CVE-2026-103603 Legion of the Bouncy Castle Inc. CVE debrief

Who should care

Defenders responsible for systems using the Bouncy Castle Inc.'s bc-csharp library for cryptographic operations should assess their exposure to this vulnerability. This includes teams handling public keys and signatures, as well as those responsible for updating and patching libraries in their systems.

Why it matters

Defenders should prioritize updating to version 2.7.0 or later of the bc-csharp library to mitigate this denial of service vulnerability. The vulnerability allows remote attackers to cause memory exhaustion through excessive memory allocation during signature parsing. Systems using this library for cryptographic operations should be assessed for exposure, particularly those handling public keys and signatures.

• Memory exhaustion through excessive public key encoding
• Potential for OutOfMemoryException during signature parsing
• Denial of service through memory allocation
• Need for verification of public key and signature handling

Technical summary

The HSS/LMS signature code in the Bouncy Castle Inc.'s bc-csharp library before version 2.7.0 is vulnerable to a denial of service attack through memory exhaustion. The vulnerability occurs because the level count L read when parsing an HSS public key is not checked against the RFC 8554 maximum of 8. This allows a remote unauthenticated attacker who can supply both an HSS public key and a signature to cause excessive memory allocation during signature parsing, potentially leading to an OutOfMemoryException.

Defensive priority

Defenders should prioritize updating to version 2.7.0 or later of the bc-csharp library to mitigate this vulnerability. Systems using this library for cryptographic operations should be assessed for exposure, particularly those handling public keys and signatures.

Recommended defensive actions

• Update to version 2.7.0 or later of the bc-csharp library
• Assess systems using this library for cryptographic operations for exposure
• Verify public key and signature handling in systems using this library
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record and NVD vulnerability detail provide information on the memory allocation vulnerability in the bc-csharp library. The vendor has released a patch, which is available in version 2.7.0 and later.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.