{"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":"2026-10-22T16:16:12.162666625Z","published":"2026-10-02T07:54:16.063Z","related":["SUSE-SU-2026:4797-1","openSUSE-SU-2026:12090-1"],"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"},{"fixed":"9c9ad88b6003bb6230d2434bd892cc1e95b294e4"}],"database_specific":{"extracted_events":[{"range":[{"introduced":"1.73"},{"fixed":"1.86"}],"source":"AFFECTED_FIELD","strategy":"StandardRange"},{"source":"REFERENCES_COMMIT","range":[{"fixed":"9c9ad88b6003bb6230d2434bd892cc1e95b294e4"}]}]}}],"versions":["r1rv73"],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-18036.json","vanir_signatures":[{"signature_version":"v1","source":"https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4","target":{"file":"core/src/main/java/org/bouncycastle/pqc/math/ntru/Polynomial.java","function":"mod3"},"deprecated":false,"digest":{"function_hash":"99335319624403829571532364691283783230","length":80},"id":"CVE-2026-18036-6c3b501b","signature_type":"Function"},{"signature_version":"v1","source":"https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4","target":{"file":"core/src/main/java/org/bouncycastle/pqc/math/ntru/Polynomial.java"},"deprecated":false,"digest":{"line_hashes":["15680412856031196787990673747701544330","115195829514276812792161094455006121373","164844410459406839810959913402674964799","222868103469363150237900034007207071107","40826237893040310746607121531876280739","57905628113648477858215661961515089202","244105881433912381496373469733194077588","102028569115762235449062903247799187456","11974242969275831223829926742786449408","300004068565516473747058896877486146916","191205219460096188324818343340589241553","166215844114616201005845394545431673852","11004915667868475128195291634305365923","213863427817204038271627700028078091036"],"threshold":0.9},"id":"CVE-2026-18036-7897e6bf","signature_type":"Line"},{"source":"https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4","target":{"file":"core/src/main/java/org/bouncycastle/pqc/math/ntru/Polynomial.java","function":"modQ"},"deprecated":false,"digest":{"function_hash":"247125669952043661407303634151120519235","length":71},"id":"CVE-2026-18036-a4c04878","signature_type":"Function","signature_version":"v1"},{"target":{"function":"mod3","file":"core/src/main/java/org/bouncycastle/pqc/math/ntru/Polynomial.java"},"deprecated":false,"digest":{"length":78,"function_hash":"323429808407840888333551253031526739826"},"id":"CVE-2026-18036-a8ee432e","signature_type":"Function","signature_version":"v1","source":"https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4"},{"signature_version":"v1","source":"https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4","target":{"file":"core/src/main/java/org/bouncycastle/pqc/crypto/ntru/NTRUSampling.java"},"deprecated":false,"digest":{"line_hashes":["25805128286082913654666991975508356010","150087098969303988676877102802038118131","61064119935604497318036595237002628304","212318202281250366308548230465613191156","148899749075524412971792461938718627083"],"threshold":0.9},"id":"CVE-2026-18036-b434da01","signature_type":"Line"},{"signature_version":"v1","source":"https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4","target":{"file":"core/src/main/java/org/bouncycastle/pqc/crypto/ntru/NTRUSampling.java","function":"mod3"},"deprecated":false,"digest":{"function_hash":"26627069544171511887990067220854408135","length":53},"id":"CVE-2026-18036-d1ebf223","signature_type":"Function"}],"vanir_signatures_modified":"2026-10-22T16:16:12Z"}}],"schema_version":"1.9.2","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"}]}