replacer: Implement `file.*` global replacements (#5463)

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>
Co-authored-by: Mohammed Al Sahaf <msaa1990@gmail.com>

4 files changed, 197 insertions, 67 deletions

diff --git a/caddytest/integration/testdata/foo.txt b/caddytest/integration/testdata/foo.txt
new file mode 100644
index 00000000..19102815
--- /dev/null
+++ b/caddytest/integration/testdata/foo.txt
@@ -0,0 +1 @@
+foo
\ No newline at end of file
diff --git a/modules/caddyhttp/templates/tplcontext.go b/modules/caddyhttp/templates/tplcontext.go
index 8ba64200..4c7c86e1 100644
--- a/modules/caddyhttp/templates/tplcontext.go
+++ b/modules/caddyhttp/templates/tplcontext.go
@@ -249,6 +249,12 @@ func (c *TemplateContext) executeTemplateInBuffer(tplName string, buf *bytes.Buf
 
 func (c TemplateContext) funcPlaceholder(name string) string {
 	repl := c.Req.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
+
+	// For safety, we don't want to allow the file placeholder in
+	// templates because it could be used to read arbitrary files
+	// if the template contents were not trusted.
+	repl = repl.WithoutFile()
+
 	value, _ := repl.GetString(name)
 	return value
 }
diff --git a/replacer.go b/replacer.go
index 2ad5b8bc..e5d2913e 100644
--- a/replacer.go
+++ b/replacer.go
@@ -16,6 +16,7 @@ package caddy
 
 import (
 	"fmt"
+	"io"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -24,6 +25,8 @@ import (
 	"strings"
 	"sync"
 	"time"
+
+	"go.uber.org/zap"
 )
 
 // NewReplacer returns a new Replacer.
@@ -32,9 +35,10 @@ func NewReplacer() *Replacer {
 		static:   make(map[string]any),
 		mapMutex: &sync.RWMutex{},
 	}
-	rep.providers = []ReplacerFunc{
-		globalDefaultReplacements,
-		rep.fromStatic,
+	rep.providers = []replacementProvider{
+		globalDefaultReplacementProvider{},
+		fileReplacementProvider{},
+		ReplacerFunc(rep.fromStatic),
 	}
 	return rep
 }
@@ -46,8 +50,8 @@ func NewEmptyReplacer() *Replacer {
 		static:   make(map[string]any),
 		mapMutex: &sync.RWMutex{},
 	}
-	rep.providers = []ReplacerFunc{
-		rep.fromStatic,
+	rep.providers = []replacementProvider{
+		ReplacerFunc(rep.fromStatic),
 	}
 	return rep
 }
@@ -56,10 +60,25 @@ func NewEmptyReplacer() *Replacer {
 // A default/empty Replacer is not valid;
 // use NewReplacer to make one.
 type Replacer struct {
-	providers []ReplacerFunc
+	providers []replacementProvider
+	static    map[string]any
+	mapMutex  *sync.RWMutex
+}
 
-	static   map[string]any
-	mapMutex *sync.RWMutex
+// WithoutFile returns a copy of the current Replacer
+// without support for the {file.*} placeholder, which
+// may be unsafe in some contexts.
+//
+// EXPERIMENTAL: Subject to change or removal.
+func (r *Replacer) WithoutFile() *Replacer {
+	rep := &Replacer{static: r.static}
+	for _, v := range r.providers {
+		if _, ok := v.(fileReplacementProvider); ok {
+			continue
+		}
+		rep.providers = append(rep.providers, v)
+	}
+	return rep
 }
 
 // Map adds mapFunc to the list of value providers.
@@ -79,7 +98,7 @@ func (r *Replacer) Set(variable string, value any) {
 // the value and whether the variable was known.
 func (r *Replacer) Get(variable string) (any, bool) {
 	for _, mapFunc := range r.providers {
-		if val, ok := mapFunc(variable); ok {
+		if val, ok := mapFunc.replace(variable); ok {
 			return val, true
 		}
 	}
@@ -298,14 +317,52 @@ func ToString(val any) string {
 	}
 }
 
-// ReplacerFunc is a function that returns a replacement
-// for the given key along with true if the function is able
-// to service that key (even if the value is blank). If the
-// function does not recognize the key, false should be
-// returned.
+// ReplacerFunc is a function that returns a replacement for the
+// given key along with true if the function is able to service
+// that key (even if the value is blank). If the function does
+// not recognize the key, false should be returned.
 type ReplacerFunc func(key string) (any, bool)
 
-func globalDefaultReplacements(key string) (any, bool) {
+func (f ReplacerFunc) replace(key string) (any, bool) {
+	return f(key)
+}
+
+// replacementProvider is a type that can provide replacements
+// for placeholders. Allows for type assertion to determine
+// which type of provider it is.
+type replacementProvider interface {
+	replace(key string) (any, bool)
+}
+
+// fileReplacementsProvider handles {file.*} replacements,
+// reading a file from disk and replacing with its contents.
+type fileReplacementProvider struct{}
+
+func (f fileReplacementProvider) replace(key string) (any, bool) {
+	if !strings.HasPrefix(key, filePrefix) {
+		return nil, false
+	}
+
+	filename := key[len(filePrefix):]
+	maxSize := 1024 * 1024
+	body, err := readFileIntoBuffer(filename, maxSize)
+	if err != nil {
+		wd, _ := os.Getwd()
+		Log().Error("placeholder: failed to read file",
+			zap.String("file", filename),
+			zap.String("working_dir", wd),
+			zap.Error(err))
+		return nil, true
+	}
+	return string(body), true
+}
+
+// globalDefaultReplacementsProvider handles replacements
+// that can be used in any context, such as system variables,
+// time, or environment variables.
+type globalDefaultReplacementProvider struct{}
+
+func (f globalDefaultReplacementProvider) replace(key string) (any, bool) {
 	// check environment variable
 	const envPrefix = "env."
 	if strings.HasPrefix(key, envPrefix) {
@@ -347,6 +404,24 @@ func globalDefaultReplacements(key string) (any, bool) {
 	return nil, false
 }
 
+// readFileIntoBuffer reads the file at filePath into a size limited buffer.
+func readFileIntoBuffer(filename string, size int) ([]byte, error) {
+	file, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	buffer := make([]byte, size)
+	n, err := file.Read(buffer)
+	if err != nil && err != io.EOF {
+		return nil, err
+	}
+
+	// slice the buffer to the actual size
+	return buffer[:n], nil
+}
+
 // ReplacementFunc is a function that is called when a
 // replacement is being performed. It receives the
 // variable (i.e. placeholder name) and the value that
@@ -363,3 +438,5 @@ var nowFunc = time.Now
 const ReplacerCtxKey CtxKey = "replacer"
 
 const phOpen, phClose, phEscape = '{', '}', '\\'
+
+const filePrefix = "file."
diff --git a/replacer_test.go b/replacer_test.go
index d18ec8ee..cf4d321b 100644
--- a/replacer_test.go
+++ b/replacer_test.go
@@ -240,9 +240,9 @@ func TestReplacerSet(t *testing.T) {
 func TestReplacerReplaceKnown(t *testing.T) {
 	rep := Replacer{
 		mapMutex: &sync.RWMutex{},
-		providers: []ReplacerFunc{
+		providers: []replacementProvider{
 			// split our possible vars to two functions (to test if both functions are called)
-			func(key string) (val any, ok bool) {
+			ReplacerFunc(func(key string) (val any, ok bool) {
 				switch key {
 				case "test1":
 					return "val1", true
@@ -255,8 +255,8 @@ func TestReplacerReplaceKnown(t *testing.T) {
 				default:
 					return "NOOO", false
 				}
-			},
-			func(key string) (val any, ok bool) {
+			}),
+			ReplacerFunc(func(key string) (val any, ok bool) {
 				switch key {
 				case "1":
 					return "test-123", true
@@ -267,7 +267,7 @@ func TestReplacerReplaceKnown(t *testing.T) {
 				default:
 					return "NOOO", false
 				}
-			},
+			}),
 		},
 	}
 
@@ -372,53 +372,99 @@ func TestReplacerMap(t *testing.T) {
 }
 
 func TestReplacerNew(t *testing.T) {
-	rep := NewReplacer()
-
-	if len(rep.providers) != 2 {
-		t.Errorf("Expected providers length '%v' got length '%v'", 2, len(rep.providers))
-	} else {
-		// test if default global replacements are added  as the first provider
-		hostname, _ := os.Hostname()
-		wd, _ := os.Getwd()
-		os.Setenv("CADDY_REPLACER_TEST", "envtest")
-		defer os.Setenv("CADDY_REPLACER_TEST", "")
-
-		for _, tc := range []struct {
-			variable string
-			value    string
-		}{
-			{
-				variable: "system.hostname",
-				value:    hostname,
-			},
-			{
-				variable: "system.slash",
-				value:    string(filepath.Separator),
-			},
-			{
-				variable: "system.os",
-				value:    runtime.GOOS,
-			},
-			{
-				variable: "system.arch",
-				value:    runtime.GOARCH,
-			},
-			{
-				variable: "system.wd",
-				value:    wd,
-			},
-			{
-				variable: "env.CADDY_REPLACER_TEST",
-				value:    "envtest",
-			},
-		} {
-			if val, ok := rep.providers[0](tc.variable); ok {
-				if val != tc.value {
-					t.Errorf("Expected value '%s' for key '%s' got '%s'", tc.value, tc.variable, val)
-				}
-			} else {
-				t.Errorf("Expected key '%s' to be recognized by first provider", tc.variable)
+	repl := NewReplacer()
+
+	if len(repl.providers) != 3 {
+		t.Errorf("Expected providers length '%v' got length '%v'", 3, len(repl.providers))
+	}
+
+	// test if default global replacements are added as the first provider
+	hostname, _ := os.Hostname()
+	wd, _ := os.Getwd()
+	os.Setenv("CADDY_REPLACER_TEST", "envtest")
+	defer os.Setenv("CADDY_REPLACER_TEST", "")
+
+	for _, tc := range []struct {
+		variable string
+		value    string
+	}{
+		{
+			variable: "system.hostname",
+			value:    hostname,
+		},
+		{
+			variable: "system.slash",
+			value:    string(filepath.Separator),
+		},
+		{
+			variable: "system.os",
+			value:    runtime.GOOS,
+		},
+		{
+			variable: "system.arch",
+			value:    runtime.GOARCH,
+		},
+		{
+			variable: "system.wd",
+			value:    wd,
+		},
+		{
+			variable: "env.CADDY_REPLACER_TEST",
+			value:    "envtest",
+		},
+	} {
+		if val, ok := repl.providers[0].replace(tc.variable); ok {
+			if val != tc.value {
+				t.Errorf("Expected value '%s' for key '%s' got '%s'", tc.value, tc.variable, val)
+			}
+		} else {
+			t.Errorf("Expected key '%s' to be recognized by first provider", tc.variable)
+		}
+	}
+
+	// test if file provider is added as the second provider
+	for _, tc := range []struct {
+		variable string
+		value    string
+	}{
+		{
+			variable: "file.caddytest/integration/testdata/foo.txt",
+			value:    "foo",
+		},
+	} {
+		if val, ok := repl.providers[1].replace(tc.variable); ok {
+			if val != tc.value {
+				t.Errorf("Expected value '%s' for key '%s' got '%s'", tc.value, tc.variable, val)
+			}
+		} else {
+			t.Errorf("Expected key '%s' to be recognized by second provider", tc.variable)
+		}
+	}
+}
+
+func TestReplacerNewWithoutFile(t *testing.T) {
+	repl := NewReplacer().WithoutFile()
+
+	for _, tc := range []struct {
+		variable string
+		value    string
+		notFound bool
+	}{
+		{
+			variable: "file.caddytest/integration/testdata/foo.txt",
+			notFound: true,
+		},
+		{
+			variable: "system.os",
+			value:    runtime.GOOS,
+		},
+	} {
+		if val, ok := repl.Get(tc.variable); ok && !tc.notFound {
+			if val != tc.value {
+				t.Errorf("Expected value '%s' for key '%s' got '%s'", tc.value, tc.variable, val)
 			}
+		} else if !tc.notFound {
+			t.Errorf("Expected key '%s' to be recognized", tc.variable)
 		}
 	}
 }
@@ -464,7 +510,7 @@ func BenchmarkReplacer(b *testing.B) {
 
 func testReplacer() Replacer {
 	return Replacer{
-		providers: make([]ReplacerFunc, 0),
+		providers: make([]replacementProvider, 0),
 		static:    make(map[string]any),
 		mapMutex:  &sync.RWMutex{},
 	}