Improper Verification of Cryptographic Signature in Nebula - CVE-2026-25793
Published: April 25, 2026
Vulnerability identifier: #VU127892
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-25793
CWE-ID: CWE-347
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: Slack Technologies, Inc.
Affected software:
Nebula
Nebula
Detailed vulnerability description
The vulnerability allows a remote user to bypass certificate blocklist enforcement.
The vulnerability exists due to improper handling of ECDSA signature malleability in certificate fingerprint blocklist validation when processing P256 certificates. A remote user can use a copy of a certificate with a different fingerprint to bypass certificate blocklist enforcement.
Exploitation requires the use of CURVE_P256 certificates, an existing blocklist entry for a certificate signed by a trusted CA that is not expired, and access to the corresponding private key and certificate.
How to mitigate CVE-2026-25793
Install security update from vendor's website.