diff --git a/bench_test.go b/bench_test.go
index 6859382..688d2b2 100644
--- a/bench_test.go
+++ b/bench_test.go
@@ -2,6 +2,7 @@ package cors
 
 import (
 	"net/http"
+	"strings"
 	"testing"
 )
 
@@ -87,7 +88,22 @@ func BenchmarkPreflightHeader(b *testing.B) {
 	req, _ := http.NewRequest(http.MethodOptions, dummyEndpoint, nil)
 	req.Header.Add(headerOrigin, dummyOrigin)
 	req.Header.Add(headerACRM, http.MethodGet)
-	req.Header.Add(headerACRH, "Accept")
+	req.Header.Add(headerACRH, "accept")
+	handler := Default().Handler(testHandler)
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		handler.ServeHTTP(resps[i], req)
+	}
+}
+
+func BenchmarkPreflightAdversarialACRH(b *testing.B) {
+	resps := makeFakeResponses(b.N)
+	req, _ := http.NewRequest(http.MethodOptions, dummyEndpoint, nil)
+	req.Header.Add(headerOrigin, dummyOrigin)
+	req.Header.Add(headerACRM, http.MethodGet)
+	req.Header.Add(headerACRH, strings.Repeat(",", 1024))
 	handler := Default().Handler(testHandler)
 
 	b.ReportAllocs()
diff --git a/cors.go b/cors.go
index 08aff84..da80d34 100644
--- a/cors.go
+++ b/cors.go
@@ -26,6 +26,8 @@ import (
 	"os"
 	"strconv"
 	"strings"
+
+	"github.com/rs/cors/internal"
 )
 
 var headerVaryOrigin = []string{"Origin"}
@@ -111,7 +113,11 @@ type Cors struct {
 	// Optional origin validator function
 	allowOriginFunc func(r *http.Request, origin string) (bool, []string)
 	// Normalized list of allowed headers
-	allowedHeaders []string
+	// Note: the Fetch standard guarantees that CORS-unsafe request-header names
+	// (i.e. the values listed in the Access-Control-Request-Headers header)
+	// are unique and sorted;
+	// see https://fetch.spec.whatwg.org/#cors-unsafe-request-header-names.
+	allowedHeaders internal.SortedSet
 	// Normalized list of allowed methods
 	allowedMethods []string
 	// Pre-computed normalized list of exposed headers
@@ -183,15 +189,19 @@ func New(options Options) *Cors {
 	}
 
 	// Allowed Headers
+	// Note: the Fetch standard guarantees that CORS-unsafe request-header names
+	// (i.e. the values listed in the Access-Control-Request-Headers header)
+	// are lowercase; see https://fetch.spec.whatwg.org/#cors-unsafe-request-header-names.
 	if len(options.AllowedHeaders) == 0 {
 		// Use sensible defaults
-		c.allowedHeaders = []string{"Accept", "Content-Type", "X-Requested-With"}
+		c.allowedHeaders = internal.NewSortedSet("accept", "content-type", "x-requested-with")
 	} else {
-		c.allowedHeaders = convert(options.AllowedHeaders, http.CanonicalHeaderKey)
+		normalized := convert(options.AllowedHeaders, strings.ToLower)
+		c.allowedHeaders = internal.NewSortedSet(normalized...)
 		for _, h := range options.AllowedHeaders {
 			if h == "*" {
 				c.allowedHeadersAll = true
-				c.allowedHeaders = nil
+				c.allowedHeaders = internal.SortedSet{}
 				break
 			}
 		}
@@ -351,10 +361,12 @@ func (c *Cors) handlePreflight(w http.ResponseWriter, r *http.Request) {
 		c.logf("  Preflight aborted: method '%s' not allowed", reqMethod)
 		return
 	}
-	reqHeadersRaw := r.Header["Access-Control-Request-Headers"]
-	reqHeaders, reqHeadersEdited := convertDidCopy(splitHeaderValues(reqHeadersRaw), http.CanonicalHeaderKey)
-	if !c.areHeadersAllowed(reqHeaders) {
-		c.logf("  Preflight aborted: headers '%v' not allowed", reqHeaders)
+	// Note: the Fetch standard guarantees that at most one
+	// Access-Control-Request-Headers header is present in the preflight request;
+	// see step 5.2 in https://fetch.spec.whatwg.org/#cors-preflight-fetch-0.
+	reqHeaders, found := first(r.Header, "Access-Control-Request-Headers")
+	if found && !c.allowedHeadersAll && !c.allowedHeaders.Subsumes(reqHeaders[0]) {
+		c.logf("  Preflight aborted: headers '%v' not allowed", reqHeaders[0])
 		return
 	}
 	if c.allowedOriginsAll {
@@ -365,14 +377,10 @@ func (c *Cors) handlePreflight(w http.ResponseWriter, r *http.Request) {
 	// Spec says: Since the list of methods can be unbounded, simply returning the method indicated
 	// by Access-Control-Request-Method (if supported) can be enough
 	headers["Access-Control-Allow-Methods"] = r.Header["Access-Control-Request-Method"]
-	if len(reqHeaders) > 0 {
+	if found && len(reqHeaders[0]) > 0 {
 		// Spec says: Since the list of headers can be unbounded, simply returning supported headers
 		// from Access-Control-Request-Headers can be enough
-		if reqHeadersEdited || len(reqHeaders) != len(reqHeadersRaw) {
-			headers.Set("Access-Control-Allow-Headers", strings.Join(reqHeaders, ", "))
-		} else {
-			headers["Access-Control-Allow-Headers"] = reqHeadersRaw
-		}
+		headers["Access-Control-Allow-Headers"] = reqHeaders
 	}
 	if c.allowCredentials {
 		headers["Access-Control-Allow-Credentials"] = headerTrue
@@ -492,24 +500,3 @@ func (c *Cors) isMethodAllowed(method string) bool {
 	}
 	return false
 }
-
-// areHeadersAllowed checks if a given list of headers are allowed to used within
-// a cross-domain request.
-func (c *Cors) areHeadersAllowed(requestedHeaders []string) bool {
-	if c.allowedHeadersAll || len(requestedHeaders) == 0 {
-		return true
-	}
-	for _, header := range requestedHeaders {
-		found := false
-		for _, h := range c.allowedHeaders {
-			if h == header {
-				found = true
-				break
-			}
-		}
-		if !found {
-			return false
-		}
-	}
-	return true
-}
diff --git a/cors_test.go b/cors_test.go
index c17dee2..a3c0aab 100644
--- a/cors_test.go
+++ b/cors_test.go
@@ -303,19 +303,19 @@ func TestSpec(t *testing.T) {
 			"AllowedHeaders",
 			Options{
 				AllowedOrigins: []string{"http://foobar.com"},
-				AllowedHeaders: []string{"X-Header-1", "x-header-2"},
+				AllowedHeaders: []string{"X-Header-1", "x-header-2", "X-HEADER-3"},
 			},
 			"OPTIONS",
 			map[string]string{
 				"Origin":                         "http://foobar.com",
 				"Access-Control-Request-Method":  "GET",
-				"Access-Control-Request-Headers": "X-Header-2, X-HEADER-1",
+				"Access-Control-Request-Headers": "x-header-1,x-header-2",
 			},
 			map[string]string{
 				"Vary":                         "Origin, Access-Control-Request-Method, Access-Control-Request-Headers",
 				"Access-Control-Allow-Origin":  "http://foobar.com",
 				"Access-Control-Allow-Methods": "GET",
-				"Access-Control-Allow-Headers": "X-Header-2, X-Header-1",
+				"Access-Control-Allow-Headers": "x-header-1,x-header-2",
 			},
 			true,
 		},
@@ -329,13 +329,13 @@ func TestSpec(t *testing.T) {
 			map[string]string{
 				"Origin":                         "http://foobar.com",
 				"Access-Control-Request-Method":  "GET",
-				"Access-Control-Request-Headers": "X-Requested-With",
+				"Access-Control-Request-Headers": "x-requested-with",
 			},
 			map[string]string{
 				"Vary":                         "Origin, Access-Control-Request-Method, Access-Control-Request-Headers",
 				"Access-Control-Allow-Origin":  "http://foobar.com",
 				"Access-Control-Allow-Methods": "GET",
-				"Access-Control-Allow-Headers": "X-Requested-With",
+				"Access-Control-Allow-Headers": "x-requested-with",
 			},
 			true,
 		},
@@ -349,13 +349,13 @@ func TestSpec(t *testing.T) {
 			map[string]string{
 				"Origin":                         "http://foobar.com",
 				"Access-Control-Request-Method":  "GET",
-				"Access-Control-Request-Headers": "X-Header-2, X-HEADER-1",
+				"Access-Control-Request-Headers": "x-header-1,x-header-2",
 			},
 			map[string]string{
 				"Vary":                         "Origin, Access-Control-Request-Method, Access-Control-Request-Headers",
 				"Access-Control-Allow-Origin":  "http://foobar.com",
 				"Access-Control-Allow-Methods": "GET",
-				"Access-Control-Allow-Headers": "X-Header-2, X-Header-1",
+				"Access-Control-Allow-Headers": "x-header-1,x-header-2",
 			},
 			true,
 		},
@@ -369,7 +369,7 @@ func TestSpec(t *testing.T) {
 			map[string]string{
 				"Origin":                         "http://foobar.com",
 				"Access-Control-Request-Method":  "GET",
-				"Access-Control-Request-Headers": "X-Header-3, X-Header-1",
+				"Access-Control-Request-Headers": "x-header-1,x-header-3",
 			},
 			map[string]string{
 				"Vary": "Origin, Access-Control-Request-Method, Access-Control-Request-Headers",
@@ -577,8 +577,8 @@ func TestDefault(t *testing.T) {
 	if !s.allowedOriginsAll {
 		t.Error("c.allowedOriginsAll should be true when Default")
 	}
-	if s.allowedHeaders == nil {
-		t.Error("c.allowedHeaders should be nil when Default")
+	if s.allowedHeaders.Size() == 0 {
+		t.Error("c.allowedHeaders should be empty when Default")
 	}
 	if s.allowedMethods == nil {
 		t.Error("c.allowedMethods should be nil when Default")
@@ -712,64 +712,6 @@ func TestOptionsSuccessStatusCodeOverride(t *testing.T) {
 	})
 }
 
-func TestCorsAreHeadersAllowed(t *testing.T) {
-	cases := []struct {
-		name             string
-		allowedHeaders   []string
-		requestedHeaders []string
-		want             bool
-	}{
-		{
-			name:             "nil allowedHeaders",
-			allowedHeaders:   nil,
-			requestedHeaders: []string{"X-PINGOTHER, Content-Type"},
-			want:             false,
-		},
-		{
-			name:             "star allowedHeaders",
-			allowedHeaders:   []string{"*"},
-			requestedHeaders: []string{"X-PINGOTHER, Content-Type"},
-			want:             true,
-		},
-		{
-			name:             "empty reqHeader",
-			allowedHeaders:   nil,
-			requestedHeaders: []string{},
-			want:             true,
-		},
-		{
-			name:             "match allowedHeaders",
-			allowedHeaders:   []string{"Content-Type", "X-PINGOTHER", "X-APP-KEY"},
-			requestedHeaders: []string{"X-PINGOTHER, Content-Type"},
-			want:             true,
-		},
-		{
-			name:             "not matched allowedHeaders",
-			allowedHeaders:   []string{"X-PINGOTHER"},
-			requestedHeaders: []string{"X-API-KEY, Content-Type"},
-			want:             false,
-		},
-		{
-			name:             "allowedHeaders should be a superset of requestedHeaders",
-			allowedHeaders:   []string{"X-PINGOTHER"},
-			requestedHeaders: []string{"X-PINGOTHER, Content-Type"},
-			want:             false,
-		},
-	}
-
-	for _, tt := range cases {
-		tt := tt
-
-		t.Run(tt.name, func(t *testing.T) {
-			c := New(Options{AllowedHeaders: tt.allowedHeaders})
-			have := c.areHeadersAllowed(convert(splitHeaderValues(tt.requestedHeaders), http.CanonicalHeaderKey))
-			if have != tt.want {
-				t.Errorf("Cors.areHeadersAllowed() have: %t want: %t", have, tt.want)
-			}
-		})
-	}
-}
-
 func TestAccessControlExposeHeadersPresence(t *testing.T) {
 	cases := []struct {
 		name    string
diff --git a/internal/sortedset.go b/internal/sortedset.go
new file mode 100644
index 0000000..513da20
--- /dev/null
+++ b/internal/sortedset.go
@@ -0,0 +1,113 @@
+// adapted from github.com/jub0bs/cors
+package internal
+
+import (
+	"sort"
+	"strings"
+)
+
+// A SortedSet represents a mathematical set of strings sorted in
+// lexicographical order.
+// Each element has a unique position ranging from 0 (inclusive)
+// to the set's cardinality (exclusive).
+// The zero value represents an empty set.
+type SortedSet struct {
+	m      map[string]int
+	maxLen int
+}
+
+// NewSortedSet returns a SortedSet that contains all of elems,
+// but no other elements.
+func NewSortedSet(elems ...string) SortedSet {
+	sort.Strings(elems)
+	m := make(map[string]int)
+	var maxLen int
+	i := 0
+	for _, s := range elems {
+		if _, exists := m[s]; exists {
+			continue
+		}
+		m[s] = i
+		i++
+		maxLen = max(maxLen, len(s))
+	}
+	return SortedSet{
+		m:      m,
+		maxLen: maxLen,
+	}
+}
+
+// Size returns the cardinality of set.
+func (set SortedSet) Size() int {
+	return len(set.m)
+}
+
+// String sorts joins the elements of set (in lexicographical order)
+// with a comma and returns the resulting string.
+func (set SortedSet) String() string {
+	elems := make([]string, len(set.m))
+	for elem, i := range set.m {
+		elems[i] = elem // safe indexing, by construction of SortedSet
+	}
+	return strings.Join(elems, ",")
+}
+
+// Subsumes reports whether csv is a sequence of comma-separated names that are
+//   - all elements of set,
+//   - sorted in lexicographically order,
+//   - unique.
+func (set SortedSet) Subsumes(csv string) bool {
+	if csv == "" {
+		return true
+	}
+	posOfLastNameSeen := -1
+	chunkSize := set.maxLen + 1 // (to accommodate for at least one comma)
+	for {
+		// As a defense against maliciously long names in csv,
+		// we only process at most chunkSize bytes per iteration.
+		end := min(len(csv), chunkSize)
+		comma := strings.IndexByte(csv[:end], ',')
+		var name string
+		if comma == -1 {
+			name = csv
+		} else {
+			name = csv[:comma]
+		}
+		pos, found := set.m[name]
+		if !found {
+			return false
+		}
+		// The names in csv are expected to be sorted in lexicographical order
+		// and appear at most once in csv.
+		// Therefore, the positions (in set) of the names that
+		// appear in csv should form a strictly increasing sequence.
+		// If that's not actually the case, bail out.
+		if pos <= posOfLastNameSeen {
+			return false
+		}
+		posOfLastNameSeen = pos
+		if comma < 0 { // We've now processed all the names in csv.
+			break
+		}
+		csv = csv[comma+1:]
+	}
+	return true
+}
+
+// TODO: when updating go directive to 1.21 or later,
+// use min builtin instead.
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+// TODO: when updating go directive to 1.21 or later,
+// use max builtin instead.
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
diff --git a/internal/sortedset_test.go b/internal/sortedset_test.go
new file mode 100644
index 0000000..9727686
--- /dev/null
+++ b/internal/sortedset_test.go
@@ -0,0 +1,119 @@
+package internal
+
+import (
+	"testing"
+)
+
+func TestSortedSet(t *testing.T) {
+	cases := []struct {
+		desc       string
+		elems      []string
+		combined   string
+		subsets    []string
+		notSubsets []string
+		wantSize   int
+	}{
+		{
+			desc:     "empty set",
+			combined: "",
+			notSubsets: []string{
+				"bar",
+				"bar,foo",
+			},
+			wantSize: 0,
+		}, {
+			desc:     "singleton set",
+			elems:    []string{"foo"},
+			combined: "foo",
+			subsets: []string{
+				"",
+				"foo",
+			},
+			notSubsets: []string{
+				"bar",
+				"bar,foo",
+			},
+			wantSize: 1,
+		}, {
+			desc:     "no dupes",
+			elems:    []string{"foo", "bar", "baz"},
+			combined: "bar,baz,foo",
+			subsets: []string{
+				"",
+				"bar",
+				"baz",
+				"foo",
+				"bar,baz",
+				"bar,foo",
+				"baz,foo",
+				"bar,baz,foo",
+			},
+			notSubsets: []string{
+				"qux",
+				"bar,baz,baz",
+				"qux,baz",
+				"qux,foo",
+				"quxbaz,foo",
+			},
+			wantSize: 3,
+		}, {
+			desc:     "some dupes",
+			elems:    []string{"foo", "bar", "bar", "foo", "e"},
+			combined: "bar,e,foo",
+			subsets: []string{
+				"",
+				"bar",
+				"e",
+				"foo",
+				"bar,foo",
+				"bar,e",
+				"e,foo",
+				"bar,e,foo",
+			},
+			notSubsets: []string{
+				"qux",
+				"qux,bar",
+				"qux,foo",
+				"qux,baz,foo",
+			},
+			wantSize: 3,
+		},
+	}
+	for _, tc := range cases {
+		f := func(t *testing.T) {
+			elems := clone(tc.elems)
+			s := NewSortedSet(tc.elems...)
+			size := s.Size()
+			if s.Size() != tc.wantSize {
+				const tmpl = "NewSortedSet(%#v...).Size(): got %d; want %d"
+				t.Errorf(tmpl, elems, size, tc.wantSize)
+			}
+			combined := s.String()
+			if combined != tc.combined {
+				const tmpl = "NewSortedSet(%#v...).String(): got %q; want %q"
+				t.Errorf(tmpl, elems, combined, tc.combined)
+			}
+			for _, sub := range tc.subsets {
+				if !s.Subsumes(sub) {
+					const tmpl = "%q is not a subset of %q, but should be"
+					t.Errorf(tmpl, sub, s)
+				}
+			}
+			for _, notSub := range tc.notSubsets {
+				if s.Subsumes(notSub) {
+					const tmpl = "%q is a subset of %q, but should not be"
+					t.Errorf(tmpl, notSub, s)
+				}
+			}
+		}
+		t.Run(tc.desc, f)
+	}
+}
+
+// adapted from https://pkg.go.dev/slices#Clone
+// TODO: when updating go directive to 1.21 or later,
+// use slices.Clone instead.
+func clone(s []string) []string {
+	// The s[:0:0] preserves nil in case it matters.
+	return append(s[:0:0], s...)
+}
diff --git a/utils.go b/utils.go
index ca9983d..7019f45 100644
--- a/utils.go
+++ b/utils.go
@@ -1,72 +1,34 @@
 package cors
 
 import (
+	"net/http"
 	"strings"
 )
 
-type converter func(string) string
-
 type wildcard struct {
 	prefix string
 	suffix string
 }
 
 func (w wildcard) match(s string) bool {
-	return len(s) >= len(w.prefix)+len(w.suffix) && strings.HasPrefix(s, w.prefix) && strings.HasSuffix(s, w.suffix)
-}
-
-// split compounded header values ["foo, bar", "baz"] -> ["foo", "bar", "baz"]
-func splitHeaderValues(values []string) []string {
-	out := values
-	copied := false
-	for i, v := range values {
-		needsSplit := strings.IndexByte(v, ',') != -1
-		if !copied {
-			if needsSplit {
-				split := strings.Split(v, ",")
-				out = make([]string, i, len(values)+len(split)-1)
-				copy(out, values[:i])
-				for _, s := range split {
-					out = append(out, strings.TrimSpace(s))
-				}
-				copied = true
-			}
-		} else {
-			if needsSplit {
-				split := strings.Split(v, ",")
-				for _, s := range split {
-					out = append(out, strings.TrimSpace(s))
-				}
-			} else {
-				out = append(out, v)
-			}
-		}
-	}
-	return out
+	return len(s) >= len(w.prefix)+len(w.suffix) &&
+		strings.HasPrefix(s, w.prefix) &&
+		strings.HasSuffix(s, w.suffix)
 }
 
 // convert converts a list of string using the passed converter function
-func convert(s []string, c converter) []string {
-	out, _ := convertDidCopy(s, c)
+func convert(s []string, f func(string) string) []string {
+	out := make([]string, len(s))
+	for i := range s {
+		out[i] = f(s[i])
+	}
 	return out
 }
 
-// convertDidCopy is same as convert but returns true if it copied the slice
-func convertDidCopy(s []string, c converter) ([]string, bool) {
-	out := s
-	copied := false
-	for i, v := range s {
-		if !copied {
-			v2 := c(v)
-			if v2 != v {
-				out = make([]string, len(s))
-				copy(out, s[:i])
-				out[i] = v2
-				copied = true
-			}
-		} else {
-			out[i] = c(v)
-		}
+func first(hdrs http.Header, k string) ([]string, bool) {
+	v, found := hdrs[k]
+	if !found || len(v) == 0 {
+		return nil, false
 	}
-	return out, copied
+	return v[:1], true
 }
diff --git a/utils_test.go b/utils_test.go
index f6a83ec..409f24e 100644
--- a/utils_test.go
+++ b/utils_test.go
@@ -1,7 +1,6 @@
 package cors
 
 import (
-	"reflect"
 	"strings"
 	"testing"
 )
@@ -24,41 +23,6 @@ func TestWildcard(t *testing.T) {
 	}
 }
 
-func TestSplitHeaderValues(t *testing.T) {
-	testCases := []struct {
-		input    []string
-		expected []string
-	}{
-		{
-			input:    []string{},
-			expected: []string{},
-		},
-		{
-			input:    []string{"foo"},
-			expected: []string{"foo"},
-		},
-		{
-			input:    []string{"foo, bar, baz"},
-			expected: []string{"foo", "bar", "baz"},
-		},
-		{
-			input:    []string{"abc", "def, ghi", "jkl"},
-			expected: []string{"abc", "def", "ghi", "jkl"},
-		},
-		{
-			input:    []string{"foo, bar", "baz, qux", "quux, corge"},
-			expected: []string{"foo", "bar", "baz", "qux", "quux", "corge"},
-		},
-	}
-
-	for _, testCase := range testCases {
-		output := splitHeaderValues(testCase.input)
-		if !reflect.DeepEqual(output, testCase.expected) {
-			t.Errorf("Input: %v, Expected: %v, Got: %v", testCase.input, testCase.expected, output)
-		}
-	}
-}
-
 func TestConvert(t *testing.T) {
 	s := convert([]string{"A", "b", "C"}, strings.ToLower)
 	e := []string{"a", "b", "c"}