Memory allocation with excessive size value in the HSS/LMS signature code (HssPublicKeyParameters…

Why it is Amber

• CVSS 8.7 high
• Remotely exploitable without logging in

References

• https://github.com/bcgit/bc-csharp/commit/f47ad47c7b5745b53d3f9ac711a5a419a272a5d7
• https://github.com/bcgit/bc-csharp/wiki/CVE-2026-103603

Original description

Memory allocation with excessive size value in the HSS/LMS signature code (HssPublicKeyParameters, HssSignature) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote unauthenticated attacker who can supply both an HSS public key and a signature to cause a denial of service through memory exhaustion via a public key encoding with an excessive level count, because the level count L read when parsing an HSS public key was not checked against the RFC 8554 maximum of 8, and signature parsing then allocated an array of L - 1 entries before reading any further signature data. A single verification can commit up to about 17 GB of memory or fail with an OutOfMemoryException.