fix: removes multiline from default regex modifiers

README.md

@@ -104,6 +104,7 @@ dictionaries to reduce memory consumption in deployments that launch several cor
 instances. For more context check [this issue](https://github.com/corazawaf/coraza-caddy/issues/76)
 * `no_fs_access` - indicates that the target environment has no access to FS in order to not leverage OS' filesystem related functionality e.g. file body buffers.
 * `coraza.rule.case_sensitive_args_keys` - enables case-sensitive matching for ARGS keys, aligning Coraza behavior with RFC 3986 specification. It will be enabled by default in the next major version.
+* `coraza.rule.no_regex_multiline` - disables enabling by default regexes multiline modifiers in `@rx` operator. It aligns with CRS expected behavior, reduces false positives and might improve performances. No multiline regexes by default will be enabled in the next major version. For more context check [this PR](https://github.com/corazawaf/coraza/pull/876)
 
 ## E2E Testing
 

internal/operators/multilineregex.go

@@ -0,0 +1,8 @@
+// Copyright 2024 Juan Pablo Tosso and the OWASP Coraza contributors
+// SPDX-License-Identifier: Apache-2.0
+
+//go:build coraza.rule.no_regex_multiline
+
+package operators
+
+var shouldNotUseMultilineRegexesOperatorByDefault = true

internal/operators/multilineregex_default.go

@@ -0,0 +1,8 @@
+// Copyright 2024 Juan Pablo Tosso and the OWASP Coraza contributors
+// SPDX-License-Identifier: Apache-2.0
+
+//go:build !coraza.rule.no_regex_multiline
+
+package operators
+
+var shouldNotUseMultilineRegexesOperatorByDefault = false

internal/operators/rx.go

@@ -24,10 +24,19 @@
 var _ plugintypes.Operator = (*rx)(nil)
 
 func newRX(options plugintypes.OperatorOptions) (plugintypes.Operator, error) {
-	// (?sm) enables multiline and dotall mode, required by some CRS rules and matching ModSec behavior, see
-	// - https://stackoverflow.com/a/27680233
-	// - https://groups.google.com/g/golang-nuts/c/jiVdamGFU9E
-	data := fmt.Sprintf("(?sm)%s", options.Arguments)
+	var data string
+	if shouldNotUseMultilineRegexesOperatorByDefault {
+		// (?s) enables dotall mode, required by some CRS rules and matching ModSec behavior, see
+		// - https://github.com/google/re2/wiki/Syntax
+		// - Flag usage: https://groups.google.com/g/golang-nuts/c/jiVdamGFU9E
+		data = fmt.Sprintf("(?s)%s", options.Arguments)
+	} else {
+		// TODO: deprecate multiline modifier set by default in Coraza v4
+		// CRS rules will explicitly set the multiline modifier when needed
+		// Having it enabled by default can lead to false positives and less performance
+		// See https://github.com/corazawaf/coraza/pull/876
+		data = fmt.Sprintf("(?sm)%s", options.Arguments)
+	}
 
 	if matchesArbitraryBytes(data) {
 		// Use binary regex matcher if expression matches non-utf8 bytes. The binary matcher does

internal/operators/rx_no_multiline_test.go

@@ -0,0 +1,119 @@
+// Copyright 2022 Juan Pablo Tosso and the OWASP Coraza contributors
+// SPDX-License-Identifier: Apache-2.0
+
+//go:build coraza.rule.no_regex_multiline
+
+package operators
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/corazawaf/coraza/v3/experimental/plugins/plugintypes"
+	"github.com/corazawaf/coraza/v3/internal/corazawaf"
+)
+
+func TestRx(t *testing.T) {
+	tests := []struct {
+		pattern string
+		input   string
+		want    bool
+	}{
+		{
+			pattern: "som(.*)ta",
+			input:   "somedata",
+			want:    true,
+		},
+		{
+			pattern: "som(.*)ta",
+			input:   "notdata",
+			want:    false,
+		},
+		{
+			pattern: "ハロー",
+			input:   "ハローワールド",
+			want:    true,
+		},
+		{
+			pattern: "ハロー",
+			input:   "グッバイワールド",
+			want:    false,
+		},
+		{
+			pattern: `\xac\xed\x00\x05`,
+			input:   "\xac\xed\x00\x05t\x00\x04test",
+			want:    true,
+		},
+		{
+			pattern: `\xac\xed\x00\x05`,
+			input:   "\xac\xed\x00t\x00\x04test",
+			want:    false,
+		},
+		{
+			// Requires dotall
+			pattern: `hello.*world`,
+			input:   "hello\nworld",
+			want:    true,
+		},
+		{
+			// Requires multiline disabled by default
+			pattern: `^hello.*world`,
+			input:   "test\nhello\nworld",
+			want:    false,
+		},
+		{
+			// Makes sure multiline can be enabled by the user
+			pattern: `(?m)^hello.*world`,
+			input:   "test\nhello\nworld",
+			want:    true,
+		},
+		{
+			// Makes sure, (?s) passed by the user does not
+			// break the regex.
+			pattern: `(?s)hello.*world`,
+			input:   "hello\nworld",
+			want:    true,
+		},
+		{
+			// Make sure user flags are also applied
+			pattern: `(?i)hello.*world`,
+			input:   "testHELLO\nworld",
+			want:    true,
+		},
+		{
+			// The so called DOLLAR_ENDONLY modifier in PCRE2 is meant to tweak the meaning of dollar '$'
+			// so that it matches only at the very end of the string (see: https://www.pcre.org/current/doc/html/pcre2pattern.html#SEC6)
+			// It seems that re2 already behaves like that by default.
+			pattern: `123$`,
+			input:   "123\n",
+			want:    false,
+		},
+		{
+			// Dollar endonly match
+			pattern: `123$`,
+			input:   "test123",
+			want:    true,
+		},
+	}
+
+	for _, tc := range tests {
+		tt := tc
+		t.Run(fmt.Sprintf("%s/%s", tt.pattern, tt.input), func(t *testing.T) {
+
+			opts := plugintypes.OperatorOptions{
+				Arguments: tt.pattern,
+			}
+			rx, err := newRX(opts)
+			if err != nil {
+				t.Error(err)
+			}
+			waf := corazawaf.NewWAF()
+			tx := waf.NewTransaction()
+			tx.Capture = true
+			res := rx.Evaluate(tx, tt.input)
+			if res != tt.want {
+				t.Errorf("want %v, got %v", tt.want, res)
+			}
+		})
+	}
+}

internal/operators/rx_test.go

@@ -1,6 +1,8 @@
 // Copyright 2022 Juan Pablo Tosso and the OWASP Coraza contributors
 // SPDX-License-Identifier: Apache-2.0
 
+//go:build !coraza.rule.no_regex_multiline
+
 package operators
 
 import (

magefile.go

@@ -130,6 +130,15 @@ func Test() error {
 		return err
 	}
 
+	// Execute FTW tests with coraza.rule.no_regex_multiline as well
+	if err := sh.RunV("go", "test", "-tags=coraza.rule.no_regex_multiline", "./testing/coreruleset"); err != nil {
+		return err
+	}
+
+	if err := sh.RunV("go", "test", "-tags=coraza.rule.no_regex_multiline", "-run=^TestRx", "./..."); err != nil {
+		return err
+	}
+
 	if err := sh.RunV("go", "test", "-tags=coraza.rule.case_sensitive_args_keys", "-run=^TestCaseSensitive", "./..."); err != nil {
 		return err
 	}
@@ -174,7 +183,7 @@ func Coverage() error {
 	if err := sh.RunV("go", "test", tagsCmd, "-coverprofile=build/coverage-ftw.txt", "-covermode=atomic", "-coverpkg=./...", "./testing/coreruleset"); err != nil {
 		return err
 	}
-	// we run tinygo tag only if memoize_builders is is not enabled
+	// we run tinygo tag only if memoize_builders is not enabled
 	if !strings.Contains(tags, "memoize_builders") {
 		if tagsCmd != "" {
 			tagsCmd += ",tinygo"
@@ -271,6 +280,7 @@ func combinations(tags []string) []string {
 func TagsMatrix() error {
 	tags := []string{
 		"coraza.rule.case_sensitive_args_keys",
+		"coraza.rule.no_regex_multiline",
 		"memoize_builders",
 		"coraza.rule.multiphase_valuation",
 		"no_fs_access",