CVE-2026-71891 BLS12-381 key validation accepts a public key built on a foreign curve
Attackers can bypass BLS12-381 signature verification by using a public key from a foreign curve, allowing phantom signers in aggregate signatures.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-71891?
CVE-2026-71891 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. BC-JAVA, published on 3 Oct 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
In Bouncy Castle for Java before 1.86, BLS12_381BasicScheme.keyValidate, and so BLSPublicKeyParameters and every BasicScheme, MessageAugmentation and ProofOfPossession verify and aggregateVerify that gate on it, accepted a public key built on a foreign ECCurve that merely shares BLS12-381's field characteristic. The prime-order subgroup check trusts a point's own curve to name its cofactor, since ECPoint.satisfiesOrder returns true outright when the curve's cofactor is one, so a point on a curve with a different equation and a cofactor forged to one passed keyValidate despite not being a G1 point at all. In BC's pairing implementation such a point contributes the identity in the target group, so an aggregate signature verified against a set of public keys including it is accepted even though it contains no signature for that key and message pair, admitting a phantom signer. keyValidate now first confirms that the point's curve carries exactly the canonical G1 field, equation, order and cofactor before any subgroup check. The issue is reachable only where an application constructs an ECPoint on an explicit, non-canonical curve and accepts it as an authority-bearing key; the standard 48-byte compressed-point decoder always supplies the canonical curve and was never affected.
Weakness: CWE-347
Which versions are affected by CVE-2026-71891?
How do I fix CVE-2026-71891?
Upgrade to a fixed version.
Is CVE-2026-71891 being exploited?
There is no known exploitation of CVE-2026-71891 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-DK36206, CLEANSTART-2026-HW07490, CLEANSTART-2026-KF30791, DEBIAN-CVE-2026-71891, GHSA-gggh-4cvh-v637, UBUNTU-CVE-2026-71891.