{"dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": {"cveId": "CVE-2026-18040", "assignerOrgId": "91579145-5d7b-4cc5-b925-a0262ff19630", "state": "PUBLISHED", "assignerShortName": "bcorg", "dateReserved": "2026-07-28T08:50:28.760Z", "datePublished": "2026-10-03T08:55:06.766Z", "dateUpdated": "2026-10-05T16:00:20.062Z"}, "containers": {"cna": {"providerMetadata": {"orgId": "91579145-5d7b-4cc5-b925-a0262ff19630", "shortName": "bcorg", "dateUpdated": "2026-10-03T08:55:06.766Z"}, "title": "HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-dependent fixed-weight sampler", "problemTypes": [{"descriptions": [{"lang": "en", "cweId": "CWE-208", "description": "CWE-208 Observable Timing Discrepancy", "type": "CWE"}]}], "affected": [{"vendor": "Legion of the Bouncy Castle Inc.", "product": "BC-JAVA", "platforms": ["all"], "collectionURL": "https://www.bouncycastle.org/download/bouncy-castle-java/", "packageName": "bcprov", "repo": "https://github.com/bcgit/bc-java", "packageURL": "pkg:maven/org.bouncycastle/bcprov-jdk18on", "modules": ["core"], "programFiles": ["GF", "HQCEngine", "ReedSolomon", "FastFourierTransform"], "versions": [{"status": "affected", "version": "1.73", "lessThan": "1.86", "versionType": "maven"}], "defaultStatus": "unaffected"}], "descriptions": [{"lang": "en", "value": "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.", "supportingMedia": [{"type": "text/html", "base64": false, "value": "<p>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.</p>"}]}], "references": [{"url": "https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018040", "tags": ["vendor-advisory"]}, {"url": "https://github.com/bcgit/bc-java/commit/283acd81048ec9e951032525279d3b58aaf0bb03", "tags": ["patch"]}], "metrics": [{"format": "CVSS", "scenarios": [{"lang": "en", "value": "GENERAL"}], "cvssV4_0": {"attackVector": "LOCAL", "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": "MEDIUM", "baseScore": 5.9, "vectorString": "CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/U:Amber"}}], "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": [{"metrics": [{"other": {"type": "ssvc", "content": {"timestamp": "2026-10-05T16:00:03.782416Z", "id": "CVE-2026-18040", "options": [{"Exploitation": "none"}, {"Automatable": "no"}, {"Technical Impact": "partial"}], "role": "CISA Coordinator", "version": "2.0.3"}}}], "title": "CISA ADP Vulnrichment", "providerMetadata": {"orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2026-10-05T16:00:20.062Z"}}]}}

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