CVE-2026-108529: SETTINGS and stream-0 WINDOW_UPDATE frames trigger quadratic work over blocked streams in Bandit

Inefficient Algorithmic Complexity vulnerability in the HTTP/2 implementation of mtrudel bandit allows an unauthenticated remote attacker to exhaust server CPU via small SETTINGS or connection-level WINDOWUPDATE frames. The attacker opens thousands of streams whose responses block on the connection send window, then sends cheap frames that each make the server walk the whole queue of blocked sends. In the reporter's run, with 3,000 blocked streams a single 9-byte SETTINGS frame took 133 ms to process.

dopendingsends/2 in lib/bandit/http2/connection.ex retries every queued send and, before each retry, removes that entry from the queue with List.delete/2, so one frame costs O(n²) list operations for n blocked streams. maxconcurrentstreams defaults to :infinity, so the attacker chooses n.

This issue affects bandit: from 0.3.4 before 1.14.0.