CVE-2026-63567: legion of the bouncy castle inc. bc-csharp Observable Discrepancy

Vulnerability summary

CVE-2026-63567 is a high (CVSS 8.2) Observable Discrepancy vulnerability in legion of the bouncy castle inc. bc-csharp. EPSS estimates a 0.47% chance of exploitation in the next 30 days, higher than 39.4% of scored vulnerabilities.

Timeline

1. 02-10-2026
   PublishedVulnerability first disclosed.
2. 02-10-2026
   Last modifiedVulnerability information updated.

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.

• CWE-203Observable DiscrepancyThe product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor.