{"id":"CVE-2026-18036","state":"PUBLISHED","title":"NTRU leaks private key information by reducing secret values with a non-constant-time integer division","description":"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.","publishedAt":"2026-10-02T07:54:16.063Z","updatedAt":"2026-10-02T17:42:56.393Z","severity":{"score":8.2,"label":"High","version":"4.0","vector":"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","source":"bcorg"},"assigner":"bcorg","products":["Legion of the Bouncy Castle Inc. BC-JAVA"],"affected":[{"vendor":"Legion of the Bouncy Castle Inc.","product":"BC-JAVA","packageName":"bcprov","packageURL":"pkg:maven/org.bouncycastle/bcprov-jdk18on","defaultStatus":"unaffected","versions":[{"version":"1.73","status":"affected","lessThan":"1.86","versionType":"maven"}],"platforms":["all"],"collectionURL":"https://www.bouncycastle.org/download/bouncy-castle-java/","repo":"https://github.com/bcgit/bc-java","modules":["core"],"programFiles":["Polynomial","NTRUSampling"]}],"references":["https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018036","https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4"],"weaknesses":["CWE-208"],"ingestedAt":"2026-10-29T20:11:10.572Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/18xxx/CVE-2026-18036.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-18036","state":"PUBLISHED","datePublished":"2026-10-02T07:54:16.063Z","dateUpdated":"2026-10-02T17:42:56.393Z","assignerShortName":"bcorg","assignerOrgId":"91579145-5d7b-4cc5-b925-a0262ff19630","dateReserved":"2026-07-28T08:28:40.900Z"},"containers":{"cna":{"title":"NTRU leaks private key information by reducing secret values with a non-constant-time integer division","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"Legion of the Bouncy Castle Inc.","product":"BC-JAVA","packageName":"bcprov","packageURL":"pkg:maven/org.bouncycastle/bcprov-jdk18on","defaultStatus":"unaffected","versions":[{"version":"1.73","status":"affected","lessThan":"1.86","versionType":"maven"}],"platforms":["all"],"collectionURL":"https://www.bouncycastle.org/download/bouncy-castle-java/","repo":"https://github.com/bcgit/bc-java","modules":["core"],"programFiles":["Polynomial","NTRUSampling"]}],"references":[{"url":"https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018036","tags":["vendor-advisory"]},{"url":"https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4","tags":["patch"]}],"metrics":[{"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}],"cvssV4_0":{"attackVector":"NETWORK","attackComplexity":"HIGH","attackRequirements":"PRESENT","privilegesRequired":"NONE","userInteraction":"NONE","vulnConfidentialityImpact":"HIGH","subConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","subIntegrityImpact":"NONE","vulnAvailabilityImpact":"NONE","subAvailabilityImpact":"NONE","exploitMaturity":"NOT_DEFINED","Safety":"NOT_DEFINED","Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","valueDensity":"NOT_DEFINED","vulnerabilityResponseEffort":"NOT_DEFINED","providerUrgency":"AMBER","version":"4.0","baseSeverity":"HIGH","baseScore":8.2,"vectorString":"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"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-208","lang":"en","description":"CWE-208 Observable Timing Discrepancy","type":"CWE"}]}],"providerMetadata":{"shortName":"bcorg","orgId":"91579145-5d7b-4cc5-b925-a0262ff19630","dateUpdated":"2026-10-02T07:54:16.063Z"},"credits":[{"lang":"en","value":"The Robusta team: Deepak Bhargavan Pillai, Anirban Chakraborty, Chitchanok Chuengsatiansup, Matthew Roughan, Peter Schwabe, and Yuval Yarom","type":"reporter"}],"source":{"discovery":"EXTERNAL"},"x_generator":{"engine":"bc release-notes template"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-10-02T17:42:43.345821Z","id":"CVE-2026-18036","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-02T17:42:56.393Z"}}]}}}