ImageMagick before 6.9.13-57 and 7.x before 7.1.2-32 lacks proper synchronization when the security policy is accessed, so simultaneous read

ImageMagick before 6.9.13-57 and 7.x before 7.1.2-32 lacks proper synchronization when the security policy is accessed, so simultaneous reads and writes of the policy by multiple threads trigger a race condition that causes a heap buffer over-write. An attacker able to induce concurrent security policy access can crash the application, causing a denial of service.