net/http: permit requests with invalid Host headers

Historically, the Transport has silently truncated invalid
Host headers at the first '/' or ' ' character. CL 506996 changed
this behavior to reject invalid Host headers entirely.
Unfortunately, Docker appears to rely on the previous behavior.

When sending a HTTP/1 request with an invalid Host, send an empty
Host header. This is safer than truncation: If you care about the
Host, then you should get the one you set; if you don't care,
then an empty Host should be fine.

Continue to fully validate Host headers sent to a proxy,
since proxies generally can't productively forward requests
without a Host.

For #60374
Fixes #61431

Change-Id: If170c7dd860aa20eb58fe32990fc93af832742b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/511155
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>

1 files changed, 22 insertions, 1 deletions

diff --git a/src/net/http/request.go b/src/net/http/request.go
index a2e8373dd5..d1fbd5df90 100644
--- a/src/net/http/request.go
+++ b/src/net/http/request.go
@@ -591,8 +591,29 @@ func (r *Request) write(w io.Writer, usingProxy bool, extraHeaders Header, waitF
 	if err != nil {
 		return err
 	}
+	// Validate that the Host header is a valid header in general,
+	// but don't validate the host itself. This is sufficient to avoid
+	// header or request smuggling via the Host field.
+	// The server can (and will, if it's a net/http server) reject
+	// the request if it doesn't consider the host valid.
 	if !httpguts.ValidHostHeader(host) {
-		return errors.New("http: invalid Host header")
+		// Historically, we would truncate the Host header after '/' or ' '.
+		// Some users have relied on this truncation to convert a network
+		// address such as Unix domain socket path into a valid, ignored
+		// Host header (see https://go.dev/issue/61431).
+		//
+		// We don't preserve the truncation, because sending an altered
+		// header field opens a smuggling vector. Instead, zero out the
+		// Host header entirely if it isn't valid. (An empty Host is valid;
+		// see RFC 9112 Section 3.2.)
+		//
+		// Return an error if we're sending to a proxy, since the proxy
+		// probably can't do anything useful with an empty Host header.
+		if !usingProxy {
+			host = ""
+		} else {
+			return errors.New("http: invalid Host header")
+		}
 	}
 
 	// According to RFC 6874, an HTTP client, proxy, or other