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

Is it exploited?

Not on CISA's list. CISA has not added it to the Known Exploited Vulnerabilities catalog as of 29 Oct 2026. If CISA adds it, this page shows it as known exploited.

How likely is exploitation in the next 30 days?

0.3%

EPSS score, percentile 22.6 of all scored records. A probability estimated by FIRST, not a report of exploitation.

How severe?

8.2 High

CVSS v4.0 base score, given by the CNA

What should I do?

CISA has set no required action, because the vulnerability is not in KEV.

Advisories and patches (3)

Which versions are affected?

Affected

• before 2.7.0

Versions by product

Source: CISA KEV 2026.10.29, as of 29 Oct 2026; FIRST EPSS, as of 29 Oct 2026; CVE List, as of 29 Oct 2026. About the data.

What to do

Advisories and patches

• github.com (Patch)
• github.com (Patch)
• github.com (Vendor advisory)

This summary is based on the CVE record as of 29 Oct 2026. Check the vendor's advisory before you act: affected versions and fixes can change.

Affected products and versions

As listed in the CVE record.

Versions not listed are: unaffected.

Source: CVE List, as of 29 Oct 2026. About the data.

About this vulnerability

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 r…

Summary: CVE record, as of 29 Oct 2026.

• CISA has not added CVE-2026-63573 to its Known Exploited Vulnerabilities (KEV) catalog, as of 29 Oct 2026.
• The CVE record was published on 2 Oct 2026.
• The CVE record carries a CVSS v4.0 base score of 8.2 (High), given by its CNA.
• FIRST's EPSS model puts the probability of exploitation in the next 30 days at 0.3% (percentile 22.6), as of 29 Oct 2026. EPSS is a statistical estimate, not a report of exploitation.
• CISA's SSVC assessment in the CVE record: exploitation none, automatable no, technical impact partial.
• The credits in the CVE record name Anthropic (Claude) as finder.

Source: CISA KEV 2026.10.29, as of 29 Oct 2026; CVE List, as of 29 Oct 2026; FIRST EPSS, as of 29 Oct 2026. About the data.

Description

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

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.

Source: CVE List, as of 29 Oct 2026. About the data.

Timeline