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CVE-2026-63573

7.5 · HIGH

Observable discrepancy in the CMS RSA PKCS#1 v1.5 key-transport unwrap (KeyTransRecipientInformation.UnwrapKey) in Legion of the Bouncy Castle Inc.

Published Oct 2, 2026bouncycastleCWE-203EPSS 0.30%

Overview

CVE-2026-63573 is a high-severity vulnerability affecting bouncycastle bc-csharp. It was published on October 2, 2026 and has a CVSS 3.1 base score of 7.5 (HIGH).

This vulnerability has a CVSS 3.1 base score of 7.5, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical 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.

Remediation

Check the references section for vendor advisories and patches from bouncycastle. Update bc-csharp to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

Affected products

VendorProductVersionsStatus
bouncycastlebc-csharp>= 0, < 2.7.0Affected

References

GitHubgithub.com/bcgit/bc-csharp/commit/85679cc7023adf4e2a0650b88a1f4f98679c7463GitHubgithub.com/bcgit/bc-csharp/commit/c38b4790e2aef68f116a551f73b94ea05cb5590cGitHubgithub.com/bcgit/bc-csharp/wiki/CVE-2026-63573

How CyberStrike helps

Frequently asked questions

What is CVE-2026-63573?

CVE-2026-63573 is a high-severity vulnerability affecting bouncycastle bc-csharp. It was published on October 2, 2026 and has a CVSS 3.1 base score of 7.5 (HIGH).

How severe is CVE-2026-63573?

This vulnerability has a CVSS 3.1 base score of 7.5, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2026-63573?

Check the references section for vendor advisories and patches from bouncycastle. Update bc-csharp to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-63573?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-63573 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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