HashDoS, or Hash Denial of Service, is a type of cyberattack that targets web applications by exploiting predictable or weak hash functions used in data structures like hash tables. Attackers send specially crafted inputs that cause many hash collisions, forcing the server to spend excessive CPU resources resolving these collisions. This can severely degrade server performance or even cause a denial of service, making the application slow or unavailable to legitimate users. HashDoS attacks highlight the importance of using strong, unpredictable hash functions and implementing rate limiting to protect against resource exhaustion.