CVE-2026-103603 Unbounded HSS public key level count allows huge array allocation during signature verification
Remote attackers can cause denial of service via memory exhaustion in Bouncy Castle C# by supplying HSS public keys with excessive level counts.
- 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-2026-103603?
CVE-2026-103603 is a high-severity denial of service 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
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.
Weakness: CWE-789
Which versions are affected by CVE-2026-103603?
It affects 1 product.
Products include Bouncycastle Bc-Csharp.
How do I fix CVE-2026-103603?
No fix has been published for CVE-2026-103603 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-103603 being exploited?
There is no known exploitation of CVE-2026-103603 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-2026-103604 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
- CVE-2026-63566 DTLS handshake reassembler allocates buffer from unchecked 24-bit length
Sources
Also known as GHSA-gpr4-c479-xrw4.