Affects BouncyCastle bc-csharp, a widely used cryptography library; vulnerability undermines PKIX certificate validation enabling broad impersonation/MITM across .NET apps.
CVE-2026-63577
Name-constraint bypass in BouncyCastle C# allows unauthorized certificates to be accepted.
Is CVE-2026-63577 being exploited?
Not confirmed. CVE-2026-63577 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked.
How severe is CVE-2026-63577?
CVE-2026-63577 is rated High with a CVSS score of 8.2. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-63577?
Yes. A fix has been recorded for CVE-2026-63577. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-63577 affect?
CVE-2026-63577 affects BouncyCastle bc-csharp < 2.7.0, .NET applications using bc-csharp, Libraries embedding bc-csharp. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-63577?
Upgrade bc-csharp to >= 2.7.0, rebuild and deploy affected apps
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Bypasses PKIX name-constraint checks to accept certs outside allowed subtrees, enabling impersonation, MITM, and trust abuse.
Immediate action required
- affected< 2.7.0
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.