CVE-2026-63573 Bleichenbacher padding oracle in CMS RSA PKCS#1 v1.5 key-transport unwrap
A Bleichenbacher padding oracle in the Bouncy Castle C# library allows attackers to decrypt CMS messages by distinguishing invalid padding errors.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-63573?
CVE-2026-63573 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. bc-csharp, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-203
Which versions are affected by CVE-2026-63573?
It affects 1 product.
Products include Bouncycastle Bc-Csharp.
How do I fix CVE-2026-63573?
No fix has been published for CVE-2026-63573 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-63573 being exploited?
There is no known exploitation of CVE-2026-63573 in the wild and no public exploit code.
Other vulnerabilities in Legion of the Bouncy Castle Inc. bc-csharp
- CVE-2026-63569 MTI/A0 DHAgreement does not validate the peer's ephemeral value
- CVE-2026-103600 Unbounded ASN.1 nesting depth causes process-terminating stack overflow
- CVE-2026-103603 Unbounded HSS public key level count allows huge array allocation during signature verification
- CVE-2026-103604 Quadratic-time escaping when converting X.509 distinguished names to strings
- CVE-2026-16000 KCcmBlockCipher (DSTU 7624 CCM) tag not bound to nonce when no associated data is used
Sources
Also known as GHSA-xx4m-xv2j-xx6g.