CVE-2026-10233

Executive Summary

CVE-2026-10233 is a locally exploitable out-of-bounds read vulnerability in the Assimp library, specifically within the Half-Life 1 MDL loader component (HL1MDLLoader.cpp). The flaw resides in the HL1MDLLoader::read_sequence_infos function, where improper handling of the aiString argument can lead to reading memory beyond the allocated buffer. This vulnerability has a CVSS score of 3.3 (Low), requires local access and user interaction, and has a public proof-of-concept (PoC) exploit available. The project has acknowledged this as a bug but has not yet released a patch. This report provides a comprehensive technical analysis, detection rules, and exploitation guidance for both Red and Blue Teams.

Supply Chain & Package Security

Assimp (Open Asset Import Library) is a widely used C++ library for importing various 3D model formats. It is a dependency in many game engines, 3D modeling tools, and rendering applications. A vulnerability in Assimp can propagate to any software that uses it to load Half-Life 1 MDL files. While the CVSS score is low, the public availability of the exploit and the lack of a patch increase the risk for supply chain attacks, especially in environments where untrusted 3D models are processed.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N
• Base Score: 3.3 (Low)
• Attack Vector (AV): Local (L) – The attacker must have local access to the system.
• Attack Complexity (AC): Low (L) – The exploit 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): Low (L) – Limited memory disclosure is possible.
• Integrity (I): None (N) – No data modification is possible.
• Availability (A): None (N) – The vulnerability does not cause a denial of service.

Threat Level: Low. The requirement for local access and user interaction significantly reduces the attack surface. However, the public exploit and lack of a patch mean that targeted attacks against specific users or systems are possible.

Deep Dive & Mechanics

The vulnerability is located in the HL1MDLLoader::read_sequence_infos function within HL1MDLLoader.cpp. This function is responsible for parsing sequence information from Half-Life 1 MDL files. The issue arises when the function reads an aiString argument from the file. An aiString is a fixed-size string buffer used by Assimp. If the MDL file contains a string that is longer than the allocated buffer, the function reads beyond the buffer's bounds, leading to an out-of-bounds read.

The root cause is a lack of proper bounds checking before copying or reading the string data. The function likely uses a memcpy or similar operation without verifying that the source data length does not exceed the destination buffer size. This allows an attacker to craft a malicious MDL file that triggers the out-of-bounds read when processed by an application using Assimp.

The exploit is triggered locally, meaning the attacker must convince a user to open a specially crafted MDL file with an application that uses the vulnerable Assimp library. The out-of-bounds read can potentially leak sensitive memory contents, though the practical impact is limited due to the low severity.

Vulnerable vs. Patched Code

Vulnerable Code (Assimp <= 6.0.4):

The vulnerable code in HL1MDLLoader.cpp likely resembles the following (simplified for illustration):

void HL1MDLLoader::read_sequence_infos(aiString &out) {
    // Read string length from file
    unsigned int len;
    mReader->Read(&len, sizeof(len));

    // Read string data directly into aiString buffer without bounds check
    mReader->Read(out.data, len);
    out.length = len;
}

The issue is that out.data has a fixed size (typically 1024 bytes), but the code reads len bytes directly into it without checking if len exceeds the buffer size.

Patched Code (Hypothetical):

A patched version would include a bounds check:

void HL1MDLLoader::read_sequence_infos(aiString &out) {
    unsigned int len;
    mReader->Read(&len, sizeof(len));

    // Bounds check: ensure len does not exceed buffer size
    if (len > AI_MAXLEN) {
        len = AI_MAXLEN - 1; // Truncate or handle error
    }

    mReader->Read(out.data, len);
    out.data[len] = '\0';
    out.length = len;
}

Note: As of this report, no official patch has been released. The above is a hypothetical fix.

Exploitation & Lab Setup

Red Team

Objective: Trigger the out-of-bounds read to leak memory or cause a controlled crash.

Lab Setup:

1. Environment: A Linux or Windows machine with Assimp 6.0.4 installed.
2. Tools: A hex editor (e.g., xxd, 010 Editor) and a compiler (e.g., g++).
3. Sample MDL File: Obtain a valid Half-Life 1 MDL file to use as a base.

Exploitation Steps:

1. Craft Malicious MDL File:

   • Locate the sequence info section in the MDL file.
   • Modify the string length field to a value larger than the aiString buffer (e.g., 2000 bytes).
   • Fill the string data with a pattern (e.g., AAAA...) to observe the out-of-bounds read.

2. Trigger the Vulnerability:

   • Use a simple C++ program that loads the malicious MDL file using Assimp:

     #include <assimp/Importer.hpp>
     #include <assimp/scene.h>
     #include <iostream>
     
     int main() {
         Assimp::Importer importer;
         const aiScene* scene = importer.ReadFile("malicious.mdl", 0);
         if (!scene) {
             std::cerr << "Error: " << importer.GetErrorString() << std::endl;
             return 1;
         }
         std::cout << "File loaded successfully." << std::endl;
         return 0;
     }
     

3. Observe the Out-of-Bounds Read:

   • Run the program under a debugger (e.g., GDB) or with AddressSanitizer enabled.
   • The out-of-bounds read will be detected, potentially leaking memory contents.

Expected Outcome: The program may crash or leak memory, depending on the environment and the size of the crafted string.

Blue Team

Objective: Detect and prevent exploitation of CVE-2026-10233.

Detection Steps:

1. Monitor for Suspicious MDL Files:

   • Look for MDL files with unusually large string fields in the sequence info section.
   • Use file integrity monitoring (FIM) to detect modifications to MDL files.

2. Application Behavior Monitoring:

   • Monitor applications that use Assimp for crashes or abnormal memory access patterns.
   • Use endpoint detection and response (EDR) tools to detect out-of-bounds read attempts.

3. User Awareness:

   • Educate users about the risks of opening untrusted 3D model files.
   • Implement strict file upload policies if users can upload MDL files.

Mitigation Steps:

1. Patch Management:

   • Since no official patch exists, consider using a workaround or disabling the Half-Life 1 MDL loader if not needed.
   • Monitor the Assimp project for updates and apply patches as soon as they are released.

2. Input Validation:

   • Implement additional input validation for MDL files before passing them to Assimp.
   • Use a sandboxed environment to process untrusted 3D models.

3. Compile with Security Flags:

   • If you compile Assimp from source, enable AddressSanitizer (-fsanitize=address) to detect out-of-bounds reads during development.

Indicators of Compromise (IOCs)

• File Hashes (MD5/SHA256): (Example hashes for a malicious MDL file)
  • malicious.mdl: e99a18c428cb38d5f260853678922e03 (MD5)
  • malicious.mdl: a665a45920422f9d417e4867efdc4fb8a04a1f3fff1fa07e998e86f7f7a27ae3 (SHA256)
• File Paths: Any .mdl file processed by an application using Assimp.
• Network Traffic: None (local vulnerability).
• Process Behavior: Applications using Assimp may crash or exhibit abnormal memory access patterns when loading malicious MDL files.
• Registry/File System Changes: None expected.

Detection Engineering

YARA Rule

rule CVE_2026_10233_Assimp_OOB_Read {
    meta:
        description = "Detects malicious Half-Life 1 MDL files exploiting CVE-2026-10233"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2026-10233"
        hash = "a665a45920422f9d417e4867efdc4fb8a04a1f3fff1fa07e998e86f7f7a27ae3"

    strings:
        $mdl_header = "IDST"  // Half-Life MDL header
        $seq_info_marker = { 00 00 00 00 }  // Placeholder for sequence info offset

    condition:
        $mdl_header at 0 and
        // Check for unusually large string length in sequence info section
        uint32(0x100) > 1024  // Example: offset 0x100 contains string length
}

Sigma Rule

title: CVE-2026-10233 - Assimp Half-Life 1 MDL Out-of-Bounds Read
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential exploitation of CVE-2026-10233 via malicious MDL files
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-10233
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\assimp.exe'  # Example application using Assimp
        CommandLine|contains: '.mdl'
    condition: selection
falsepositives:
    - Legitimate use of Assimp with valid MDL files
level: low

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2026-10233 - Assimp Half-Life 1 MDL Out-of-Bounds Read"; flow:to_server,established; content:"|49 44 53 54|"; offset:0; depth:4; content:"|00 00 00 00|"; within:1024; byte_test:4,>,1024,0x100; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Actions:

   • Disable the Half-Life 1 MDL loader in Assimp if not required. This can be done by modifying the build configuration or using a wrapper that rejects MDL files.
   • Implement strict input validation for MDL files before processing them with Assimp.

2. Long-Term Fix:

   • Apply the official patch once released by the Assimp project.
   • If you maintain a fork of Assimp, apply the hypothetical patch described in the "Vulnerable vs. Patched Code" section.

3. Compensating Controls:

   • Use application whitelisting to prevent untrusted applications from loading MDL files.
   • Deploy endpoint protection solutions that can detect and block exploitation attempts.

4. Monitoring:

   • Continuously monitor for new CVEs related to Assimp and apply patches promptly.
   • Review logs for crashes or abnormal behavior in applications using Assimp.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert