AIKIDO-2026-617103

TL;DR

hpax decodes HPACK variable-length integers with no upper bound on the decoded value or the number of continuation octets. HPAX.Types.decode_remaining_integer/3 in lib/hpax/types.ex accumulates the integer as int + (value <<< m), shifting by 7 more bits for each continuation octet and stopping only on a terminating octet or truncated input, never because the integer grew too large. Because BEAM integers are arbitrary precision, a run of N continuation octets builds an O(N)-bit bignum and re-adds into an ever-larger bignum on each step, so the total decoding cost is superlinear (about O(N^2)). An unauthenticated attacker who can send an HTTP/2 header block to a server using this decoder (reached through HPAX.decode/2) can supply a small header block that forces a large, attacker-controlled amount of CPU (and transient memory), a denial-of-service amplification.