HTTP/2 cookie crumb join expands a small HPACK header block into an unbounded allocation in Bandit

Improper Handling of Highly Compressed Data (Data Amplification) vulnerability in the HTTP/2 implementation of mtrudel bandit allows an unauthenticated remote attacker to exhaust server memory via a small HPACK-compressed HEADERS block. The block stores one large cookie value in the HPACK dynamic table and then references it tens of thousands of times with 1-byte indexed fields. In the reporter's run, a 49 KB block made a server with default settings allocate a 183 MB cookie binary before any application code ran, and every concurrent stream can repeat the cost.

Bandit.HTTP2.Connection caps only the compressed block (max_header_block_size), and its check of the decoded list against max_header_list_size never fires because that setting defaults to :infinity. combine_cookie_crumbs/1 in lib/bandit/http2/stream.ex then joins every crumb into one binary.

This issue affects bandit: from 0.3.3 before 1.14.0.

1. Compression

The attacker's header block, split across HEADERS and CONTINUATION frames, stays under the 50,000-byte max_header_block_size. It inserts one 4,058-byte cookie into the 4,096-byte dynamic table, the largest value that fits, then references it about 45,000 times with 1-byte indexed representations. HPAX.decode returns a list whose entries all point at the same binary, so decoding itself stays cheap.

2. Missing decoded limit (lib/bandit/http2/connection.ex, lib/bandit/http2/settings.ex)

After decoding, the connection compares header_list_size(headers) against local_settings.max_header_list_size. That setting defaults to :infinity, and in Erlang term order every integer sorts below an atom, so the comparison never rejects a list. The limit is enforced only when an operator sets it.

3. Expansion (lib/bandit/http2/stream.ex)

read_headers calls combine_cookie_crumbs/1, which joins every crumb with Enum.map_join/3 into a single binary of about 183 MB before Plug.call/2 runs. max_concurrent_streams also defaults to :infinity, so N streams allocate N times as much.

1. Against a Bandit server with default HTTP/2 settings, build an HPACK header block with the request pseudo-headers plus a 4,058-byte cookie stored in the dynamic table and then referenced about 45,000 times with indexed representations. The block is about 49 KB.
2. Send it on one stream as a HEADERS frame followed by CONTINUATION frames of at most 16,384 bytes, with END_HEADERS on the last.
3. The request reaches the plug with a cookie header of about 183 MB, and VM memory grows by about the same amount. Repeat on further streams to multiply it.

On 1.12.2 and later, set a finite decoded header list limit, for example http_2_options: [default_local_settings: [max_header_list_size: 65_536]], which resets the stream before the cookie crumbs are joined. From 0.7.0, disabling HTTP/2 with http_2_options: [enabled: false] removes the vulnerable path.