CVE-2020-7212

The _encode_invalid_chars 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 percent_encodings
array contains all matches of percent encodings. It is not deduplicated.
For a URL of length N, the size of percent_encodings 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 percent_encodings were
deduplicated, the time to compute _encode_invalid_chars would be O(kN),
where k is at most 484 ((10+6*2)^2).

Notes