Missing CR/LF/NUL validation in header values

Summary

SwiftNIO HTTP/2 was missing validation on inbound HEADERS frames that let
CR, LF, NUL, SP and other control characters reach an HTTP/1.1 backend
through NIOHTTP2's HTTP/2-to-HTTP/1 codec, enabling HTTP request smuggling
or response splitting.

Impact

Two related gaps in inbound header validation, against any application
using HTTP2ToHTTP1Codec (or HTTP2FramePayloadToHTTP1Codec) to front an
HTTP/1.1 backend:

Regular header field values were only checked against a forbidden-name
list (connection, transfer-encoding, proxy-connection, keep-alive,
upgrade); the value itself was never inspected. An attacker-controlled
regular header field value containing CR or LF passed validation and, once
serialized as name: value CRLF by the codec, terminated the field early
and injected extra header lines into the outbound HTTP/1.1 message.

Pseudo-header values (:path in particular) were only checked against
CR, LF and NUL. A :path value containing SP serializes into the
request-target of METHOD SP request-target SP HTTP-version CRLF, so a
value like /a HTTP/1.1 produces GET /a HTTP/1.1 HTTP/1.1 — a
parser-differential request line depending on whether a downstream reader
takes the first or last SP-delimited token as the version.

Neither of these is reachable on a stock pipeline: NIOHTTP1's outbound
validator (enableOutboundHeaderValidation, on by default) already rejects
these characters on write. The exposure is pipelines that skip outbound
validation, or any code that reads validated-looking HTTPRequestHead.headers
and forwards the values on trusting that HTTP/2 already checked them.

Fix

Fixed in 48bfd90 and 45bdf67.

Mitigation

Upgrade to 1.45.0