{"id":"CVE-2026-18036","summary":"NTRU leaks private key information by reducing secret values with a non-constant-time integer division","details":"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 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.","modified":"2026-10-04T02:46:52.549369220Z","published":"2026-10-02T07:54:16.063Z","database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/18xxx/CVE-2026-18036.json","cna_assigner":"bcorg","cwe_ids":["CWE-208"]},"references":[{"type":"WEB","url":"https://www.bouncycastle.org/download/bouncy-castle-java/"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/18xxx/CVE-2026-18036.json"},{"type":"ADVISORY","url":"https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018036"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-18036"},{"type":"FIX","url":"https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4"},{"type":"PACKAGE","url":"https://github.com/bcgit/bc-java"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/bcgit/bc-java","events":[{"introduced":"9032821b326fba9fec7528899dfffa79f9f19aef"},{"fixed":"eacda831fbe15b272933e40841d8b0827b556fae"}],"database_specific":{"extracted_events":[{"introduced":"1.73"},{"fixed":"1.86"}],"source":"AFFECTED_FIELD"}}],"versions":["r1rv73"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-18036.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}]}