate buffer based on num_total without validation
    float* buffer = new float[num_total];
    // Copy animation data into buffer - potential overflow if num_total is larger than actual data
    for (int i = 0; i < num_total; i++) {
        buffer[i] = *(float*)(data + data_offset + i * sizeof(float));
    }
}

Patched Code (Conceptual):

void HL1MDLLoader::extract_anim_value(const unsigned char* data, size_t data_size) {
    // Read num.total from the data
    int num_total = *(int*)(data + offset);
    // Validate num_total against available data size
    if (num_total <= 0 || num_total > (data_size - data_offset) / sizeof(float)) {
        // Handle error - invalid num_total
        return;
    }
    // Allocate buffer with validated size
    float* buffer = new float[num_total];
    // Copy animation data safely
    for (int i = 0; i < num_total; i++) {
        buffer[i] = *(float*)(data + data_offset + i * sizeof(float));
    }
}

The patch adds a validation check to ensure num_total does not exceed the available data size, preventing the buffer overflow.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A system running an application that uses Assimp <= 6.0.4.
• Ability to deliver a malicious Half-Life 1 MDL file to the target user.

Exploitation Steps:

1. Craft Malicious MDL File: Create a specially crafted Half-Life 1 MDL file with an oversized num.total value in the animation data section. This can be done using a hex editor or a custom script.
2. Deliver the File: Send the malicious MDL file to the target via email, download link, or physical media.
3. Trigger the Vulnerability: The target user opens the MDL file in an application that uses the vulnerable Assimp library (e.g., a 3D model viewer, game, or editor).
4. Achieve Impact: The heap-based buffer overflow occurs, potentially crashing the application or allowing arbitrary code execution.

Example Exploit Code (Conceptual):

import struct

def create_malicious_mdl():
    # Minimal MDL header (simplified)
    mdl_data = b'\x00' * 100  # Placeholder header
    # Offset to animation data
    anim_offset = 100
    # Set num.total to a large value to trigger overflow
    num_total = 10000
    mdl_data += struct.pack('<I', num_total)
    # Add more data to fill the rest of the file
    mdl_data += b'\x41' * 1000
    with open('malicious.mdl', 'wb') as f:
        f.write(mdl_data)

create_malicious_mdl()

Blue Team

Detection and Mitigation:

1. Monitor for Suspicious MDL Files: Use file integrity monitoring (FIM) to detect the creation or modification of MDL files, especially those with unusual sizes or structures.
2. Application Sandboxing: Run applications that use Assimp in a sandboxed environment to limit the impact of exploitation.
3. User Awareness: Educate users about the risks of opening untrusted 3D model files.
4. Network Segmentation: Limit the ability to deliver malicious files to critical systems.
5. Patch Management: Monitor for official patches from the Assimp project and apply them as soon as they are available.

Indicators of Compromise (IOCs)

• File Hashes (MD5/SHA1/SHA256): (Example - replace with actual hashes if available)
  • malicious.mdl: d41d8cd98f00b204e9800998ecf8427e (MD5)
• File Names: *.mdl files with unusual sizes or structures.
• Process Behavior: Application crashes or abnormal memory access patterns when loading MDL files.
• Network Traffic: Unusual outbound connections from applications processing MDL files (potential C2).
• Registry/File System Changes: Modifications to application configuration files or creation of new executable files.

Detection Engineering

YARA Rule

rule CVE_2026_10231_Assimp_HL1_MDL_Overflow {
    meta:
        description = "Detects malicious Half-Life 1 MDL files exploiting CVE-2026-10231"
        author = "Valters IT"
        date = "2025-04-14"
        reference = "CVE-2026-10231"
    strings:
        $mdl_header = "IDST"  // Half-Life MDL header magic bytes
        $large_num_total = { [0-3] [0-3] [0-3] [0-3] }  // Pattern for large num.total values
    condition:
        $mdl_header at 0 and uint32(0x64) > 1000  // Check num.total at offset 100 (example)
}

Sigma Rule

title: CVE-2026-10231 - Assimp HL1 MDL Heap Buffer Overflow
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects attempts to exploit a heap-based buffer overflow in Assimp's HL1 MDL loader.
author: Valters IT
date: 2025-04-14
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-10231
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\assimp.dll'
        CommandLine|contains: '*.mdl'
    condition: selection
falsepositives:
    - Legitimate use of MDL files
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-10231 - Assimp HL1 MDL Heap Buffer Overflow"; flow:to_server,established; content:"IDST"; offset:0; depth:4; content:"|00 00 00 00|"; within:100; byte_test:4,>,1000,100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Actions:

   • Disable HL1 MDL Loading: If possible, disable the Half-Life 1 MDL loader in applications using Assimp until a patch is available.
   • Input Validation: Implement additional input validation for MDL files at the application level, checking for suspicious num.total values.
   • Use Memory Safe Languages: Consider using memory-safe languages or libraries for parsing untrusted data.

2. Long-term Fix:

   • Apply Official Patch: Monitor the Assimp project (https://github.com/assimp/assimp) for an official patch and apply it as soon as it is released.
   • Upgrade Assimp: Upgrade to a patched version of Assimp once available.
   • Code Review: Conduct a thorough code review of the HL1 MDL loader and other parsers for similar vulnerabilities.

3. Workarounds:

   • Application Whitelisting: Only allow trusted applications to process MDL files.
   • File Reputation Services: Use file reputation services to scan MDL files before opening them.
   • User Training: Train users to avoid opening MDL files from untrusted sources.

Valters IT
Securing Digital Frontiers