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CVE-2026-63573

bc-csharp: Bleichenbacher padding oracle in CMS RSA PKCS#1 v1.5 key-transport unwrap

Legion of the Bouncy Castle Inc. bc-csharp
Open in ThreatlineRecord JSON
CVSS
8.2High
ExploitationNo known exploitationNot in CISA KEV, which does not prove it is unexploitedCISA KEV
Exploit odds (EPSS)0.31%Chance of exploitation activity in the next 30 days, higher than 22% of CVEs. Scored 23 Oct 2026FIRST EPSS
Fixed in2.7.0Fixed versionCVE List
Published2 Oct 2026Updated 2 Oct 2026CVE List

What fixes it

A fixed version on one release line is not the upgrade target for every line. Check your exact version and any vendor backports.

Description

Observable discrepancy in the CMS RSA PKCS#1 v1.5 key-transport unwrap (KeyTransRecipientInformation.UnwrapKey) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote attacker who holds a captured CMS EnvelopedData message, and who can submit many modified messages to an application that decrypts them with the recipient's RSA private key and reveals how decryption failed, to recover the captured message's content-encryption key and so its content, via a Bleichenbacher-style adaptive chosen-ciphertext attack, because a key-transport ciphertext with invalid PKCS#1 v1.5 padding is rejected during unwrap with a distinct "bad padding in message." CmsException instead of being replaced by a random key, so it can be told apart from a correctly padded ciphertext, which fails only later at content decryption.

Text from CVE List

Affected versions 1

  • bc-csharpaffected: all versions before 2.7.0CVE List

References 3