# CVE-2026-71891: Bouncy Castle for Java BLS signature phantom signer

Source: https://junglewise.ai/threats/cve-2026-71891-in-bouncy-castle-for-java-before-1-86-bls12-381basicscheme  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-03. Updated: 2026-10-05  
License: CC BY 4.0 with a link back

CVE-2026-71891 · Severity: info · Published 2026-10-03

Bouncy Castle, a widely-used cryptographic library for Java, contains a flaw in its BLS12-381 signature scheme implementation. An attacker can construct a specially-crafted public key on a non-standard elliptic curve that passes validation checks but does not correspond to a legitimate signer. This allows forged aggregate signatures to be accepted as valid, effectively allowing an attacker to impersonate a signer without providing a real signature.

Technologies: [Bouncy Castle for Java](https://junglewise.ai/threats/technologies/bouncy-castle-for-java). Vendors: [Bouncy Castle](https://junglewise.ai/threats/vendors/bouncy-castle).

## Quick answers

**Is CVE-2026-71891 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-71891 being exploited in the wild.

**How severe is CVE-2026-71891?** Informational severity, published 3 October 2026.

**Which products and versions are affected?** Bouncy Castle for Java before 1.86.

**Is there a fix for CVE-2026-71891?** Yes: fixed in Bouncy Castle for Java 1.86. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-71891 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-71891?** Check whether you run Bouncy Castle for Java at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in Bouncy Castle for Java 1.86

## Technical details

The BLS12_381BasicScheme.keyValidate function fails to verify that public keys use the canonical G1 curve before performing subgroup checks. An attacker can supply an ECPoint on a foreign curve with the same field characteristic but different equation and cofactor, which will pass validation because ECPoint.satisfiesOrder returns true when cofactor is one. In pairing operations, such a point contributes the identity element, allowing an aggregate signature to be accepted without an actual signature from that key-message pair. The vulnerability requires an application to explicitly construct ECPoints on non-canonical curves and accept them as keys; standard 48-byte compressed-point decoding is unaffected.

## Affected products

- Bouncy Castle Bouncy Castle for Java before 1.86

## Timeline

- 2026-10-03: disclosed

## References

- [https://www.bouncycastle.org/download/bouncy-castle-java/](https://www.bouncycastle.org/download/bouncy-castle-java/)
- [https://github.com/bcgit/bc-java/commit/c16bcfdce1f96230261b7c2c8c1e044bfc9d1deb](https://github.com/bcgit/bc-java/commit/c16bcfdce1f96230261b7c2c8c1e044bfc9d1deb)

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