CVE-2026-103603
HIGHDescription
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.
How to fix
No published remediation has been found for this vulnerability's affected products yet.
Mitigation guidance may be in the linked vendor advisories in the References section below.
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CVSS v4 Vector
Exploitability
Impact
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Exploit Intelligence
Low risk: more likely to be exploited than 33% of all known CVEs.
References
Related Vulnerabilities
Other CWE-789 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2022-30522 | High | 7.5 | 90% | - | Fix |
| CVE-2025-27533 | High | 7.5 | 9.0% | - | Fix |
| CVE-2017-7651 | High | 7.5 | 5.1% | - | Fix |
| CVE-2026-49975 | High | 7.5 | 4.2% | - | Fix |
| CVE-2021-31811 | Medium | 5.5 | 3.6% | - | - |
| CVE-2021-27906 | Medium | 5.5 | 3.3% | - | Fix |
Common questions
How do I fix CVE-2026-103603?
No published fix for CVE-2026-103603 has been found yet. Check the linked vendor advisories for guidance as it becomes available.
Is CVE-2026-103603 being actively exploited?
Not that we know of. CVE-2026-103603 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.41% is the estimated probability that it will be exploited in the next 30 days. That is higher than 33% of all scored CVEs.
How severe is CVE-2026-103603?
CVE-2026-103603 has a CVSS v4 base score of 8.7, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
Embed a live status badge for CVE-2026-103603
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-10-02.