High 8.2Published Oct 2, 2026

CVE-2026-63573Bleichenbacher padding oracle in CMS RSA PKCS#1 v1.5 key-transport unwrap

Updated under an hour ago (2026-10-19 07:00 UTC)

Does this affect you?

It affects Legion of the Bouncy Castle Inc. bc-csharp. Add what you run to find out whether that includes you.

EPSS puts the chance of exploitation in the next 30 days at 0.31%, higher than 22% of scored CVEs.

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Every CVE for Legion of the Bouncy Castle Inc. bc-csharp

Exploited in the last 30 days

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.

CVSS: 8.2 HIGH (v4.0, CNA)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
Weakness (CWE): CWE-203
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): 0.31% (percentile 22)
Assigned by: bcorg
NVD analysis: Undergoing Analysis
Record last updated: Oct 2, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

Legion of the Bouncy Castle Inc. bc-csharp legion_of_the_bouncy_castle:bc_csharp (from the name)	before 2.7.0	CNA record

References

https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63573vendor-advisory
https://github.com/bcgit/bc-csharp/commit/85679cc7023adf4e2a0650b88a1f4f98679c7463patch
https://github.com/bcgit/bc-csharp/commit/c38b4790e2aef68f116a551f73b94ea05cb5590cpatch

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.