Netty: HttpObjectDecoder skips arbitrary initial control characters when only initial CRLF characters are permitted
Summary
Before reading the first request-line, HttpObjectDecoder skips every byte for which
Character.isISOControl(b) is true (0x00–0x1F and 0x7F) as well as all whitespace.
RFC 9112 §2.2 only asks servers to ignore empty CRLF lines preceding the request-line —
a carefully scoped robustness allowance intended to handle HTTP/1.0 POST workarounds.
Silently absorbing NUL bytes, SOH, STX, and other non-CRLF control characters goes
significantly beyond this, and can be exploited for request-boundary confusion in pipelined
or multiplexed transports where a front-end component treats those bytes differently.
Affected Code
| File | Lines | Role |
|------|-------|------|
| codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectDecoder.java | 1298–1313 | ISO_CONTROL_OR_WHITESPACE static initialiser — marks all ISO control chars |
| codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectDecoder.java | 1307–1313 | SKIP_CONTROL_CHARS_BYTES ByteProcessor — skips the entire set |
| codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectDecoder.java | 1275–1289 | LineParser.skipControlChars — advances readerIndex past all matching bytes |
Specification Analysis
RFC 9112 §2.2 — Message Parsing
> In the interest of robustness, a server that is expecting to receive and parse a
> request-line SHOULD ignore at least one empty line (CRLF) received prior to the
> request-line.
> An HTTP/1.1 user agent MUST NOT preface or follow a request with an extra CRLF.
Deviation
The RFC names a single permitted exception: an empty line (bare CRLF, i.e. the two-byte
sequence \r\n). The ISO_CONTROL_OR_WHITESPACE table is initialised as:
```java
for (byte b = Byte.MIN_VALUE; b < Byte.MAX_VALUE; b++) {
ISO_CONTROL_OR_WHITESPACE[128 + b] =
Character.isISOControl(b) || isWhitespace(b);
}
```
Character.isISOControl returns true for 0x00–0x1F and 0x7F. This includes NUL
(0x00), SOH (0x01), STX (0x02), BEL (0x07), DEL (0x7F), and every other non-CRLF
control character. The SKIP_CONTROL_CHARS state runs this scan unconditionally before the
first READ_INITIAL, meaning any sequence of such bytes prepended to a request is silently
consumed.
A load balancer or TLS terminator that does not perform the same scan sees a different
message boundary than Netty does, which is the basis of a request-desync / smuggling attack.
Suggested Unit Test
Add to HttpRequestDecoderTest.java.
```java
@Test
public void testNonCrlfControlBytesPrecedingRequestLineAreRejected() {
// RFC 9112 §2.2: servers SHOULD ignore "at least one empty line (CRLF)" before the
// request-line. Non-CRLF control bytes are not part of this robustness allowance
// and must not be silently swallowed.
EmbeddedChannel channel = new EmbeddedChannel(new HttpRequestDecoder());
ByteBuf buf = Unpooled.buffer();
buf.writeByte(0x00); // NUL — not an empty CRLF line
buf.writeByte(0x01); // SOH — not an empty CRLF line
buf.writeCharSequence(
"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n",
CharsetUtil.US_ASCII);
channel.writeInbound(buf);
HttpRequest req = channel.readInbound();
// Current behaviour: NUL and SOH are in ISO_CONTROL_OR_WHITESPACE, so they are
// silently skipped; the request decodes successfully and isFailure() == false.
//
// RFC-correct behaviour: only empty CRLF lines should be ignored; NUL/SOH must
// cause a parse error — isFailure() == true.
assertTrue(
req.decoderResult().isFailure(),
"Non-CRLF control bytes before the request-line must not be silently skipped " +
"(RFC 9112 §2.2 allows only empty CRLF lines)");
assertFalse(channel.finish());
}
```
Current behaviour (unfixed): skipControlChars advances past 0x00 and 0x01 because
both are in ISO_CONTROL_OR_WHITESPACE; the request parses normally, isFailure() is
false → test fails.
Expected behaviour after fix: only CRLF empty lines are tolerated; non-CRLF control
bytes produce an error, isFailure() is true → test passes.
