diff --git a/base_lexer.go b/base_lexer.go
index 0261c37..79c1546 100644
--- a/base_lexer.go
+++ b/base_lexer.go
@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"slices"
 	"strconv"
 )
@@ -107,30 +108,16 @@ func (l *baseLexer) readUint64Field() (i uint64, err error) { //nolint:cyclop
 			break
 		}
 
-		switch ch {
-		case '0':
-			i *= 10
-		case '1':
-			i = i*10 + 1
-		case '2':
-			i = i*10 + 2
-		case '3':
-			i = i*10 + 3
-		case '4':
-			i = i*10 + 4
-		case '5':
-			i = i*10 + 5
-		case '6':
-			i = i*10 + 6
-		case '7':
-			i = i*10 + 7
-		case '8':
-			i = i*10 + 8
-		case '9':
-			i = i*10 + 9
-		default:
+		if ch < '0' || ch > '9' {
 			return i, l.syntaxError()
 		}
+
+		digit := uint64(ch - '0')
+		// Check for overflow and return syntax error if it would occur.
+		if i > (math.MaxUint64-digit)/10 {
+			return i, l.syntaxError()
+		}
+		i = i*10 + digit
 	}
 
 	return i, nil
diff --git a/base_lexer_test.go b/base_lexer_test.go
index 0ac1daa..2aa9c34 100644
--- a/base_lexer_test.go
+++ b/base_lexer_test.go
@@ -31,6 +31,14 @@ func TestLexer(t *testing.T) {
 		assert.Error(t, err)
 	})
 
+	t.Run("uint64 overflow returns syntax error", func(t *testing.T) {
+		// 18446744073709551616 == 2^64, wraps to 0 without overflow check
+		l := &baseLexer{value: "18446744073709551616"}
+		_, err := l.readUint64Field()
+		var se syntaxError
+		assert.ErrorAs(t, err, &se, "overflow should return a syntaxError")
+	})
+
 	t.Run("many fields", func(t *testing.T) {
 		lex := &baseLexer{value: "aaa  123\nf1 f2\nlast"}
 
diff --git a/extmap.go b/extmap.go
index fdd1d81..50defef 100644
--- a/extmap.go
+++ b/extmap.go
@@ -52,12 +52,12 @@ func (e *ExtMap) Unmarshal(raw string) error {
 
 	valdir := strings.Split(fields[0], "/")
 	value, err := strconv.ParseInt(valdir[0], 10, 64)
-	if (value < 1) || (value > 246) {
-		return fmt.Errorf("%w: %v -- extmap key must be in the range 1-256", errSyntaxError, valdir[0])
-	}
 	if err != nil {
 		return fmt.Errorf("%w: %v", errSyntaxError, valdir[0])
 	}
+	if (value < 1) || (value > 256) {
+		return fmt.Errorf("%w: %v -- extmap key must be in the range 1-256", errSyntaxError, valdir[0])
+	}
 
 	var direction Direction
 	if len(valdir) == 2 {
diff --git a/extmap_test.go b/extmap_test.go
index 086b787..3c82d0a 100644
--- a/extmap_test.go
+++ b/extmap_test.go
@@ -94,3 +94,27 @@ func TestExtMap_Unmarshal_Error_URLParse(t *testing.T) {
 	err := em.Unmarshal("extmap:1 http://example.com/%zz")
 	assert.Error(t, err)
 }
+
+func TestExtMap_Unmarshal_ValueBounds(t *testing.T) {
+	t.Run("value 1 accepted (lower bound)", func(t *testing.T) {
+		var em ExtMap
+		assert.NoError(t, em.Unmarshal("extmap:1 http://example.com"))
+		assert.Equal(t, 1, em.Value)
+	})
+
+	t.Run("value 0 rejected (below lower bound)", func(t *testing.T) {
+		var em ExtMap
+		assert.ErrorIs(t, em.Unmarshal("extmap:0 http://example.com"), errSyntaxError)
+	})
+
+	t.Run("value 256 accepted (upper bound)", func(t *testing.T) {
+		var em ExtMap
+		assert.NoError(t, em.Unmarshal("extmap:256 http://example.com"))
+		assert.Equal(t, 256, em.Value)
+	})
+
+	t.Run("value 257 rejected (above upper bound)", func(t *testing.T) {
+		var em ExtMap
+		assert.ErrorIs(t, em.Unmarshal("extmap:257 http://example.com"), errSyntaxError)
+	})
+}
diff --git a/unmarshal.go b/unmarshal.go
index 2f011a8..9271b4a 100644
--- a/unmarshal.go
+++ b/unmarshal.go
@@ -6,6 +6,7 @@ package sdp
 import (
 	"errors"
 	"fmt"
+	"math"
 	"net/url"
 	"strconv"
 	"strings"
@@ -998,18 +999,23 @@ func parseTimeUnits(value string) (num int64, err error) {
 	if len(value) == 0 {
 		return 0, fmt.Errorf("%w `%v`", errSDPInvalidValue, value)
 	}
-	k := timeShorthand(value[len(value)-1])
-	if k > 0 {
+	factor := timeShorthand(value[len(value)-1])
+	if factor > 0 {
 		num, err = strconv.ParseInt(value[:len(value)-1], 10, 64)
 	} else {
-		k = 1
+		factor = 1
 		num, err = strconv.ParseInt(value, 10, 64)
 	}
 	if err != nil {
 		return 0, fmt.Errorf("%w `%v`", errSDPInvalidValue, value)
 	}
 
-	return num * k, nil
+	// Check for overflow and return error if it would occur.
+	if factor > 1 && (num > math.MaxInt64/factor || num < math.MinInt64/factor) {
+		return 0, fmt.Errorf("%w `%v`", errSDPInvalidValue, value)
+	}
+
+	return num * factor, nil
 }
 
 func timeShorthand(b byte) int64 {
diff --git a/unmarshal_test.go b/unmarshal_test.go
index 2e0ada7..d10fe8d 100644
--- a/unmarshal_test.go
+++ b/unmarshal_test.go
@@ -1909,3 +1909,18 @@ func TestTimeShorthand_MinutesAndSeconds(t *testing.T) {
 		assert.Equal(t, int64(1), timeShorthand('s'))
 	})
 }
+
+func TestParseTimeUnits_Overflow(t *testing.T) {
+	// math.MaxInt64 = 9223372036854775807; multiplied by 3600 (h) overflows int64
+	_, err := parseTimeUnits("9223372036854775807h")
+	assert.Error(t, err, "positive overflow with 'h' shorthand should return error")
+
+	// math.MinInt64 = -9223372036854775808; multiplied by 3600 (h) overflows int64
+	_, err = parseTimeUnits("-9223372036854775808h")
+	assert.Error(t, err, "negative overflow with 'h' shorthand should return error")
+
+	// Largest value that does NOT overflow with 'd' (86400): math.MaxInt64/86400 = 106751991167300
+	v, err := parseTimeUnits("106751991167300d")
+	assert.NoError(t, err)
+	assert.Equal(t, int64(106751991167300)*86400, v)
+}