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CVE-2026-71891— BLS12-381 key validation accepts a public key built on a foreign curve

Quick assessment

Affected
Legion of the Bouncy Castle Inc. BC-JAVA
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

在 Bouncy Castle for Java 1.86 版本之前的实现中， 方法（以及 和所有 、 和 中依赖该方法进行验证和聚合验证的逻辑），接受了一个基于“外来”椭圆曲线（ECCurve）构建的公钥。该外来曲线仅与 BLS12-381 共享相同的域特征（field characteristic），但并非标准曲线。 由于质数阶子群检查（prime-order subgroup check）依赖于点自身所在曲线的协因子（cofactor）名称，而 在曲线的协因子为 1 时直接返回 true，因此，即使某个点实际

CVSS 7.1 · High EPSS 0.17% · P6

Possible ATT&CK Techniques 1 AI

T1556 · Modify Authentication Process

Affected Version Matrix 1

VendorProduct Version RangeStatus
Legion of the Bouncy Castle Inc. BC-JAVA 1.85< 1.86 affected
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I. Basic Information for CVE-2026-71891

Vulnerability Information
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Vulnerability Title
BLS12-381 key validation accepts a public key built on a foreign curve
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
密码学签名的验证不恰当
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Legion of the Bouncy Castle Inc. BC-JAVA 1.85 ~ 1.86 -

II. Public POCs for CVE-2026-71891

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III. Intelligence Information for CVE-2026-71891

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Patches & Fixes for CVE-2026-71891 (1)

News Coverage for CVE-2026-71891 (1)

Same Patch Batch · Legion of the Bouncy Castle Inc. · 2026-10-03 · 12 CVEs total

CVE-2026-71885 9.2 CRITICAL MLS X.509 credential not bound to the LeafNode signature key
CVE-2026-71888 8.7 HIGH CMS AuthenticatedData exposes attacker-inserted authAttrs when digestAlgorithm is absent
CVE-2026-71889 8.7 HIGH PKIXCertPathReviewer does not apply X.509 name constraints to the target certificate
CVE-2026-71890 8.7 HIGH MLS external commit can remove an arbitrary group member
CVE-2026-85515 8.2 HIGH OpenPGP message truncation not reported, bypassing the SEIPDv1 integrity check
CVE-2026-71887 8.2 HIGH OpenPGP data signature accepted from a signing subkey without cross-certification
CVE-2026-71883 8.2 HIGH Native AES packet cipher returns the raw AES key on an alias
CVE-2026-71886 8.2 HIGH OpenPGP certification accepted from a subkey without certification authority
CVE-2026-71892 6.9 MEDIUM CMS key-transport recipient key-size validation never runs for RFC 9709 HKDF-derived keys
CVE-2026-18040 5.9 MEDIUM HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-depen
CVE-2026-97873 5.3 MEDIUM Legacy PBES1 and PKCS#12 PBE iteration count honoured unbounded in the raw JCA provider

IV. Related Vulnerabilities

Same product: BC-JAVA
Same vendor: Legion of the Bouncy Castle Inc.
Same weakness: CWE-347

V. Comments for CVE-2026-71891

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