CVE-2026-71891: BLS12-381 key validation accepts a public key built on a foreign curve
In Bouncy Castle for Java before 1.86, BLS12381BasicScheme.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.
Affected Software
Event History
Frequently Asked Questions
Which applications are realistically exposed?
The issue is reachable only when an application constructs an ECPoint on an explicit, non-canonical curve and accepts that point as an authority-bearing BLS public key. Standard handling that does not permit such foreign-curve point construction is not described as reachable.
What does an attacker need to exploit this?
An attacker needs a public key built on a different ECCurve with the same field characteristic as BLS12-381, a different equation, and a cofactor set to one. The application must then use that key in validation or signature aggregation as an authority-bearing key.
What is the practical impact during aggregate signature verification?
A foreign-curve key can act as a phantom signer: an aggregate signature may verify for a public-key set that includes it even though no signature was supplied for that key and its corresponding message. In the pairing implementation, the invalid point contributes the identity in the target group.
How can teams tell whether they are affected?
Review whether Bouncy Castle for Java versions before 1.86 are used for BLS12-381 BasicScheme, MessageAugmentation, or ProofOfPossession verification or aggregateVerify operations. Then determine whether untrusted or externally supplied public keys can be represented as ECPoint objects on explicit non-canonical curves and accepted as authority-bearing keys.
What remediation is available?
Upgrade to Bouncy Castle for Java 1.86 or later. The corrected key validation confirms that a point's curve has the canonical G1 field, equation, order, and cofactor before performing the subgroup check.