bc-csharp: Denial of service via unchecked HSS level count in signature verification
Summary
A denial-of-service flaw in the HSS/LMS signature code of bc-csharp (Bouncy Castle for .NET) allows memory exhaustion. An unauthenticated attacker who can supply both an HSS public key and a signature to an application that decodes and verifies them can make a single verification allocate up to about 17 GB, or throw an OutOfMemoryException, which can exhaust server memory. Only applications that call these classes directly with untrusted input are affected; the vendor states that BC C# .NET's own X.509 and CMS verification does not use HSS/LMS.
Affected
- BC C# .NET 2.6.2 and earlier (CNA range: before 2.7.0)
- BC C# .NET 2.7.0-beta.98 pre-release
Fixed in
- BC C# .NET 2.7.0 or later
Workaround
- None documented by the vendor. Upgrading is the only fix named in the sources.
- Until patched, do not accept HSS/LMS public keys from untrusted sources.
- If upgrading is not possible, check each decoded HssPublicKeyParameters and reject keys whose Level is less than 1 or greater than 8 before using them for verification.
Exploitation
- CISA SSVC: exploitation none, automatable yes, technical impact partial.
- No exploitation in the wild and no public proof of concept were found in the sources.
Our recommendation
- Check whether any in-house .NET software or third-party component uses BouncyCastle.Cryptography, including transitive dependencies.
- Update to BC C# .NET 2.7.0 or later as part of the normal update cycle.
- Until then, do not verify HSS/LMS material from untrusted sources and monitor for OutOfMemory errors.
- Escalate only if an internet-facing service verifies HSS/LMS keys or signatures from outside.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Vendor wiki CVE-2026-103603 github.com
- Fix commit f47ad47c7b5745b53d3f9ac711a5a419a272a5d7 github.com
- OSV CVE-2026-103603 osv.dev
- VulDB CVE-2026-103603 vuldb.com
- Tenable CVE-2026-103603 www.tenable.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 06:13.
Original description (NVD)
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.
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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