CVE-2021-43523
📋 TL;DR
This vulnerability in uClibc and uClibc-ng allows DNS responses containing special characters to bypass validation, potentially leading to incorrect hostname resolution or injection attacks. It affects systems using these C libraries for DNS lookups via functions like gethostbyname and getaddrinfo. The lack of proper validation in the stub resolver enables various injection scenarios.
💻 Affected Systems
- uClibc
- uClibc-ng
📦 What is this software?
Uclibc by Uclibc
Uclibc Ng by Uclibc Ng Project
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution through application injection, domain hijacking enabling man-in-the-middle attacks, or complete system compromise via malicious DNS responses.
Likely Case
Application crashes, cross-site scripting (XSS) in web applications using affected DNS functions, or incorrect domain resolution leading to service disruption.
If Mitigated
Limited impact with proper network segmentation, DNS filtering, and application-level input validation preventing exploitation.
🎯 Exploit Status
Exploitation requires control over DNS responses or ability to poison DNS cache. Public proof-of-concept demonstrates the vulnerability.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.0.39 and later
Vendor Advisory: https://uclibc-ng.org/
Restart Required: Yes
Instructions:
1. Update uClibc/uClibc-ng to version 1.0.39 or later. 2. Recompile affected applications with updated library. 3. Restart services using the patched library. 4. For embedded systems, update firmware or rebuild system image.
🔧 Temporary Workarounds
DNS Response Filtering
allImplement DNS filtering at network perimeter to block responses containing special characters in hostnames
Application Input Validation
allAdd application-level validation of hostnames returned by DNS functions before processing
🧯 If You Can't Patch
- Implement strict DNS filtering at network boundaries to block malicious responses
- Isolate affected systems in segmented networks with limited external DNS access
🔍 How to Verify
Check if Vulnerable:
Check uClibc/uClibc-ng version: ldd --version or check library files. If version < 1.0.39, system is vulnerable.Check Version:
ldd --version | grep -i uclibc || strings /lib/libc.so.* | grep -i uclibcVerify Fix Applied:
Verify uClibc/uClibc-ng version is 1.0.39 or later. Test DNS functions with specially crafted responses to ensure proper validation.📡 Detection & Monitoring
Log Indicators:
- Unusual DNS query patterns
- Application crashes after DNS lookups
- Unexpected hostname resolutions in application logs
Network Indicators:
- DNS responses containing special characters (non-alphanumeric) in hostname fields
- Unusual DNS traffic to/from embedded devices
SIEM Query:
dns.query.name contains special characters AND (device.vendor contains "embedded" OR os.name contains "uclibc")🔗 References
- https://github.com/wbx-github/uclibc-ng/commit/0f822af0445e5348ce7b7bd8ce1204244f31d174
- https://uclibc-ng.org/
- https://www.openwall.com/lists/oss-security/2021/11/09/1
- https://github.com/wbx-github/uclibc-ng/commit/0f822af0445e5348ce7b7bd8ce1204244f31d174
- https://uclibc-ng.org/
- https://www.openwall.com/lists/oss-security/2021/11/09/1
