Flash5/flash/docs/http2/HTTP1-HARDENING.md

HTTP/1.1 hardening

Audience: operators. This is the document to read when a 400/413/414/431/501 shows up in the logs and it isn't obvious why. Every rejection rule Flash's HTTP/1.1 parser enforces is listed here with its RFC citation and the status it produces. Contributor-level detail (why each check is implemented the way it is, the exact code paths) lives in the Javadoc of RequestParser, ChunkedInputStream, and dev.relism.flash.exceptions.MalformedRequestException.

Every rejection in this document has one thing in common: the connection is always closed afterwards, never kept alive. A rejected request is exactly the situation a smuggling attack needs a reusable connection for, so none of these rejections offer one — see MalformedRequestException's Javadoc.

Request-smuggling defenses (RFC 9112 §6.1)

Rule	Status	Detail
Content-Length and Transfer-Encoding both present	400	The canonical CL.TE/TE.CL smuggling vector. Rejected regardless of which header appears first.
Multiple Content-Length lines with differing values	400	Identical repeated values are tolerated (RFC 9110 §8.6 permits treating them as one).
Transfer-Encoding whose final coding is not chunked	501	Flash implements only chunked; anything else (gzip alone, or chunked, gzip — chunked must be last) is unsupported.

Strict Content-Length parsing (RFC 9110 §8.6)

Input	Status
Empty value	400
Any non-digit byte (including a leading + or -)	400
More than 19 digits	400
Value overflows Long.MAX_VALUE	400
Value exceeds Http1Limits.MAX_CONTENT_LENGTH (4 GiB by default)	413

The previous parser silently skipped non-digit characters ("5abc" parsed as 5; "-1" parsed as 1) instead of rejecting them — this is the fix.

Header and request-line limits (Http1Limits)

Limit	Default	Status when exceeded
MAX_HEADER_COUNT	100	431 Request Header Fields Too Large
MAX_HEADER_NAME_LENGTH	256 B	431
MAX_HEADER_VALUE_LENGTH	8192 B	431
MAX_REQUEST_LINE_LENGTH	8192 B	431
Header block exceeds maxHeaderBufferSize (or the connection ends before it completes)	configurable, default 64 KiB	431

Line-terminator and header-syntax correctness (RFC 9112 §5)

Rule	Status
A \r not immediately followed by \n (bare CR)	400 — a known desynchronization/smuggling surface
A header line beginning with whitespace (obsolete line folding, RFC 9112 §5.2)	400
A header name containing a byte outside RFC 9110 §5.6.2's tchar set	400
A header line with no :	400

Chunked transfer safety (RFC 9112 §7.1, Http1Limits)

Limit	Default	Status when exceeded
MAX_CHUNK_SIZE	16 MiB	413
Chunk-size line longer than 16 hex digits	—	400
MAX_CHUNK_EXT_LENGTH (the optional ;name=value after a chunk size)	256 B	400
MAX_CHUNKS_PER_BODY	100 000	413
MAX_TRAILER_COUNT	50	431
A chunk's data not followed by \r\n, or a malformed chunk-size/trailer terminator	—	400

Trailers are consumed within the bounds above and exposed through Request.trailers() on both HTTP/1.1 and HTTP/2.

Timeouts (FlashConfiguration)

Setting	Default	Covers
idleKeepAliveTimeoutMs	60 000	How long a keep-alive connection may sit idle waiting for its next request.
headerReadTimeoutMs	10 000	Once the first byte of a request arrives, how long the full header block may take.
bodyReadTimeoutMs	30 000	How long reading the body (by the handler, or the automatic post-response drain) may take.
shutdownDrainTimeoutMs	15 000	How long graceful shutdown waits for in-flight requests before force-closing.

These are enforced by an absolute deadline, not merely Socket.setSoTimeout. A per-read socket timeout alone never trips against a peer that sends one byte just often enough to keep each individual read alive (the classic slowloris shape) — see dev.relism.flash.transport.BufferedByteSource's Javadoc for how the absolute deadline is implemented on top of the JDK's per-read-only timeout API.

TLS (RFC 9113 §9.2.2, applies once a listener offers h2 over ALPN)

• The TLS handshake is forced explicitly (not left to the JDK's lazy on-first-read trigger) before any protocol decision is made, and is bounded by headerReadTimeoutMs.
• When a listener's TlsConfig.applicationProtocols includes "h2", the enabled TLS 1.2 cipher suite list is filtered against the RFC 9113 Appendix A blocklist (TlsConfig.TLS12_H2_BLOCKED_CIPHERS, ~280 entries). TLS 1.3 is never affected — none of its cipher suites are on that list.
• FlashConfiguration.http2Enabled advertises h2 on TLS listeners. The independent http2CleartextEnabled switch accepts the h2c prior-knowledge preface on plaintext listeners.