BouncyCastle .NET CVE-2026-103600 ASN.1 Unbounded Recursion Stack Overflow Fix

CVE-2026-103600 Vulnerability Overview Description: The ASN.1 parser imposes no limit on the depth of nested constructed encodings (SEQUENCE, SET, constructed tag objects, and constructed strings). and recursively invoke processing for each nested element; the recursion depth is determined by the input for both fixed-length (DER) and indefinite-length (BER) encodings. Impact: A nested SEQUENCE header is sufficient to exhaust the thread stack: approximately 2,000 levels or 8 KB of DER data can exhaust a 1.5 MB stack. On Windows, .NET throws a that cannot be caught, resulting in the termination of the entire process rather than just the operation handling the input. For threads with larger stacks, the same input consumes CPU time, which grows with the square of the nesting depth; for a 64 KB input, this may take up to 9 seconds. Attack Surface: Any application parsing ASN.1 from untrusted sources is exposed, regardless of higher-level APIs, as long as it receives DER-encoded data in TLS handshake messages via X.509 certificates and CRLs, CMS/PKCS#7 and S/MIME, PKCS#8 and PKCS#12, OCSP, timestamps, and certificates. Scope of Impact Affected Versions: BC#.NET 2.6.2 and earlier, as well as the 2.7.0-beta.98 pre-release version. Platform Impact: All CLR platforms. Remediation Fixed Version: BC#.NET 2.7.0. Fix Details: BC#.NET 2.7.0 limits the nesting depth of constructed encodings to 64 by default. Inputs with deeper nesting are rejected and throw an , which is a subclass of ; therefore, code already handling malformed input does not need to be modified. Configuration Adjustment: The limit can be adjusted per thread via the environment variable or through . Recommendation: Earlier versions lack a setting to prevent this issue; upgrading is recommended. If an immediate upgrade is not possible, applications should inspect input for nested constructed elements that are deeper than expected before passing it to Bouncy Castle, and reject such input. Legal structures remain within the new default limit of 64. Limiting input size may not be sufficient, as less input is required to trigger the issue. Parsing on threads with larger stacks can avoid process termination but will not prevent CPU consumption. Related Vulnerabilities The corresponding BC Java issue was resolved in BC Java 1.84, with subsequent fixes in BC Java 1.85 (CVE-2026-13598). Remediation Commits https://github.com/bcgit/bc-csharp/commit/89c81b0da7fe1cf70013285560bbc472cb73901 (Added depth limit) https://github.com/bcgit/bc-csharp/commit/b99f4d3ceaa05eb57becc73dfa6ad91a54c778d65 (Set default value to 64) Contributors Yt.

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BouncyCastle .NET CVE-2026-103600 ASN.1 Unbounded Recursion Stack Overflow Fix

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