CVE-2026-103603: Unbounded HSS public key level count allows huge array allocation during signature verification

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-02 · updated 2026-10-28
Weakness: CWE-789

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.

Affected products

Legion of the Bouncy Castle Inc. bc-csharp: before 2.7.0 — fixed in 2.7.0
Legion of the Bouncy Castle Inc. bc-csharp: before 2.7.0 — fixed in 2.7.0

How to fix

Upgrade to a fixed version: Legion of the Bouncy Castle Inc. bc-csharp 2.7.0; Legion of the Bouncy Castle Inc. bc-csharp 2.7.0. Check the vendor advisory for mitigations.

References

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