CVE-2021-3751
📋 TL;DR
CVE-2021-3751 is an out-of-bounds write vulnerability in libmobi, a library for handling MOBI eBook files. This vulnerability allows attackers to execute arbitrary code or cause denial of service by processing specially crafted MOBI files. Any application or system using vulnerable versions of libmobi is affected.
💻 Affected Systems
- libmobi
- Applications using libmobi (e.g., eBook readers, document converters)
📦 What is this software?
Libmobi by Libmobi Project
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data theft, or ransomware deployment.
Likely Case
Application crash (denial of service) or limited code execution within the application context.
If Mitigated
Application crash with no further impact if proper sandboxing and privilege separation are implemented.
🎯 Exploit Status
Exploitation requires the victim to open a malicious MOBI file. The vulnerability is in file parsing, so no authentication is needed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Commit ab5bf0e37e540eac682a14e628853b918626e72b and later
Vendor Advisory: https://github.com/bfabiszewski/libmobi/commit/ab5bf0e37e540eac682a14e628853b918626e72b
Restart Required: No
Instructions:
1. Update libmobi to the latest version from the official repository. 2. Recompile any applications that statically link libmobi. 3. For dynamically linked applications, ensure the updated library is loaded.
🔧 Temporary Workarounds
Disable MOBI file processing
allTemporarily disable or block MOBI file processing in affected applications.
Use application sandboxing
allRun applications that process MOBI files in restricted environments (containers, sandboxes) to limit impact.
🧯 If You Can't Patch
- Implement strict file upload controls to block MOBI files from untrusted sources.
- Use endpoint detection and response (EDR) tools to monitor for exploitation attempts and application crashes.
🔍 How to Verify
Check if Vulnerable:
Check libmobi version or commit hash. If using a package manager, check the installed version against the patched commit.Check Version:
For source installations: git log --oneline -1. For packages: dpkg -s libmobi (Linux) or equivalent package manager command.Verify Fix Applied:
Verify that libmobi is updated to commit ab5bf0e37e540eac682a14e628853b918626e72b or later. Test with known malicious MOBI files if available.📡 Detection & Monitoring
Log Indicators:
- Application crashes or abnormal termination when processing MOBI files
- Unexpected process spawning from eBook reader applications
Network Indicators:
- Unusual outbound connections from applications that handle MOBI files
SIEM Query:
Example: process_name:"ebook_reader" AND event_type:"crash" OR process_name:"libmobi" AND event_type:"malicious_file_detected"🔗 References
- https://github.com/bfabiszewski/libmobi/commit/ab5bf0e37e540eac682a14e628853b918626e72b
- https://huntr.dev/bounties/fcb4383c-bc27-4b89-bfce-6b041f0cb769
- https://github.com/bfabiszewski/libmobi/commit/ab5bf0e37e540eac682a14e628853b918626e72b
- https://huntr.dev/bounties/fcb4383c-bc27-4b89-bfce-6b041f0cb769
