CVE-2026-25793
📋 TL;DR
This vulnerability in Nebula overlay networking tool allows attackers to bypass certificate blocklist entries when using P256 certificates (non-default configuration). By exploiting ECDSA signature malleability, an attacker can create a certificate copy with a different fingerprint to evade blocklist restrictions. Only users with P256 certificates configured are affected.
💻 Affected Systems
- Nebula
📦 What is this software?
Nebula by Slack
⚠️ Risk & Real-World Impact
Worst Case
Malicious nodes could bypass certificate blocklists and join the Nebula network, potentially gaining unauthorized access to internal resources and performing lateral movement.
Likely Case
Attackers with compromised certificates could evade blocklist enforcement and maintain persistent access to the Nebula network.
If Mitigated
With proper network segmentation and monitoring, impact is limited to potential unauthorized network access that can be detected and contained.
🎯 Exploit Status
Exploitation requires access to a certificate that has been blocklisted and knowledge of ECDSA signature malleability techniques.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.10.3
Vendor Advisory: https://github.com/slackhq/nebula/security/advisories/GHSA-69x3-g4r3-p962
Restart Required: Yes
Instructions:
1. Download Nebula version 1.10.3 or later from GitHub releases. 2. Replace existing Nebula binaries with patched version. 3. Restart all Nebula services. 4. Verify new version is running.
🔧 Temporary Workarounds
Switch to Curve25519 certificates
allMigrate from P256 certificates to Curve25519 certificates (default configuration) which are not vulnerable.
nebula-cert sign -name "hostname" -ip "10.0.0.1/24" -groups "default" -curve 25519
Implement additional network controls
allUse firewall rules or network policies to restrict Nebula traffic in addition to certificate blocklists.
🧯 If You Can't Patch
- Monitor for unusual certificate fingerprint changes in Nebula logs
- Implement additional authentication layers beyond certificate validation
🔍 How to Verify
Check if Vulnerable:
Check Nebula version with 'nebula -version' and verify if using P256 certificates by examining certificate configuration.Check Version:
nebula -versionVerify Fix Applied:
Confirm version is 1.10.3 or later with 'nebula -version' and verify P256 certificates are properly validated.📡 Detection & Monitoring
Log Indicators:
- Certificate validation failures
- Multiple certificate fingerprints for same identity
- Blocklist bypass attempts
Network Indicators:
- Unexpected Nebula handshakes from previously blocked nodes
- Traffic from nodes with modified certificate signatures
SIEM Query:
source="nebula.log" AND ("certificate validation" OR "fingerprint mismatch" OR "blocklist bypass")