# CVE-2026-18036: Bouncy Castle for Java NTRU timing side-channel in modular reduction

Source: https://junglewise.ai/threats/cve-2026-18036-in-bouncy-castle-for-java-before-1-86-ntru-reduced-secret-values  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-02. Updated: 2026-10-05  
License: CC BY 4.0 with a link back

CVE-2026-18036 · Severity: info · Published 2026-10-02

Bouncy Castle for Java, a widely-used cryptographic library, contains a timing-based information leak in its NTRU post-quantum key encapsulation implementation. An attacker who can precisely measure the timing of decapsulation operations can extract information about NTRU private keys. This vulnerability was fixed in version 1.86.

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-2026-18036 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-18036 being exploited in the wild.

**How severe is CVE-2026-18036?** Informational severity, published 2 October 2026.

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

**Is there a fix for CVE-2026-18036?** Yes: fixed in 1.86. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-18036 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-18036?** 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 1.86

## Technical details

NTRU's mod3 and modQ reduction helpers used the % operator on secret values, whose integer division latency depends on the secret operand, instead of implementing the reference's division-free algorithms. The modQ reduction in Polynomial was exposed on the decapsulation path where the dividend derives from the private key; mod3 reductions in Polynomial and NTRUSampling were exposed during key generation, encapsulation, and decapsulation. Bouncy Castle 1.86 replaces these with masking for modQ and division-free fold-and-select for mod3.

## Affected products

- Bouncy Castle Bouncy Castle for Java before 1.86

## Timeline

- 2026-07-28: patched
- 2026-10-02: 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/9c9ad88b6003bb6230d2434bd892cc1e95b294e4](https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4)

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