diff --git a/oauthex/resource_meta.go b/oauthex/resource_meta.go
index 880182bc2..2869c82f9 100644
--- a/oauthex/resource_meta.go
+++ b/oauthex/resource_meta.go
@@ -180,6 +180,11 @@ func splitChallenges(header string) ([]string, error) {
 	inQuotes := false
 	escaped := false
 	start := 0
+	// Every comma in a run of commas and blanks sees the same text after the
+	// run, so the lookahead is computed once per run. Rescanning per comma
+	// makes a long header take quadratic time.
+	runEnd := 0
+	runIsParam := false
 	for i, r := range header {
 		if escaped {
 			// The previous backslash began a quoted-pair, so this character is
@@ -198,20 +203,13 @@ func splitChallenges(header string) ([]string, error) {
 			// This is a potential challenge separator.
 			// A new challenge does not start with `key=value`.
 			// We check if the part after the comma looks like a parameter.
-			lookahead := trimEmptyListElements(header[i+1:])
-			eqPos := strings.Index(lookahead, "=")
-
-			isParam := false
-			if eqPos > 0 {
-				// Check if the part before '=' is a single token (no spaces).
-				// Whitespace between the token and '=' is allowed (BWS, RFC 9110 section 11.2).
-				token := strings.TrimRight(lookahead[:eqPos], " \t")
-				if strings.IndexFunc(token, unicode.IsSpace) == -1 {
-					isParam = true
-				}
+			if i >= runEnd {
+				lookahead := trimEmptyListElements(header[i+1:])
+				runEnd = len(header) - len(lookahead)
+				runIsParam = startsWithAuthParam(lookahead)
 			}
 
-			if !isParam {
+			if !runIsParam {
 				// The part after the comma does not look like a parameter,
 				// so this comma separates challenges.
 				challenges = append(challenges, header[start:i])
@@ -224,6 +222,26 @@ func splitChallenges(header string) ([]string, error) {
 	return challenges, nil
 }
 
+// startsWithAuthParam reports whether s begins with a token followed by '='.
+// Whitespace between the token and '=' is allowed (BWS, RFC 9110 section 11.2).
+// It reads no further than that token, so its cost does not depend on the
+// length of the rest of the header.
+func startsWithAuthParam(s string) bool {
+	for i, r := range s {
+		switch {
+		case r == '=':
+			return i > 0
+		case r == ',':
+			return false
+		case r == ' ' || r == '\t':
+			return strings.HasPrefix(strings.TrimLeft(s[i:], " \t"), "=")
+		case unicode.IsSpace(r):
+			return false
+		}
+	}
+	return false
+}
+
 // parseSingleChallenge parses a string containing exactly one challenge.
 // challenge   = auth-scheme [ 1*SP ( token68 / #auth-param ) ]
 func parseSingleChallenge(s string) (Challenge, error) {
diff --git a/oauthex/resource_meta_test.go b/oauthex/resource_meta_test.go
index c262bffc7..c6678006c 100644
--- a/oauthex/resource_meta_test.go
+++ b/oauthex/resource_meta_test.go
@@ -9,7 +9,9 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"reflect"
+	"strings"
 	"testing"
+	"time"
 )
 
 func TestSplitChallenges(t *testing.T) {
@@ -68,6 +70,31 @@ func TestSplitChallenges(t *testing.T) {
 			input: "",
 			want:  []string{""},
 		},
+		{
+			name:  "challenges without params",
+			input: `Basic,Bearer`,
+			want:  []string{`Basic`, `Bearer`},
+		},
+		{
+			name:  "blank between token and equals",
+			input: "Bearer a=1, b \t=2",
+			want:  []string{"Bearer a=1, b \t=2"},
+		},
+		{
+			name:  "whitespace inside the name starts a new challenge",
+			input: `Basic realm="a", Bearer x y=1`,
+			want:  []string{`Basic realm="a"`, ` Bearer x y=1`},
+		},
+		{
+			name:  "non-blank whitespace before equals starts a new challenge",
+			input: "Basic, Bearer\u00a0=1",
+			want:  []string{"Basic", " Bearer\u00a0=1"},
+		},
+		{
+			name:  "comma inside the name starts a new challenge",
+			input: `Bearer x, a,b=c`,
+			want:  []string{`Bearer x`, ` a,b=c`},
+		},
 	}
 
 	for _, tt := range tests {
@@ -83,6 +110,28 @@ func TestSplitChallenges(t *testing.T) {
 	}
 }
 
+// The lookahead after a comma must not rescan the rest of the header, or a
+// long header from a server takes quadratic time that no context deadline can
+// interrupt. The input is kept small: the test only needs the old code to take
+// several times longer than the deadline, and it must not load the machine,
+// because other packages' tests run in parallel with it.
+func TestParseWWWAuthenticateLinearTime(t *testing.T) {
+	header := "Bearer " + strings.Repeat(",", 100_000)
+	done := make(chan error, 1)
+	go func() {
+		_, err := ParseWWWAuthenticate([]string{header})
+		done <- err
+	}()
+	select {
+	case err := <-done:
+		if err != nil {
+			t.Fatal(err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatalf("ParseWWWAuthenticate did not finish within 2s on a %d byte header", len(header))
+	}
+}
+
 // A quoted string may end in an escaped backslash. The quote after it closes
 // the string, so the comma that follows still separates two challenges and
 // the Bearer challenge the auth handlers look for is not lost.