From c48da250e237b8355b2943875449ef071bee07bb Mon Sep 17 00:00:00 2001
From: "Nicholas S. Husin" <nsh@golang.org>
Date: Wed, 18 Mar 2026 15:34:19 -0400
Subject: [PATCH] internal/http3: panic in server handler if given status that
 is not 3 digits

Our HTTP/1 and HTTP/2 server implementations will panic when WriteHeader
is called from a handler with a status code that is not 3 digits. For
consistency, do this too for HTTP/3.

For golang/go#70914

Change-Id: I65aca23ea186481ae06c7388db487e476a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/759300
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
---
 internal/http3/server.go      | 19 ++++++++++++++++
 internal/http3/server_test.go | 42 +++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)

diff --git a/internal/http3/server.go b/internal/http3/server.go
index d6a366be..7b68684e 100644
--- a/internal/http3/server.go
+++ b/internal/http3/server.go
@@ -7,6 +7,7 @@ package http3
 import (
 	"context"
 	"crypto/tls"
+	"fmt"
 	"io"
 	"maps"
 	"net/http"
@@ -485,6 +486,23 @@ func isInfoStatus(status int) bool {
 	return status >= 100 && status < 200
 }
 
+// checkWriteHeaderCode is a copy of net/http's checkWriteHeaderCode.
+func checkWriteHeaderCode(code int) {
+	// Issue 22880: require valid WriteHeader status codes.
+	// For now we only enforce that it's three digits.
+	// In the future we might block things over 599 (600 and above aren't defined
+	// at http://httpwg.org/specs/rfc7231.html#status.codes).
+	// But for now any three digits.
+	//
+	// We used to send "HTTP/1.1 000 0" on the wire in responses but there's
+	// no equivalent bogus thing we can realistically send in HTTP/3,
+	// so we'll consistently panic instead and help people find their bugs
+	// early. (We can't return an error from WriteHeader even if we wanted to.)
+	if code < 100 || code > 999 {
+		panic(fmt.Sprintf("invalid WriteHeader code %v", code))
+	}
+}
+
 func (rw *responseWriter) WriteHeader(statusCode int) {
 	// TODO: handle sending informational status headers (e.g. 103).
 	rw.mu.Lock()
@@ -492,6 +510,7 @@ func (rw *responseWriter) WriteHeader(statusCode int) {
 	if rw.statusCodeSet {
 		return
 	}
+	checkWriteHeaderCode(statusCode)
 
 	// Informational headers can be sent multiple times, and should be flushed
 	// immediately.
diff --git a/internal/http3/server_test.go b/internal/http3/server_test.go
index 6d6fec62..9529d624 100644
--- a/internal/http3/server_test.go
+++ b/internal/http3/server_test.go
@@ -6,6 +6,7 @@ package http3
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"maps"
 	"net/http"
@@ -163,6 +164,47 @@ func TestServerInvalidHeader(t *testing.T) {
 	})
 }
 
+func TestServerInvalidStatus(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		gotpanic := make(chan bool)
+		ts := newTestServer(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			defer close(gotpanic)
+			defer func() {
+				if e := recover(); e != nil {
+					got := fmt.Sprintf("%T, %v", e, e)
+					want := "string, invalid WriteHeader code 0"
+					if got != want {
+						t.Errorf("unexpected panic value:\n got: %v\nwant: %v\n", got, want)
+					}
+					gotpanic <- true
+					// Set an explicit 503. This also tests that the
+					// WriteHeader call panics before it recorded that an
+					// explicit value was set.
+					w.WriteHeader(503)
+
+					// Verify that writing invalid status will not panic if a
+					// status is already set anyways.
+					w.WriteHeader(0)
+				}
+			}()
+			w.WriteHeader(0) // Invalid. Will panic.
+		}))
+		tc := ts.connect()
+		tc.greet()
+
+		reqStream := tc.newStream(streamTypeRequest)
+		reqStream.writeHeaders(requestHeader(nil))
+		if !<-gotpanic {
+			t.Error("expected panic in handler")
+		}
+		synctest.Wait()
+		reqStream.wantSomeHeaders(http.Header{
+			":status": {"503"},
+		})
+		reqStream.wantClosed("request is complete")
+	})
+}
+
 func TestServerBody(t *testing.T) {
 	synctest.Test(t, func(t *testing.T) {
 		ts := newTestServer(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {