CVE-2026-18040 Legion of the Bouncy Castle Inc. CVE debrief

Who should care

Defenders responsible for Java deployments using Bouncy Castle, particularly those using versions before 1.86, should assess exposure and prioritize remediation. This includes reviewing and updating inventory to ensure affected versions are identified and remediated, and monitoring for potential side-channel attacks. Additionally, defenders should verify and apply the patch for Bouncy Castle for Java versions before 1.86, and plan vendor-supported updates

Why it matters

The vulnerability in Bouncy Castle for Java before 1.86 allows an attacker to recover information about the HQC private key through side channels, requiring defenders to verify and apply the patch.

• An attacker may be able to recover information about the HQC private key through side-channel attacks
• Defenders need to verify and apply the patch to prevent potential attacks
• Remediation requires updating Bouncy Castle for Java to version 1.86 or later

Technical summary

The vulnerability in Bouncy Castle for Java before 1.86 allows an attacker to recover information about the HQC private key through two side channels: its GF(2^8) arithmetic used lookup tables indexed by field elements, and its fixed-weight support sampler. The changes made to fix the issue include making the field arithmetic table-free and the sampler branch-free within a batch of candidates. The vulnerability can be mitigated by updating Bouncy Castle for Java to version 1.86 or later. The attack requires the ability to observe cache behavior or decapsulation timing, and the sampler re-expands the secret key from its seed on every decapsulation.

Defensive priority

Defenders should prioritize verifying and applying the patch for Bouncy Castle for Java versions before 1.86, as the vulnerability allows an attacker to recover information about the HQC private key through side channels.

Recommended defensive actions

• Verify and apply the patch for Bouncy Castle for Java versions before 1.86
• Review and update inventory to ensure affected versions are identified and remediated
• Monitor for potential side-channel attacks
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed
• Check relevant monitoring, detection, and logs for exposed assets that need extra review

Evidence notes

The CVE record and source item provide details about the vulnerability in Bouncy Castle for Java, including the affected versions and the changes made to fix the issue. However, the corpus does not establish versions, exploitation, impact, or remediation beyond the provided information.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.