An unbounded iteration count in the PKMacBuilder verifier (PKMacBuilder) of the Legion of the Bouncy Castle Inc. bc-csharp library allows a remote unauthenticated attacker to trigger a denial of service through CPU exhaustion. By forging a CMP message or a CRMF certificate request whose PBMParameter declares a very large iteration count, the verifier performs hash iterations up to about 2^31. The library enforces a ceiling only when an explicit maximum is supplied via the PKMacBuilder(IPKMacPrimitivesProvider, int) constructor; otherwise the default constructors permit any count. The resulting CPU over‑utilisation can cause the hosting process to become unresponsive or crash, compromising availability.

The affected product is the Bouncy Castle C# cryptographic library (bc-csharp) from Legion of the Bouncy Castle Inc. Any deployment using a version prior to 2.7.0 is vulnerable. Users of earlier releases that employ CMP or CRMF message verification via PKMacBuilder without specifying a custom maximum are at risk.

The vulnerability has a CVSS score of 8.7, indicating high severity. The EPSS score is not available, but the lack of a listed KEV status suggests no known exploitation at the time of reporting. An attacker can remotely supply a crafted message to any application that processes CMP or CRMF messages using the vulnerable library, without needing authentication. The main attack vector is an unauthenticated network request that includes a PBMParameter with an extreme iteration count, leading to excessive CPU usage and service disruption.