The encodeinvalidchars function in util/url.py in the urllib3 library 1.25.2 through 1.25.7 for Python allows a denial of service (CPU consumption) because of an inefficient algorithm. The percentencodings array contains all matches of percent encodings. It is not deduplicated. For a URL of length N, the size of percentencodings may be up to O(N). The next step (normalize existing percent-encoded bytes) also takes up to O(N) for each step, so the total time is O(N^2). If percentencodings were deduplicated, the time to compute encodeinvalidchars would be O(kN), where k is at most 484 ((10+62)^2).