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CVE-2026-25793: Doppelgänger Certificates: Bypassing Nebula Blocklists with ECDSA Magic

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Doppelgänger Certificates: Bypassing Nebula Blocklists with ECDSA Magic

Vulnerability ID: CVE-2026-25793
CVSS Score: 7.6
Published: 2026-02-06

In the world of cryptography, two things can be mathematically identical yet look completely different to a computer. CVE-2026-25793 is a fascinating logic flaw in Slack's Nebula overlay network that exploits the malleability of ECDSA signatures on the NIST P-256 curve. By flipping the 's' value of a cryptographic signature, an attacker can generate a new binary representation of a certificate that remains cryptographically valid but results in a completely different file hash. Since Nebula's blocklist mechanism relied solely on SHA-256 fingerprints of the raw certificate bytes, this allowed banned attackers to simply 'flip a switch' and re-enter the network.

TL;DR

Nebula versions 1.7.0 through 1.10.2 using non-default P256 settings allow blocklist evasion. Because ECDSA signatures are malleable, an attacker can modify a blocked certificate's signature to generate a different SHA-256 fingerprint while maintaining validity. This bypasses blocklists that rely on static file hashes.

⚠️ Exploit Status: POC

Technical Details

  • CWE ID: CWE-347
  • Attack Vector: Network
  • CVSS v4.0: 7.6 (High)
  • Impact: Security Bypass / Revocation Failure
  • EPSS Score: 0.00017 (Low)
  • Configuration: Requires non-default P256 curve

Affected Systems

  • Slack Nebula (v1.7.0 - v1.10.2) configured with P256 curves
  • Nebula: >= 1.7.0, <= 1.10.2 (Fixed in: 1.10.3)

Code Analysis

Commit: f573e8a

Fix ECDSA P-256 signature malleability in blocklist verification

func VerifyCertificate(...) {
+ fp2, err := CalculateAlternateFingerprint(c)
+ if fp2 != "" && ncp.IsBlocklisted(fp2) { return nil, ErrBlockListed }
}
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Exploit Details

  • Hypothetical: Exploitation involves local manipulation of ASN.1/DER certificate structures to invert the ECDSA 's' value.

Mitigation Strategies

  • Upgrade to Nebula v1.10.3 immediately.
  • Migrate from NIST P-256 to Curve25519 (default) if compliance rules allow.
  • Rotate the Certificate Authority (CA) if active attackers are suspected.

Remediation Steps:

  1. Download the v1.10.3 binary from the official GitHub release.
  2. Replace the nebula binary on all lighthouses and nodes.
  3. Restart the nebula service.
  4. Verify P256 certificates are now checked against alternate fingerprints.

References

Read the full report for CVE-2026-25793 on our website for more details including interactive diagrams and full exploit analysis.

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