{"id":"CVE-2026-18036","title":"In Bouncy Castle for Java before 1.86, NTRU reduced secret values with the % operator in three helpers whose reference implementations are deliberately division-free, so each reduction was carried out by an integer division whose latency…","summary":"In Bouncy Castle for Java before 1.86, NTRU reduced secret values with the % operator in three helpers whose reference implementations are deliberately division-free, so each reduction was carried out by an integer division whose latency…","severity":"high","cvss":8.2,"cvssVector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/U:Amber","cwe":["CWE-208"],"vendor":"Legion of the Bouncy Castle Inc.","product":"bcprov","affected":["bcprov >= 1.73 < 1.86"],"published":"2026-10-02","updated":"2026-10-02","sourceUpdated":"2026-10-02T18:17:03.507","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-18036","references":[{"url":"https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4","label":"91579145-5d7b-4cc5-b925-a0262ff19630"},{"url":"https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018036","label":"91579145-5d7b-4cc5-b925-a0262ff19630"},{"url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-18036.json"},{"url":"https://access.redhat.com/security/cve/CVE-2026-18036"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-18036"}],"tags":["nvd","cve.org","csaf","vex","red-hat","score-dispute"],"ssvc":{"exploitation":"none","automatable":"no","technicalImpact":"partial","timestamp":"2026-10-02T17:42:43.345821Z"},"cvssSource":"cna","epss":0.00282,"epssPercentile":0.1876,"ingestedAt":"2026-10-02T08:14:06.214Z","scores":{"cna":8.2,"vendor":5.9},"slug":"CVE-2026-18036","body":"## Overview\n\nIn Bouncy Castle for Java before 1.86, NTRU reduced secret values with the % operator in three helpers whose reference implementations are deliberately division-free, so each reduction was carried out by an integer division whose latency depends on the secret operand. Polynomial.modQ divided by a variable divisor, which a compiler cannot strength-reduce to a multiply the way it can a constant one, so it emitted a division on every call including on the decapsulation path where the dividend derives from the private key; Polynomial.mod3 and NTRUSampling.mod3 divided the secret key polynomials f and g during key generation, the message polynomials r and m during encapsulation, and coefficients recovered during decapsulation. An attacker able to measure that timing can recover information about the NTRU private key. modQ now masks, which is exact because q is always a power of two, and mod3 uses the reference implementation's division-free fold and select; the results are unchanged.\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.\n\n## Vendor advisories\n\n- **Red Hat VEX** · updated 2026-10-02 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-18036.json)","depth":"twilight","depthScore":45,"depthScoreParts":{"impact":45.1,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}