---
cve: "CVE-2026-63573"
severity: "HIGH"
cvss: 8.2
epss: "0.3%"
vendor: "Legion of the Bouncy Castle Inc."
kev: false
exploited: false
published: "2026-10-02 08:17:02"
tags: [cve, security, high]
source: tsecurity.de CVE-Dossier
exported: "2026-10-08T01:20:26+02:00"
---

# CVE-2026-63573

> 8.2 HIGH · 🧪 PoC

## Beschreibung

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.

## CNA-Record (Kanon, cvelistV5)

- CNA: **bcorg**
- State: PUBLISHED
- Stand: 2026-10-02 19:20:19
- CNA-CVSS: **8.2** (`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`)

## CISA-Anreicherung (vulnrichment/ADP)

- CISA-SSVC (Coordinator, v2.0.3):
  - Exploitation: **none**
  - Automatable: **no**
  - Technical Impact: **partial**

## CVSS-Vektor

```
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
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Netzwerk | bad |
| AC Komplexität | Gering | bad |
| PR Privilegien | Keine | bad |
| UI Interaktion | Keine | bad |

## Patch verfügbar (OSV)

- 4007498b13582d90ee1eda5d9920c324428b98b3 (Commit)
- 85679cc7023adf4e2a0650b88a1f4f98679c7463 (Commit)

## BSI-Hinweise (deutsch)

- [Bouncy Castle: Mehrere Schwachstellen](https://wid.cert-bund.de/portal/wid/sicherheitshinweis/WID-SEC-W-2026-3713) — _BSI-Einstufung: mittel_
  Ein entfernter, anonymer Angreifer kann mehrere Schwachstellen in Bouncy Castle ausnutzen, um Sicherheitsvorkehrungen zu umgehen, Daten zu manipulieren oder offenzulegen und einen Denial-of-Service-Zustand auszulösen.

## Referenzen

- <https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63573>
- <https://github.com/bcgit/bc-csharp/commit/85679cc7023adf4e2a0650b88a1f4f98679c7463>
- <https://github.com/bcgit/bc-csharp/commit/c38b4790e2aef68f116a551f73b94ea05cb5590c>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/sicherheitsluecken/cve-2026-63573/) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI WID (CSAF)_