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Fencer Vulns
high 7.1

CVE-2026-71891 json

BLS12-381 key validation accepts a public key built on a foreign curve

published 2026-10-03 36 d ago modified 2026-10-06 33 d ago also EUVD-2026-92005

cvss
7.1
version 4.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 6%
public exploits
0
none known

Summary

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.

Patch / fix

  • Legion of the Bouncy Castle Inc. BC-JAVA 1.86
  • bouncycastle Debian 14 1.86-1

debian advisories DEBIAN-CVE-2026-71891

  • bouncycastle SLES for SAP applications 16.0, SLES 16.0 1.86-160000.1.1
  • bouncycastle SLES for SAP 15 SP6, SLES for SAP 15 SP5, SLES for SAP 15 SP4 +8 more 1.86-150200.3.41.1

suse advisories SUSE-SU-2026:24461-1SUSE-SU-2026:4797-1

  • bouncycastle openSUSE Leap 16.0 1.86-160000.1.1
  • bouncycastle openSUSE Tumbleweed 1.86-1.1

opensuse advisories openSUSE-SU-2026:22169-1openSUSE-SU-2026:12090-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-71891 updated cvss 5.9

bouncycastle: bouncycastle: Signature verification bypass via improper public key curve validation

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
34 d ago

Exploit prediction

0.17% chance of exploitation in the next 30 days, higher than 6% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 7.1 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
CNA 4.0 7.1 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber

Affected products

  • Legion of the Bouncy Castle Inc. BC-JAVA

Classification

Weakness
  • CWE-347 Improper Verification of Cryptographic Signature

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.