# 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.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 2 Oct 2026
- **Updated:** 2 Oct 2026
- **Also known as:** GHSA-gpr4-c479-xrw4

## 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 (Memory Allocation with Excessive Size Value)

## Which versions are affected by CVE-2026-103603?

It affects 1 product.


Products: 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.

## 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](https://insights.sbomify.com/CVE-2026-63569/): MTI/A0 DHAgreement does not validate the peer's ephemeral value (critical 9.1)
- [CVE-2026-103600](https://insights.sbomify.com/CVE-2026-103600/): Unbounded ASN.1 nesting depth causes process-terminating stack overflow (high 8.7)
- [CVE-2026-103604](https://insights.sbomify.com/CVE-2026-103604/): Quadratic-time escaping when converting X.509 distinguished names to strings (high 8.7)
- [CVE-2026-16000](https://insights.sbomify.com/CVE-2026-16000/): KCcmBlockCipher (DSTU 7624 CCM) tag not bound to nonce when no associated data is used (high 8.7)
- [CVE-2026-63566](https://insights.sbomify.com/CVE-2026-63566/): DTLS handshake reassembler allocates buffer from unchecked 24-bit length (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-103603 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-103603 in the wild and no public exploit code.

Affected product: Legion of the Bouncy Castle Inc. bc-csharp. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-103603/ (Markdown: https://insights.sbomify.com/CVE-2026-103603/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-103603
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-103603
- OSV: https://osv.dev/vulnerability/CVE-2026-103603

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-103603/
Summaries are generated and may contain mistakes; the official description above is the record.