Limit memory usage in templates

docs/publish.md

@@ -3227,7 +3227,10 @@ your templates there first ([example for Grafana alert](https://repeatit.io/#/sh
 !!! info
     A few Go template features are disabled for user-supplied templates: `{{define}}`, `{{template}}`,
     `{{block}}`, and `{{call}}` are not allowed. Templates also run with a short execution time limit --
-    a template that loops too long is stopped and rejected with an HTTP 400 error.
+    a template that loops too long is stopped and rejected with an HTTP 400 error. Templates are
+    limited to 32 KB in size, `printf` widths and precisions must be below 1000 (`%999d` is
+    allowed, `%1000d` is not), including the `%*d` form that takes the width from an argument, and
+    `indent`/`nindent` are limited to 100 spaces.
 
 ### Template functions
 ntfy supports a subset of the **[Sprig template functions](publish/template-functions.md)** (originally copied from [Sprig](https://github.com/Masterminds/sprig),

docs/releases.md

@@ -2056,6 +2056,7 @@ and the [ntfy Android app](https://github.com/binwiederhier/ntfy-android/release
 **Security:**
 
 * Exclude secrets (Stripe/Twilio/web push keys, SMTP password, provisioned users and tokens) from the config hash served to the web app
+* Limit message templates (`Template: yes`) to 32 KB, limit `printf` widths and precisions to below 1000, and limit `indent`/`nindent` to 100 spaces, preventing excessive memory use from a single small template
 
 **Features:**
 

server/errors.go

@@ -148,6 +148,7 @@ var (
 	errHTTPBadRequestEmailAddressNotVerified         = &errHTTP{40052, http.StatusBadRequest, "invalid request: email address not verified", "https://ntfy.sh/docs/publish/#e-mail-notifications", nil}
 	errHTTPBadRequestAnonymousEmailNotAllowed        = &errHTTP{40053, http.StatusBadRequest, "invalid request: anonymous email sending is not allowed", "https://ntfy.sh/docs/publish/#e-mail-notifications", nil}
 	errHTTPBadRequestResetLinkInvalid                = &errHTTP{40054, http.StatusBadRequest, "invalid request: password reset link invalid or expired", "", nil}
+	errHTTPBadRequestTemplateTooLarge                = &errHTTP{40056, http.StatusBadRequest, "invalid request: template too large", "https://ntfy.sh/docs/publish/#message-templating", nil}
 	errHTTPNotFound                                  = &errHTTP{40401, http.StatusNotFound, "page not found", "", nil}
 	errHTTPUnauthorized                              = &errHTTP{40101, http.StatusUnauthorized, "unauthorized", "https://ntfy.sh/docs/publish/#authentication", nil}
 	errHTTPForbidden                                 = &errHTTP{40301, http.StatusForbidden, "forbidden", "https://ntfy.sh/docs/publish/#authentication", nil}

server/server.go

@@ -150,17 +150,6 @@ var (
 	//go:embed docs
 	docsStaticFs     embed.FS
 	docsStaticCached = &util.CachingEmbedFS{ModTime: time.Now(), FS: docsStaticFs}
-
-	//go:embed templates
-	templatesFs  embed.FS // Contains template config files (e.g. grafana.yml, github.yml, ...)
-	templatesDir = "templates"
-
-	templateNameRegex = regexp.MustCompile(`^[-_A-Za-z0-9]+$`)
-
-	// templateMaxExecutionTime is the wall-clock deadline for a single template render, a DoS guard
-	// (GHSA-rhwf-xgc9-m9fp). It is a var (not a const) solely so tests can raise it; it is never
-	// mutated in production.
-	templateMaxExecutionTime = 100 * time.Millisecond
 )
 
 const (
@@ -174,8 +163,6 @@ const (
 	unifiedPushTopicPrefix   = "up"                      // Temporarily, we rate limit all "up*" topics based on the subscriber
 	unifiedPushTopicLength   = 14                        // Length of UnifiedPush topics, including the "up" part
 	messagesHistoryMax       = 10                        // Number of message count values to keep in memory
-	templateMaxOutputBytes   = 1024 * 1024               // Maximum number of bytes a template can output, used to prevent DoS attacks
-	templateFileExtension    = ".yml"                    // Template files must end with this extension
 )
 
 // WebSocket constants

server/server_template.go

@@ -3,12 +3,16 @@ package server
 import (
 	"bytes"
 	"context"
+	"embed"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"os"
 	"path/filepath"
+	"regexp"
 	"strings"
 	"text/template/parse"
+	"time"
 
 	"gopkg.in/yaml.v2"
 	"heckel.io/ntfy/v2/model"
@@ -17,6 +21,32 @@ import (
 	"heckel.io/ntfy/v2/util/sprig"
 )
 
+var (
+	//go:embed templates
+	templatesFs  embed.FS // Contains template config files (e.g. grafana.yml, github.yml, ...)
+	templatesDir = "templates"
+
+	templateNameRegex = regexp.MustCompile(`^[-_A-Za-z0-9]+$`)
+
+	// templatePrintfLargeSizeRegex matches a printf directive whose width or precision is a star
+	// (taken from an argument) or has four or more digits, i.e. is at least 1000. It deliberately
+	// scans the flag/width/precision characters after a % without requiring a well-formed
+	// directive: fmt pads even malformed ones (e.g. "%000 9999999#" emits 10 MB), so anything
+	// unrecognized must still be caught.
+	templatePrintfLargeSizeRegex = regexp.MustCompile(`%[-+# 0-9.*\[\]]*(\*|[0-9]{4})`)
+
+	// templateMaxExecutionTime is the wall-clock deadline for a single template render, a DoS guard
+	// (GHSA-rhwf-xgc9-m9fp). It is a var (not a const) solely so tests can raise it; it is never
+	// mutated in production.
+	templateMaxExecutionTime = 100 * time.Millisecond
+)
+
+const (
+	templateMaxOutputBytes   = 1024 * 1024 // Maximum number of bytes a template can output, used to prevent DoS attacks
+	templateMaxTemplateBytes = 32 * 1024   // Maximum size of a template (inline or from a template file), used to prevent DoS attacks
+	templateFileExtension    = ".yml"      // Template files must end with this extension
+)
+
 func (s *Server) handleBodyAsTemplatedTextMessage(ctx context.Context, m *model.Message, template templateMode, body *util.PeekedReadCloser, priorityStr string) error {
 	body, err := util.Peek(body, max(s.config.MessageSizeLimit, jsonBodyBytesLimit))
 	if err != nil {
@@ -106,11 +136,14 @@ func (s *Server) renderTemplateFromParams(ctx context.Context, m *model.Message,
 
 // renderTemplate renders a template with the given JSON source data.
 func (s *Server) renderTemplate(ctx context.Context, name, tpl, source string) (string, error) {
+	if len(tpl) > templateMaxTemplateBytes {
+		return "", errHTTPBadRequestTemplateTooLarge
+	}
 	var data any
 	if err := json.Unmarshal([]byte(source), &data); err != nil {
 		return "", errHTTPBadRequestTemplateMessageNotJSON
 	}
-	t, err := gotext.New("").Funcs(sprig.TxtFuncMap()).Parse(tpl)
+	t, err := gotext.New("").Funcs(sprig.TxtFuncMap()).Funcs(gotext.FuncMap{"printf": templatePrintf}).Parse(tpl)
 	if err != nil {
 		return "", errHTTPBadRequestTemplateInvalid.Wrap("%s", err.Error())
 	}
@@ -168,6 +201,8 @@ func treeContainsDisallowedNode(node parse.Node) bool {
 		return treeContainsDisallowedNode(n.Pipe) || treeContainsDisallowedNode(n.List) || treeContainsDisallowedNode(n.ElseList)
 	case *parse.TemplateNode: // {{template}} or {{block}} invocation
 		return true
+	case *parse.ChainNode: // A term followed by field accesses, e.g. (call .x).y
+		return treeContainsDisallowedNode(n.Node)
 	case *parse.PipeNode:
 		if n == nil {
 			return false
@@ -188,3 +223,18 @@ func treeContainsDisallowedNode(node parse.Node) bool {
 	}
 	return false
 }
+
+// templatePrintf is the template builtin printf, guarded against memory amplification: fmt
+// allows widths and precisions up to 1e6 per verb, so a small template like
+// {{printf "%999999d%999999d..." ...}} can allocate gigabytes inside a single fmt call -- and the
+// executor's cancellation context is only checked between template nodes, never inside one.
+// Widths and precisions of 1000 or more are therefore rejected, as is the star (*) form, which
+// takes the width from an argument. Combined with the template size limit, this bounds a single
+// render to a few MB. Registered via Funcs, which takes precedence over the builtin, and checked
+// at call time so a format string assembled during execution is covered too.
+func templatePrintf(format string, args ...any) (string, error) {
+	if templatePrintfLargeSizeRegex.MatchString(strings.ReplaceAll(format, "%%", "")) { // Strip escaped percent signs, they take no width
+		return "", errors.New("printf width or precision too large")
+	}
+	return fmt.Sprintf(format, args...), nil
+}

server/server_template_test.go

@@ -0,0 +1,131 @@
+package server
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestServer_MessageTemplate_TooLarge(t *testing.T) {
+	forEachBackend(t, func(t *testing.T, databaseURL string) {
+		t.Parallel()
+		s := newTestServer(t, newTestConfig(t, databaseURL))
+		response := request(t, s, "PUT", "/mytopic", `{"foo":"bar"}`, map[string]string{
+			"X-Message":  "{{.foo}}" + strings.Repeat("x", 33*1024),
+			"X-Template": "1",
+		})
+		require.Equal(t, 400, response.Code)
+		require.Equal(t, 40056, toHTTPError(t, response.Body.String()).Code)
+	})
+}
+
+func TestServer_MessageTemplate_PrintfWidthTooLarge(t *testing.T) {
+	forEachBackend(t, func(t *testing.T, databaseURL string) {
+		t.Parallel()
+		s := newTestServer(t, newTestConfig(t, databaseURL))
+		// A handful of 1MB-wide verbs would allocate several MB inside a single fmt call, where
+		// the executor's context is never checked; the printf guard must reject the call before
+		// fmt runs, not after the limit writer sees the output
+		response := request(t, s, "PUT", "/mytopic", `{"n":1}`, map[string]string{
+			"X-Message":  `{{printf "%1000000d%1000000d%1000000d" .n .n .n}}`,
+			"X-Template": "1",
+		})
+		require.Equal(t, 400, response.Code)
+		require.Equal(t, 40045, toHTTPError(t, response.Body.String()).Code)
+		require.Contains(t, response.Body.String(), "printf width or precision too large")
+	})
+}
+
+func TestServer_MessageTemplate_PrintfWidthTooLarge_DynamicFormat(t *testing.T) {
+	forEachBackend(t, func(t *testing.T, databaseURL string) {
+		t.Parallel()
+		s := newTestServer(t, newTestConfig(t, databaseURL))
+		// The format string is assembled at execution time, so the guard must inspect the actual
+		// argument, not the template source
+		response := request(t, s, "PUT", "/mytopic", `{"n":1}`, map[string]string{
+			"X-Message":  `{{$f := print "%" "999999" "d" "%" "999999" "d"}}{{printf $f .n .n}}`,
+			"X-Template": "1",
+		})
+		require.Equal(t, 400, response.Code)
+		require.Contains(t, response.Body.String(), "printf width or precision too large")
+	})
+}
+
+func TestServer_MessageTemplate_PrintfStarWidthTooLarge(t *testing.T) {
+	forEachBackend(t, func(t *testing.T, databaseURL string) {
+		t.Parallel()
+		s := newTestServer(t, newTestConfig(t, databaseURL))
+		// Star widths take the width from an argument; sprig's math functions (int64 results)
+		// make large integer arguments reachable from a template
+		response := request(t, s, "PUT", "/mytopic", `{"n":1}`, map[string]string{
+			"X-Message":  `{{printf "%*d" (mul 1000 2000) 1}}`,
+			"X-Template": "1",
+		})
+		require.Equal(t, 400, response.Code)
+		require.Contains(t, response.Body.String(), "printf width or precision too large")
+	})
+}
+
+func TestServer_MessageTemplate_PrintfSmallWidthStillWorks(t *testing.T) {
+	forEachBackend(t, func(t *testing.T, databaseURL string) {
+		t.Parallel()
+		s := newTestServer(t, newTestConfig(t, databaseURL))
+		response := request(t, s, "PUT", "/mytopic", `{"n":7}`, map[string]string{
+			"X-Message":  `{{printf "%05d" 7}}`,
+			"X-Template": "1",
+		})
+		require.Equal(t, 200, response.Code)
+		require.Equal(t, "00007", toMessage(t, response.Body.String()).Message)
+	})
+}
+
+func Test_templatePrintf(t *testing.T) {
+	tests := []struct {
+		format string
+		args   []any
+		want   string // Empty means the call must be rejected
+	}{
+		{"%d", []any{5}, "5"},
+		{"%05d", []any{5}, "00005"},
+		{"%-8.3f|", []any{1.5}, "1.500   |"},
+		{"%1000d", []any{1}, ""}, // Rejected: four digits
+		{"%.1000s", []any{"x"}, ""},
+		{"%*d", []any{500, 1}, ""},                           // Rejected: star width
+		{"%.*s", []any{400, "x"}, ""},                        // Rejected: star precision
+		{"%[1]1000000d", []any{1}, ""},                       // Rejected: explicit arg index does not hide the width
+		{"%[2]*[1]d", []any{6, 12}, ""},                      // Rejected: star width behind an arg index
+		{"100%% of 2024 values", nil, "100% of 2024 values"}, // Literal digits are not a width
+	}
+	for _, test := range tests {
+		out, err := templatePrintf(test.format, test.args...)
+		if test.want == "" {
+			require.Error(t, err, "format %q must be rejected", test.format)
+			require.Contains(t, err.Error(), "too large")
+		} else {
+			require.Nil(t, err, "format %q", test.format)
+			require.Equal(t, test.want, out)
+		}
+	}
+
+	// The largest allowed width still produces bounded output
+	out, err := templatePrintf("%999d", 1)
+	require.Nil(t, err)
+	require.Len(t, out, 999)
+}
+
+func TestServer_MessageTemplate_DisallowedCallInChain(t *testing.T) {
+	forEachBackend(t, func(t *testing.T, databaseURL string) {
+		t.Parallel()
+		s := newTestServer(t, newTestConfig(t, databaseURL))
+		// {{call}} behind a field access parses into a ChainNode. JSON data cannot produce a
+		// function value, so this cannot be exploited today, but the ban must catch every
+		// syntactic form rather than relying on the call failing at runtime.
+		response := request(t, s, "PUT", "/mytopic", `{"fn":1}`, map[string]string{
+			"X-Message":  `{{(call .fn).x}}`,
+			"X-Template": "1",
+		})
+		require.Equal(t, 400, response.Code)
+		require.Equal(t, 40044, toHTTPError(t, response.Body.String()).Code)
+	})
+}

util/sprig/functions.go

@@ -12,6 +12,7 @@ const (
 	loopExecutionLimit = 10_000  // Limit the number of loop executions to prevent execution from taking too long
 	stringLengthLimit  = 100_000 // Limit the length of strings to prevent memory issues
 	sliceSizeLimit     = 10_000  // Limit the size of slices to prevent memory issues
+	indentSpacesLimit  = 100     // Limit indentation width to prevent memory issues; indent allocates spaces*lines bytes
 )
 
 // TxtFuncMap produces the function map.

util/sprig/strings.go

@@ -116,6 +116,7 @@ func cat(v ...any) string {
 }
 
 // indent adds a specified number of spaces at the beginning of each line in a string.
+// It has a safety limit to prevent excessive memory usage.
 //
 // Parameters:
 //   - spaces: The number of spaces to add
@@ -123,7 +124,13 @@ func cat(v ...any) string {
 //
 // Returns:
 //   - string: The indented string
+//
+// Panics:
+//   - If spaces exceeds indentSpacesLimit
 func indent(spaces int, v string) string {
+	if spaces > indentSpacesLimit {
+		panic(fmt.Sprintf("indent %d exceeds limit of %d", spaces, indentSpacesLimit))
+	}
 	pad := strings.Repeat(" ", spaces)
 	return pad + strings.Replace(v, "\n", "\n"+pad, -1)
 }

util/sprig/strings_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/base32"
 	"encoding/base64"
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -214,6 +215,25 @@ func TestNindent(t *testing.T) {
 	}
 }
 
+func TestIndentLimit(t *testing.T) {
+	// Indentation beyond a sane width is an amplification attempt: indent allocates
+	// spaces * lines bytes in a single uninterruptible call
+	if err := runt(`{{indent 100 "a"}}`, strings.Repeat(" ", 100)+"a"); err != nil {
+		t.Error(err)
+	}
+	for _, tpl := range []string{
+		`{{indent 101 "a"}}`,
+		`{{nindent 101 "a"}}`,
+		`{{indent 1000000000 "a"}}`,
+	} {
+		if _, err := runRaw(tpl, nil); err == nil {
+			t.Errorf("expected %s to be rejected", tpl)
+		} else if !strings.Contains(err.Error(), "exceeds limit") {
+			t.Errorf("expected limit error for %s, got: %v", tpl, err)
+		}
+	}
+}
+
 func TestReplace(t *testing.T) {
 	tpl := `{{"I Am Henry VIII" | replace " " "-"}}`
 	if err := runt(tpl, "I-Am-Henry-VIII"); err != nil {