CVE-2026-71891BLS12-381 key validation accepts a public key built on a foreign curve
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Does this affect you?
It affects Legion of the Bouncy Castle Inc. BC-JAVA. Add what you run to find out whether that includes you.
EPSS puts the chance of exploitation in the next 30 days at 0.17%, higher than 6% of scored CVEs.
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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.
- CVSS
- 7.1 HIGH (v4.0, CNA)CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/U:Amber
- Weakness (CWE)
- CWE-347
- Known exploited
- Not in CISA's KEV catalog
- EPSS (exploitation in the next 30 days)
- 0.17% (percentile 6)
- Assigned by
- bcorg
- NVD analysis
- Awaiting Analysis
- Record last updated
- Oct 5, 2026
- First seen by InfoSec Signals
- Oct 9, 2026
Affected products
Legion of the Bouncy Castle Inc. BC-JAVA legion_of_the_bouncy_castle:bc_java (from the name) | from 1.85 before 1.86 | CNA record |
References
- https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071891vendor-advisory
- https://github.com/bcgit/bc-java/commit/c16bcfdce1f96230261b7c2c8c1e044bfc9d1debpatch
The CVE record on cve.org NVD's entry
The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.