Unbounded decoded HTTP/2 header-list size in cowlib cow_http2_machine enables HPACK amplification memory exhaustion

Overview

Allocation of Resources Without Limits or Throttling vulnerability in ninenines cowlib allows an unauthenticated remote peer to exhaust the memory of an HTTP/2 endpoint via HPACK-indexed header fields.

cow_http2_machine:headers_decode/4 in src/cow_http2_machine.erl decodes every HEADERS block with cow_hpack:decode/3. It bounds the compressed block size and the number of fields, but not the size of the decoded header list that RFC 9113 section 6.5.2 places under SETTINGS_MAX_HEADER_LIST_SIZE. A peer stores one header value of up to the dynamic table size (4096 bytes by default) and then references it with one-byte indexed fields, so about 100 bytes on the wire decode to a header list of about 390 KB. A consumer that copies the decoded values, as Cowboy does when cowboy_http2:headers_to_map/2 joins repeated cookie fields into one binary, allocates that amount for every stream. The total across streams and connections is not bounded.

This issue affects cowlib: from 2.7.0 onward.

Affected

• cowlib >= 2.7.0 < *
• ninenines/cowlib >= b461b119e78e4e09bb28b186b09da7ed4a86a0dd < a3b15e29b3067644bfd580246c87f7af7a90b3b8

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Workarounds

Cowboy servers that do not need HTTP/2 can set protocols => [http] in the protocol options. The listener then answers the HTTP/2 preface with 501 Not Implemented and the decoder is never reached.

Lowering max_headers in the HTTP/2 protocol options shrinks the per-stream allocation in proportion, to about max_headers times the 4096-byte table entry. This reduces the amplification but does not remove it.