CVE-2026-18040

Description

In Bouncy Castle for Java before 1.86, HQC leaked secret-derived data through two side channels: its GF(2^8) arithmetic used lookup tables indexed by field elements, making the cache line touched a function of the operand, and its fixed-weight support sampler left its duplicate scan as soon as a collision was found and stored accepted positions at a secret index. Both run on secret inputs during encapsulation and decapsulation, and the sampler re-expands the secret key from its seed on every decapsulation, so an attacker able to observe cache behaviour or decapsulation timing can recover information about the HQC private key. The field arithmetic is now table-free and the sampler branch-free within a batch of candidates, with output and randomness consumption unchanged.

Is your site exposed to CVE-2026-18040?

Run a free security scan — no signup, results in seconds.

EPSS — Exploit Prediction

0.0011

Probability of exploitation

0.01%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-208 CWE-208

References

Other References

https://github.com/bcgit/bc-java/commit/283acd81048ec9e951032525279d3b58aaf0bb03 https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018040

Frequently Asked Questions

What is CVE-2026-18040? +

In Bouncy Castle for Java before 1.86, HQC leaked secret-derived data through two side channels: its GF(2^8) arithmetic used lookup tables indexed by field elements, making the cache line touched a function of the operand, and its fixed-weight support sampler left its duplicate scan as soon as a collision was found and stored accepted positions at a secret index. Both run on secret inputs during encapsulation and decapsulation, and the sampler re-expands the secret key from its seed on every decapsulation, so an attacker able to observe cache behaviour or decapsulation timing can recover information about the HQC private key. The field arithmetic is now table-free and the sampler branch-free within a batch of candidates, with output and randomness consumption unchanged.

How do I check if I'm vulnerable to CVE-2026-18040? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-8794

PaperCut NG/MF contains an observable timing discrepancy in its authentication component. An unauthenticated remote attacker can exploit this vulnerability to …

CVE-2026-69247

cryptography is a package designed to expose cryptographic primitives and recipes to Python developers. From 44.0.0 until 50.0.0, pkcs7_decrypt_der, pkcs7_decrypt_pem, …

CVE-2026-88010

Traefik is an open source HTTP reverse proxy and load balancer. From 3.6.11 until 3.7.13, checkPassword in pkg/middlewares/auth/basic_auth.go constructs the …

CVE-2026-77582

Tinyauth is an authentication and authorization server. Prior to 5.1.0, Tinyauth exposes a remotely observable timing difference between authentication attempts …

CVE-2026-15432

When verifying a mac with a ChunkedMacVerification object, Tink compares the resulting tag with non constant time comparison. This potentially …

CVE-2024-14041

In Bouncy Castle for Java from 1.73 to before 1.78, three ML-KEM (CRYSTALS-Kyber) routines divided secret-derived polynomial coefficients by the …