Http4s Ember HTTP/2: unbounded continuation frame accumulation

When Ember receives an HTTP/2 HEADERS or PUSH_PROMISE frame without the END_HEADERS flag, it buffers the header block fragment and waits for subsequent CONTINUATION frames. These accumulate unbounded until the connection closes.

Impact

A remote, unauthenticated peer can exhaust the heap on any Ember endpoint that has HTTP/2 enabled:

• ember-server with .withHttp2: any HTTP/2 client can trigger this against any reachable path (including paths that return 404). No authentication is required because the attack completes before the request is decoded.
• ember-client with .withHttp2: a malicious or compromised origin server can trigger this via the response header block. A single in-flight request is sufficient.

Memory consumption is bounded only by the attacker's upload bandwidth and the connection lifetime.

Prerequisites

• EmberServerBuilder configured with .withHttp2.
  • For the server: the attacker can establish an HTTP/2 connection
  • For the client: the application makes a request to an attacker-controlled origin. HTTP/2 incoming headers exceeding the configured size limit are buffer in order to generate an informative HTTP 413 response. If a client does not stop sending headers, this leads to memory exhaustion.

Patches

The fix bounds the accumulated header-block size at SETTINGS_MAX_HEADER_LIST_SIZE (derived from EmberServerBuilder.maxHeaderSize / EmberClientBuilder.maxResponseHeaderSize). When a CONTINUATION frame would push the accumulated block over that limit, the connection is terminated with GOAWAY. The receiveHeadersTimeout additionally bounds how long an incomplete header block may remain open.

Workarounds

• Disable HTTP/2 (.withHttp2) until upgraded (default). It is off by default.
• If HTTP/2 must remain enabled, place ember behind a reverse proxy that terminates HTTP/2 and speaks HTTP/1.1 to ember.