diff --git a/jwt.go b/jwt.go
index 875e4da..4ad463d 100644
--- a/jwt.go
+++ b/jwt.go
@@ -322,6 +322,9 @@ func verifyTimeClaims(claims map[string]interface{}) error {
 	return nil
 }
 
+// ecdsaAlgBits maps an ES* algorithm to its curve size (RFC 7518 3.4).
+var ecdsaAlgBits = map[string]int{"ES256": 256, "ES384": 384, "ES512": 521}
+
 // verifySignatureWithKey verifies a JWT signature using an already-parsed
 // public key, skipping the PEM-encode/decode round trip. This is the hot
 // path used by VerifyJWTSignatureAndClaims and verifyLogoutTokenSignature.
@@ -369,7 +372,13 @@ func verifySignatureWithKey(tokenString string, pubKey crypto.PublicKey, alg str
 		if strings.HasPrefix(alg, "ES") {
 			var r, s big.Int
 			sigLen := len(signature)
-			if sigLen%2 != 0 {
+			// RFC 7518 3.4: the curve is fixed by the algorithm and the
+			// signature is R||S at exactly the curve's octet length.
+			bits := ecdsaAlgBits[alg]
+			if bits == 0 || pubKey.Curve.Params().BitSize != bits {
+				return fmt.Errorf("ECDSA key curve does not match algorithm %s", alg)
+			}
+			if sigLen != 2*((bits+7)/8) {
 				return fmt.Errorf("invalid ECDSA signature length")
 			}
 			r.SetBytes(signature[:sigLen/2])
diff --git a/jwt_ecdsa_strict_test.go b/jwt_ecdsa_strict_test.go
new file mode 100644
index 0000000..aeb7a98
--- /dev/null
+++ b/jwt_ecdsa_strict_test.go
@@ -0,0 +1,53 @@
+package traefikoidc
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/sha512"
+	"encoding/base64"
+	"testing"
+)
+
+func signES(t *testing.T, key *ecdsa.PrivateKey, signed string, h256 bool, size int) (sigRS []byte) {
+	t.Helper()
+	var digest []byte
+	if h256 {
+		d := sha256.Sum256([]byte(signed))
+		digest = d[:]
+	} else {
+		d := sha512.Sum384([]byte(signed))
+		digest = d[:]
+	}
+	r, s, err := ecdsa.Sign(rand.Reader, key, digest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	out := make([]byte, 2*size)
+	r.FillBytes(out[:size])
+	s.FillBytes(out[size:])
+	return out
+}
+
+func TestECDSAVerifyIsStrict(t *testing.T) {
+	const signed = "eyJhbGciOiJFUzI1NiJ9.eyJzdWIiOiJ1In0"
+	enc := base64.RawURLEncoding.EncodeToString
+	p256, _ := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	p384, _ := ecdsa.GenerateKey(elliptic.P384(), rand.Reader)
+
+	if err := verifySignatureWithKey(signed+"."+enc(signES(t, p256, signed, true, 32)), &p256.PublicKey, "ES256"); err != nil {
+		t.Fatalf("valid ES256 rejected: %v", err)
+	}
+	if err := verifySignatureWithKey(signed+"."+enc(signES(t, p384, signed, false, 48)), &p384.PublicKey, "ES384"); err != nil {
+		t.Fatalf("valid ES384 rejected: %v", err)
+	}
+	// ES256 token verified against a P-384 key.
+	if err := verifySignatureWithKey(signed+"."+enc(signES(t, p384, signed, true, 48)), &p384.PublicKey, "ES256"); err == nil {
+		t.Error("ES256 accepted with a P-384 key")
+	}
+	// Zero-padded halves: 66 bytes for ES256.
+	if err := verifySignatureWithKey(signed+"."+enc(signES(t, p256, signed, true, 33)), &p256.PublicKey, "ES256"); err == nil {
+		t.Error("ES256 accepted a 66-byte signature")
+	}
+}