From 013b5103527cb1646dc717cadf48ad34aceaa77e Mon Sep 17 00:00:00 2001
From: Matthew Holt <mholt@users.noreply.github.com>
Date: Wed, 28 Sep 2022 00:13:12 -0600
Subject: [PATCH] rewrite: Only trim prefix if matched

See #5073
---
 modules/caddyhttp/rewrite/rewrite.go      |  9 +++++++--
 modules/caddyhttp/rewrite/rewrite_test.go | 10 ++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/modules/caddyhttp/rewrite/rewrite.go b/modules/caddyhttp/rewrite/rewrite.go
index 95bc5046..31c97788 100644
--- a/modules/caddyhttp/rewrite/rewrite.go
+++ b/modules/caddyhttp/rewrite/rewrite.go
@@ -383,8 +383,13 @@ func trimPathPrefix(escapedPath, prefix string) string {
 		iPrefix++
 	}
 
-	// found matching prefix, trim it
-	return escapedPath[iPath:]
+	// if we iterated through the entire prefix, we found it, so trim it
+	if iPath >= len(prefix) {
+		return escapedPath[iPath:]
+	}
+
+	// otherwise we did not find the prefix
+	return escapedPath
 }
 
 func reverse(s string) string {
diff --git a/modules/caddyhttp/rewrite/rewrite_test.go b/modules/caddyhttp/rewrite/rewrite_test.go
index bc20c853..5875983b 100644
--- a/modules/caddyhttp/rewrite/rewrite_test.go
+++ b/modules/caddyhttp/rewrite/rewrite_test.go
@@ -225,6 +225,16 @@ func TestRewrite(t *testing.T) {
 			input:  newRequest(t, "GET", "/prefix/foo/bar"),
 			expect: newRequest(t, "GET", "/foo/bar"),
 		},
+		{
+			rule:   Rewrite{StripPathPrefix: "/prefix"},
+			input:  newRequest(t, "GET", "/prefix"),
+			expect: newRequest(t, "GET", ""),
+		},
+		{
+			rule:   Rewrite{StripPathPrefix: "/prefix"},
+			input:  newRequest(t, "GET", "/"),
+			expect: newRequest(t, "GET", "/"),
+		},
 		{
 			rule:   Rewrite{StripPathPrefix: "/prefix"},
 			input:  newRequest(t, "GET", "/prefix/foo%2Fbar"),