System.Security.Cryptography.Pkcs EUVD-2026-100199

Exploitation requires a target application that uses the System.Security.Cryptography.Pkcs package (or affected .NET Framework versions) and invokes SignedCms.CheckSignature on attacker-controlled CMS/PKCS#7 signed data containing malicious X.509 content, which triggers repeated reparsing and quadratic CPU cost. Per CVSS AV:N/AC:L/PR:N/UI:N it is remote and unauthenticated with no user interaction, but exploitation is LIMITED to deployments that actually accept and validate untrusted signed CMS messages via this API; services that never process external signed data, or that run patched versions (Pkcs 10.0.13/9.0.21/8.0.2/11.0.0-rc.2 or the listed .NET Framework updates), are not exposed. Impact is availability-only (C:N/I:N) resource exhaustion, not code execution or data disclosure.

Signals are consistent and point to a genuine but bounded DoS risk. The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, score 7.5 High) indicates a remotely reachable, low-complexity, unauthenticated attack with availability-only impact - confirmed by the dotnet/runtime GHSA advisory describing unauthenticated quadratic CPU exhaustion in SignedCms.CheckSignature. SSVC aligns: Exploitation 'none' (no observed exploitation), Automatable 'yes' (the trigger is a single crafted message that scales), Technical Impact 'partial' (service degradation, not full compromise), which matches C:N/I:N but tempers A:H since the impact is resource exhaustion rather than system takeover. No EPSS score, no POC, and no CISA KEV entry are present, so this is 'no public exploit identified at time of analysis' - not weaponized or in-the-wild. Net: a credible, automatable DoS worth prioritizing for any service that validates attacker-supplied CMS/PKCS#7 signatures, but its real-world blast radius is gated by whether an application actually exposes SignedCms.CheckSignature to untrusted input, so it is not a drop-everything issue for most deployments. Note: the input Tag 'Authentication Bypass' conflicts with the CWE-407 (Inefficient Algorithmic Complexity) classification and the advisory - this is a denial-of-service issue, not an auth bypass; treat that tag as erroneous.

An attacker sends a specially crafted CMS/PKCS#7 signed message containing malicious X.509 data to a .NET service that verifies signatures via SignedCms.CheckSignature. The method repeatedly reparses the attacker-controlled certificate data, driving quadratic CPU consumption that exhausts the host and denies service to legitimate users. No authentication or user interaction is required (AV:N/AC:L/PR:N/UI:N), and no public exploit code is identified at time of analysis.