net/http: add AllowQuerySemicolons

Fixes #45973

Change-Id: I6cbe05f5d1d3c324900c74314b0ea0e12524d7f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/326309
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Katie Hockman <katie@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

2 files changed, 125 insertions, 4 deletions

diff --git a/src/net/http/serve_test.go b/src/net/http/serve_test.go
index a9714682c7..c2f8811469 100644
--- a/src/net/http/serve_test.go
+++ b/src/net/http/serve_test.go
@@ -6524,3 +6524,87 @@ func TestMuxRedirectRelative(t *testing.T) {
 		t.Errorf("Expected response code %d; got %d", want, got)
 	}
 }
+
+// TestQuerySemicolon tests the behavior of semicolons in queries. See Issue 25192.
+func TestQuerySemicolon(t *testing.T) {
+	t.Cleanup(func() { afterTest(t) })
+
+	tests := []struct {
+		query           string
+		xNoSemicolons   string
+		xWithSemicolons string
+		warning         bool
+	}{
+		{"?a=1;x=bad&x=good", "good", "bad", true},
+		{"?a=1;b=bad&x=good", "good", "good", true},
+		{"?a=1%3Bx=bad&x=good%3B", "good;", "good;", false},
+		{"?a=1;x=good;x=bad", "", "good", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.query+"/allow=false", func(t *testing.T) {
+			allowSemicolons := false
+			testQuerySemicolon(t, tt.query, tt.xNoSemicolons, allowSemicolons, tt.warning)
+		})
+		t.Run(tt.query+"/allow=true", func(t *testing.T) {
+			allowSemicolons, expectWarning := true, false
+			testQuerySemicolon(t, tt.query, tt.xWithSemicolons, allowSemicolons, expectWarning)
+		})
+	}
+}
+
+func testQuerySemicolon(t *testing.T, query string, wantX string, allowSemicolons, expectWarning bool) {
+	setParallel(t)
+
+	writeBackX := func(w ResponseWriter, r *Request) {
+		x := r.URL.Query().Get("x")
+		if expectWarning {
+			if err := r.ParseForm(); err == nil || !strings.Contains(err.Error(), "semicolon") {
+				t.Errorf("expected error mentioning semicolons from ParseForm, got %v", err)
+			}
+		} else {
+			if err := r.ParseForm(); err != nil {
+				t.Errorf("expected no error from ParseForm, got %v", err)
+			}
+		}
+		if got := r.FormValue("x"); x != got {
+			t.Errorf("got %q from FormValue, want %q", got, x)
+		}
+		fmt.Fprintf(w, "%s", x)
+	}
+
+	h := Handler(HandlerFunc(writeBackX))
+	if allowSemicolons {
+		h = AllowQuerySemicolons(h)
+	}
+
+	ts := httptest.NewUnstartedServer(h)
+	logBuf := &bytes.Buffer{}
+	ts.Config.ErrorLog = log.New(logBuf, "", 0)
+	ts.Start()
+	defer ts.Close()
+
+	req, _ := NewRequest("GET", ts.URL+query, nil)
+	res, err := ts.Client().Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	slurp, _ := io.ReadAll(res.Body)
+	res.Body.Close()
+	if got, want := res.StatusCode, 200; got != want {
+		t.Errorf("Status = %d; want = %d", got, want)
+	}
+	if got, want := string(slurp), wantX; got != want {
+		t.Errorf("Body = %q; want = %q", got, want)
+	}
+
+	if expectWarning {
+		if !strings.Contains(logBuf.String(), "semicolon") {
+			t.Errorf("got %q from ErrorLog, expected a mention of semicolons", logBuf.String())
+		}
+	} else {
+		if strings.Contains(logBuf.String(), "semicolon") {
+			t.Errorf("got %q from ErrorLog, expected no mention of semicolons", logBuf.String())
+		}
+	}
+}
diff --git a/src/net/http/server.go b/src/net/http/server.go
index 8a1847e67a..50fab4520d 100644
--- a/src/net/http/server.go
+++ b/src/net/http/server.go
@@ -2862,12 +2862,49 @@ func (sh serverHandler) ServeHTTP(rw ResponseWriter, req *Request) {
 	if req.RequestURI == "*" && req.Method == "OPTIONS" {
 		handler = globalOptionsHandler{}
 	}
-	handler.ServeHTTP(rw, req)
+
 	if req.URL != nil && strings.Contains(req.URL.RawQuery, ";") {
-		// TODO(filippo): update this not to log if the special
-		// semicolon handler was called.
-		sh.srv.logf("http: URL query contains semicolon, which is no longer a supported separator; parts of the query may be stripped when parsed; see golang.org/issue/25192")
+		var allowQuerySemicolonsInUse int32
+		req = req.WithContext(context.WithValue(req.Context(), silenceSemWarnContextKey, func() {
+			atomic.StoreInt32(&allowQuerySemicolonsInUse, 1)
+		}))
+		defer func() {
+			if atomic.LoadInt32(&allowQuerySemicolonsInUse) == 0 {
+				sh.srv.logf("http: URL query contains semicolon, which is no longer a supported separator; parts of the query may be stripped when parsed; see golang.org/issue/25192")
+			}
+		}()
 	}
+
+	handler.ServeHTTP(rw, req)
+}
+
+var silenceSemWarnContextKey = &contextKey{"silence-semicolons"}
+
+// AllowQuerySemicolons returns a handler that serves requests by converting any
+// unescaped semicolons in the URL query to ampersands, and invoking the handler h.
+//
+// This restores the pre-Go 1.17 behavior of splitting query parameters on both
+// semicolons and ampersands. (See golang.org/issue/25192). Note that this
+// behavior doesn't match that of many proxies, and the mismatch can lead to
+// security issues.
+//
+// AllowQuerySemicolons should be invoked before Request.ParseForm is called.
+func AllowQuerySemicolons(h Handler) Handler {
+	return HandlerFunc(func(w ResponseWriter, r *Request) {
+		if silenceSemicolonsWarning, ok := r.Context().Value(silenceSemWarnContextKey).(func()); ok {
+			silenceSemicolonsWarning()
+		}
+		if strings.Contains(r.URL.RawQuery, ";") {
+			r2 := new(Request)
+			*r2 = *r
+			r2.URL = new(url.URL)
+			*r2.URL = *r.URL
+			r2.URL.RawQuery = strings.ReplaceAll(r.URL.RawQuery, ";", "&")
+			h.ServeHTTP(w, r2)
+		} else {
+			h.ServeHTTP(w, r)
+		}
+	})
 }
 
 // ListenAndServe listens on the TCP network address srv.Addr and then