CVE-2026-25793High▾ TwilightBlocklist Bypass possible via ECDSA Signature Malleability
▾ Twilight zone — High severity, or a signal on a lesser flaw
impact 41.3 · likelihood 0 · exploitation 0
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Exploit-prediction probability, daily snapshots since Sep 12.
Disclosure to exploitation, from the record and what we observed since indexing it.
Disclosed via OSV
Last analysed / modified upstream
0.1%
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:
CURVE_P256 certificates are being usedSee attached
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.
github.com/slackhq/nebula >= 1.7.0, < 1.10.3Upgrade to a patched release:
github.com/slackhq/nebula 1.10.3