CVE-2026-71891 Affecting bouncycastle package, versions <1.86-1

Note: Versions mentioned in the description apply only to the upstream bouncycastle package and not the bouncycastle package as distributed by Debian. See How to fix? for Debian:14 relevant fixed versions and status.

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.

• https://security-tracker.debian.org/tracker/CVE-2026-71891
• https://github.com/bcgit/bc-java/commit/c16bcfdce1f96230261b7c2c8c1e044bfc9d1deb
• https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071891