CVE-2025-40776
📋 TL;DR
A cache-poisoning vulnerability in BIND 9's named resolver when configured with ECS (EDNS Client Subnet) options allows attackers to inject malicious DNS records into the cache. This affects BIND 9 installations running as caching resolvers with ECS enabled. Successful exploitation could redirect users to malicious sites or disrupt DNS resolution.
💻 Affected Systems
- ISC BIND 9
⚠️ Manual Verification Required
This CVE does not have specific version information in our database, so automatic vulnerability detection cannot determine if your system is affected.
Why? The CVE database entry doesn't specify which versions are vulnerable (no version ranges provided by the vendor/NVD).
🔒 Custom verification scripts are available for registered users. Sign up free to download automated test scripts.
- Review the CVE details at NVD
- Check vendor security advisories for your specific version
- Test if the vulnerability is exploitable in your environment
- Consider updating to the latest version as a precaution
⚠️ Risk & Real-World Impact
Worst Case
Attackers could poison DNS caches to redirect all users of the vulnerable resolver to malicious websites, enabling phishing, malware distribution, or service disruption across entire networks.
Likely Case
Targeted DNS cache poisoning affecting specific domains, potentially redirecting users to malicious sites for credential theft or malware delivery.
If Mitigated
Limited impact with proper network segmentation, monitoring, and ECS disabled where not required.
🎯 Exploit Status
Exploitation requires network access to the vulnerable resolver and knowledge of ECS behavior. No public exploit code is currently available.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 9.16.51-S1, 9.18.38-S1, 9.20.11-S1 and later
Vendor Advisory: https://kb.isc.org/docs/cve-2025-40776
Restart Required: Yes
Instructions:
1. Download patched BIND version from ISC website. 2. Stop named service. 3. Install updated packages. 4. Restart named service. 5. Verify version and functionality.
🔧 Temporary Workarounds
Disable ECS (EDNS Client Subnet)
allDisable ECS functionality in BIND configuration if not required for your use case.
Add 'edns-client-subnet no;' to options section in named.conf
Restrict Recursive Queries
allLimit recursive queries to trusted networks only.
Add 'allow-recursion { trusted_nets; };' to options section in named.conf
🧯 If You Can't Patch
- Disable ECS functionality in BIND configuration immediately
- Implement network segmentation to restrict access to caching resolvers
🔍 How to Verify
Check if Vulnerable:
Check BIND version with 'named -v' and verify if ECS is enabled in named.conf with 'grep -i edns-client-subnet named.conf'Check Version:
named -vVerify Fix Applied:
Verify BIND version is patched with 'named -v' and check that service is running normally📡 Detection & Monitoring
Log Indicators:
- Unusual query patterns, unexpected cache entries, increased query volume from single sources
Network Indicators:
- DNS responses with unexpected IP addresses, TTL anomalies, unusual ECS option usage
SIEM Query:
source="bind" AND (message="cache" OR message="poison" OR message="ECS")