diff --git a/backend/shared/infra/security/sanitize.go b/backend/shared/infra/security/sanitize.go
index 8813822..56785fa 100644
--- a/backend/shared/infra/security/sanitize.go
+++ b/backend/shared/infra/security/sanitize.go
@@ -8,6 +8,42 @@ import (
 	"unicode"
 )
 
+const (
+	// PlayerNameMaxLength defines the maximum length for player names.
+	PlayerNameMaxLength = 50
+	// AnswerMaxLength defines the maximum length for answer submissions.
+	AnswerMaxLength = 500
+	// QuestionTextMaxLength defines the maximum length for admin-authored question text.
+	QuestionTextMaxLength = 1000
+	// ThemeMaxLength defines the maximum length for theme names.
+	ThemeMaxLength = 100
+)
+
+var (
+	// Sanitization order is trim -> collapse spaces -> HTML escape -> clamp -> allowed pattern.
+	spaceRegex = regexp.MustCompile(`\s+`)
+
+	// Player names allow letters, numbers, spaces, '-', '_' and '.'.
+	playerNameAllowedPattern = regexp.MustCompile(`^[a-zA-Z0-9\s\-_.]+$`)
+	// Themes allow letters, numbers, spaces, '-' and '_'.
+	themeAllowedPattern = regexp.MustCompile(`^[a-zA-Z0-9\s\-_]+$`)
+
+	tagRegex    = regexp.MustCompile(`<[^>]*>`)
+	scriptRegex = regexp.MustCompile(`(?i)<script[^>]*>.*?</script>`)
+
+	dangerousPatterns = []string{
+		"javascript:",
+		"data:",
+		"vbscript:",
+		"<script",
+		"</script",
+		"onerror",
+		"onload",
+		"onclick",
+		"onmouseover",
+	}
+)
+
 // SanitizeOptions configures sanitization behavior.
 type SanitizeOptions struct {
 	TrimWhitespace       bool
@@ -38,7 +74,6 @@ func Sanitize(input string, opts SanitizeOptions) string {
 
 	// Remove multiple consecutive spaces
 	if opts.RemoveMultipleSpaces {
-		spaceRegex := regexp.MustCompile(`\s+`)
 		result = spaceRegex.ReplaceAllString(result, " ")
 	}
 
@@ -48,8 +83,8 @@ func Sanitize(input string, opts SanitizeOptions) string {
 	}
 
 	// Apply maximum length
-	if opts.MaxLength > 0 && len(result) > opts.MaxLength {
-		result = result[:opts.MaxLength]
+	if opts.MaxLength > 0 {
+		result = clampByRunes(result, opts.MaxLength)
 	}
 
 	// Validate against allowed pattern
@@ -66,8 +101,8 @@ func SanitizePlayerName(input string) string {
 		TrimWhitespace:       true,
 		RemoveMultipleSpaces: true,
 		HTMLEscape:           true,
-		MaxLength:            50,
-		AllowedPattern:       regexp.MustCompile(`^[a-zA-Z0-9\s\-_.]+$`),
+		MaxLength:            PlayerNameMaxLength,
+		AllowedPattern:       playerNameAllowedPattern,
 	}
 	return Sanitize(input, opts)
 }
@@ -78,7 +113,7 @@ func SanitizeAnswer(input string) string {
 		TrimWhitespace:       true,
 		RemoveMultipleSpaces: true,
 		HTMLEscape:           true,
-		MaxLength:            500,
+		MaxLength:            AnswerMaxLength,
 	}
 
 	result := Sanitize(input, opts)
@@ -95,13 +130,12 @@ func SanitizeQuestionText(input string) string {
 		TrimWhitespace:       true,
 		RemoveMultipleSpaces: true,
 		HTMLEscape:           true,
-		MaxLength:            1000,
+		MaxLength:            QuestionTextMaxLength,
 	}
 
 	result := Sanitize(input, opts)
 
 	// Remove potential script content
-	scriptRegex := regexp.MustCompile(`(?i)<script[^>]*>.*?</script>`)
 	result = scriptRegex.ReplaceAllString(result, "")
 
 	return result
@@ -113,8 +147,8 @@ func SanitizeTheme(input string) string {
 		TrimWhitespace:       true,
 		RemoveMultipleSpaces: true,
 		HTMLEscape:           true,
-		MaxLength:            100,
-		AllowedPattern:       regexp.MustCompile(`^[a-zA-Z0-9\s\-_]+$`),
+		MaxLength:            ThemeMaxLength,
+		AllowedPattern:       themeAllowedPattern,
 	}
 
 	result := Sanitize(input, opts)
@@ -137,24 +171,11 @@ func SanitizeTheme(input string) string {
 
 // RemoveHTMLTags removes all HTML tags from a string.
 func RemoveHTMLTags(input string) string {
-	tagRegex := regexp.MustCompile(`<[^>]*>`)
 	return tagRegex.ReplaceAllString(input, "")
 }
 
 // ContainsDangerousPatterns checks if input contains potentially dangerous patterns.
 func ContainsDangerousPatterns(input string) bool {
-	dangerousPatterns := []string{
-		"javascript:",
-		"data:",
-		"vbscript:",
-		"<script",
-		"</script",
-		"onerror",
-		"onload",
-		"onclick",
-		"onmouseover",
-	}
-
 	lowerInput := strings.ToLower(input)
 	for _, pattern := range dangerousPatterns {
 		if strings.Contains(lowerInput, pattern) {
@@ -170,3 +191,16 @@ func IsValidEmail(email string) bool {
 	emailRegex := regexp.MustCompile(`^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`)
 	return emailRegex.MatchString(email)
 }
+
+func clampByRunes(input string, max int) string {
+	if max <= 0 {
+		return input
+	}
+
+	runes := []rune(input)
+	if len(runes) <= max {
+		return input
+	}
+
+	return string(runes[:max])
+}
diff --git a/backend/shared/infra/security/sanitize_test.go b/backend/shared/infra/security/sanitize_test.go
index 7598076..27324fc 100644
--- a/backend/shared/infra/security/sanitize_test.go
+++ b/backend/shared/infra/security/sanitize_test.go
@@ -4,7 +4,9 @@ package security
 
 import (
 	"regexp"
+	"strings"
 	"testing"
+	"unicode/utf8"
 
 	"github.com/stretchr/testify/require"
 )
@@ -51,6 +53,29 @@ func TestSanitizeTheme(t *testing.T) {
 	require.Equal(t, "Science Fiction", SanitizeTheme("  science fiction "))
 }
 
+// TestSanitizeLengthBoundaries verifies exact max length is preserved and over-limit input is truncated.
+func TestSanitizeLengthBoundaries(t *testing.T) {
+	playerExact := strings.Repeat("a", PlayerNameMaxLength)
+	playerOver := strings.Repeat("a", PlayerNameMaxLength+1)
+	require.Equal(t, PlayerNameMaxLength, len(SanitizePlayerName(playerExact)))
+	require.Equal(t, PlayerNameMaxLength, len(SanitizePlayerName(playerOver)))
+
+	answerExact := strings.Repeat("x", AnswerMaxLength)
+	answerOver := strings.Repeat("x", AnswerMaxLength+1)
+	require.Equal(t, AnswerMaxLength, len(SanitizeAnswer(answerExact)))
+	require.Equal(t, AnswerMaxLength, len(SanitizeAnswer(answerOver)))
+
+	questionExact := strings.Repeat("q", QuestionTextMaxLength)
+	questionOver := strings.Repeat("q", QuestionTextMaxLength+1)
+	require.Equal(t, QuestionTextMaxLength, len(SanitizeQuestionText(questionExact)))
+	require.Equal(t, QuestionTextMaxLength, len(SanitizeQuestionText(questionOver)))
+
+	themeExact := strings.Repeat("z", ThemeMaxLength)
+	themeOver := strings.Repeat("z", ThemeMaxLength+1)
+	require.Equal(t, ThemeMaxLength, len(SanitizeTheme(themeExact)))
+	require.Equal(t, ThemeMaxLength, len(SanitizeTheme(themeOver)))
+}
+
 // TestRemoveHTMLTags verifies all HTML tags are stripped from input.
 func TestRemoveHTMLTags(t *testing.T) {
 	require.Equal(t, "Hi", RemoveHTMLTags("<b>Hi</b>"))
@@ -58,8 +83,29 @@ func TestRemoveHTMLTags(t *testing.T) {
 
 // TestContainsDangerousPatterns detects known dangerous substrings.
 func TestContainsDangerousPatterns(t *testing.T) {
-	require.True(t, ContainsDangerousPatterns("javascript:alert(1)"))
-	require.False(t, ContainsDangerousPatterns("hello"))
+	cases := []struct {
+		name     string
+		input    string
+		expected bool
+	}{
+		{name: "javascript", input: "javascript:alert(1)", expected: true},
+		{name: "mixed_case_javascript", input: "JaVaScRiPt:alert(1)", expected: true},
+		{name: "data", input: "data:text/html;base64,abc", expected: true},
+		{name: "vbscript", input: "vbscript:msgbox(1)", expected: true},
+		{name: "onerror", input: `<img src=x onerror="alert(1)">`, expected: true},
+		{name: "onload", input: `<body onload="x()">`, expected: true},
+		{name: "onclick", input: `<a onclick="x()">link</a>`, expected: true},
+		{name: "onmouseover", input: `<div onmouseover="x()">`, expected: true},
+		{name: "benign", input: "hello", expected: false},
+		{name: "benign_near_miss_1", input: "onboarding flow", expected: false},
+		{name: "benign_near_miss_2", input: "scripture reference", expected: false},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			require.Equal(t, tc.expected, ContainsDangerousPatterns(tc.input))
+		})
+	}
 }
 
 // TestIsValidEmail verifies basic email pattern validation.
@@ -67,3 +113,19 @@ func TestIsValidEmail(t *testing.T) {
 	require.True(t, IsValidEmail("a@b.com"))
 	require.False(t, IsValidEmail("bad@"))
 }
+
+// TestSanitizeRuneSafeClamping verifies rune-based truncation remains valid UTF-8 and deterministic.
+func TestSanitizeRuneSafeClamping(t *testing.T) {
+	input := strings.Repeat("é", 5)
+	opts := SanitizeOptions{
+		TrimWhitespace:       false,
+		RemoveMultipleSpaces: false,
+		HTMLEscape:           false,
+		MaxLength:            3,
+	}
+
+	result := Sanitize(input, opts)
+	require.True(t, utf8.ValidString(result))
+	require.Equal(t, 3, utf8.RuneCountInString(result))
+	require.Equal(t, "ééé", result)
+}