{"id":"CVE-2026-63567","state":"PUBLISHED","title":"IesEngine block-cipher mode checks padding before MAC (CBC padding oracle)","description":"Observable discrepancy in IesEngine.DecryptBlock in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote attacker who has captured an IES or ECIES ciphertext, and who can submit modified ciphertexts for decryption under the same key pair, to recover its plaintext via a CBC padding-oracle attack, because in block-cipher mode the engine decrypts the ciphertext and removes its padding before verifying the MAC. A padding failure is therefore reported with a different error message, and without the MAC computation, compared with a MAC failure. Only applications that construct IesEngine directly with a padded block cipher, such as AES in CBC mode with PKCS#7 padding, are affected; stream-mode IES is not.","publishedAt":"2026-10-02T06:58:32.487Z","updatedAt":"2026-10-02T18:09:54.933Z","severity":{"score":8.2,"label":"High","version":"4.0","vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N","source":"bcorg"},"assigner":"bcorg","products":["Legion of the Bouncy Castle Inc. bc-csharp"],"affected":[{"vendor":"Legion of the Bouncy Castle Inc.","product":"bc-csharp","packageName":"BouncyCastle.Cryptography","defaultStatus":"unaffected","versions":[{"version":"0","status":"affected","lessThan":"2.7.0","versionType":"semver"}],"collectionURL":"https://www.nuget.org/packages/BouncyCastle.Cryptography","repo":"https://github.com/bcgit/bc-csharp","programFiles":["crypto/src/crypto/engines/IesEngine.cs"]}],"references":["https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63567","https://github.com/bcgit/bc-csharp/commit/f881400b761134160ec2a05fd27c3e569453788d","https://github.com/bcgit/bc-csharp/commit/eaa42e1e8125b3e2e25e365c4194c7cd5932360b"],"weaknesses":["CWE-203"],"ingestedAt":"2026-10-29T20:12:41.951Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/63xxx/CVE-2026-63567.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-63567","state":"PUBLISHED","datePublished":"2026-10-02T06:58:32.487Z","dateUpdated":"2026-10-02T18:09:54.933Z","assignerShortName":"bcorg","assignerOrgId":"91579145-5d7b-4cc5-b925-a0262ff19630","dateReserved":"2026-07-16T23:48:46.076Z"},"containers":{"cna":{"title":"IesEngine block-cipher mode checks padding before MAC (CBC padding oracle)","descriptions":[{"lang":"en","value":"Observable discrepancy in IesEngine.DecryptBlock in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote attacker who has captured an IES or ECIES ciphertext, and who can submit modified ciphertexts for decryption under the same key pair, to recover its plaintext via a CBC padding-oracle attack, because in block-cipher mode the engine decrypts the ciphertext and removes its padding before verifying the MAC. A padding failure is therefore reported with a different error message, and without the MAC computation, compared with a MAC failure. Only applications that construct IesEngine directly with a padded block cipher, such as AES in CBC mode with PKCS#7 padding, are affected; stream-mode IES is not."}],"affected":[{"vendor":"Legion of the Bouncy Castle Inc.","product":"bc-csharp","packageName":"BouncyCastle.Cryptography","defaultStatus":"unaffected","versions":[{"version":"0","status":"affected","lessThan":"2.7.0","versionType":"semver"}],"collectionURL":"https://www.nuget.org/packages/BouncyCastle.Cryptography","repo":"https://github.com/bcgit/bc-csharp","programFiles":["crypto/src/crypto/engines/IesEngine.cs"]}],"references":[{"url":"https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63567","tags":["vendor-advisory"]},{"url":"https://github.com/bcgit/bc-csharp/commit/f881400b761134160ec2a05fd27c3e569453788d","tags":["patch"]},{"url":"https://github.com/bcgit/bc-csharp/commit/eaa42e1e8125b3e2e25e365c4194c7cd5932360b","tags":["patch"]}],"metrics":[{"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}],"cvssV4_0":{"attackVector":"NETWORK","attackComplexity":"LOW","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":"NOT_DEFINED","version":"4.0","baseSeverity":"HIGH","baseScore":8.2,"vectorString":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-203","lang":"en","description":"CWE-203 Observable Discrepancy","type":"CWE"}]}],"providerMetadata":{"shortName":"bcorg","orgId":"91579145-5d7b-4cc5-b925-a0262ff19630","dateUpdated":"2026-10-02T06:58:32.487Z"},"impacts":[{"descriptions":[{"lang":"en","value":"Confidentiality: recovery of IES/ECIES plaintext via a chosen-ciphertext padding oracle"}]}],"credits":[{"lang":"en","value":"Discovered by Claude, Anthropic's AI assistant, and triaged by the Anthropic security team in collaboration with Anthropic Research.","type":"finder"}],"source":{"discovery":"EXTERNAL"},"x_generator":{"engine":"Claude Opus 5.5"},"x_antReportIds":["ANT-2026-J7SXX2NS","ANT-2026-SNRMZ9MS"]},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-10-02T18:09:42.725922Z","id":"CVE-2026-63567","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":"2026-10-02T18:09:54.933Z"}}]}}}