CVE-2026-25793: Nebula Has Possible Blocklist Bypass via ECDSA Signature Malleability
Impact
When using P256 certificates (which is not the default configuration), it is possible to evade a blocklist entry created against the fingerprint of a certificate by using ECDSA Signature Malleability to use a copy of the certificate with a different fingerprint.
In order for this to affect a user or network, all of the following must be true: CURVEP256 certificates are being used There are one or more entries on the blocklist The certificates for those entries are signed by a trusted CA and not expired An attacker has a copy of the private key, and corresponding certificate, for one of those blocklist entries
Patches
See attached
Workarounds
If full copies of each certificate on the existing blocklist are available, it is possible to compute their opposite-chirality signature, and then the appropriate second fingerprint to list in the blocklist.
Rotating out all CAs that have signed hosts on the blocklist will also prevent exploitation of this vulnerability.
Other sources
Nebula is a scalable overlay networking tool. In versions from 1.7.0 to 1.10.2, when using P256 certificates (which is not the default configuration), it is possible to evade a blocklist entry created against the fingerprint of a certificate by using ECDSA Signature Malleability to use a copy of the certificate with a different fingerprint. This issue has been patched in version 1.10.3.
— MITRE
Affected Software
Remediation
Event History
Frequently Asked Questions
What is the severity of CVE-2026-25793?
CVE-2026-25793 has a moderate severity level due to the potential for certificate fingerprint evasion.
How do I fix CVE-2026-25793?
To fix CVE-2026-25793, upgrade to version 1.10.3 or later of the affected software.
Which software is affected by CVE-2026-25793?
CVE-2026-25793 affects versions 1.7.0 to 1.10.2 of the Nebula software from Slack.
What vulnerability does CVE-2026-25793 address?
CVE-2026-25793 addresses ECDSA Signature Malleability in P256 certificates, allowing blocklist evasion.
Is CVE-2026-25793 exploitable under default configurations?
No, CVE-2026-25793 is only exploitable when using P256 certificates, which is not the default configuration.