diff --git a/pkcs12/crypto.go b/pkcs12/crypto.go
index 212538c..3f30732 100644
--- a/pkcs12/crypto.go
+++ b/pkcs12/crypto.go
@@ -80,6 +80,10 @@ func pbDecrypterFor(algorithm pkix.AlgorithmIdentifier, password []byte) (cipher
 		return nil, 0, err
 	}
 
+	if params.Iterations < 0 || params.Iterations > maxIterations {
+		return nil, 0, NotImplementedError("iteration count is invalid or too high")
+	}
+
 	key := cipherType.deriveKey(params.Salt, password, params.Iterations)
 	iv := cipherType.deriveIV(params.Salt, password, params.Iterations)
 
diff --git a/pkcs12/crypto_test.go b/pkcs12/crypto_test.go
index eb4dae8..29962ec 100644
--- a/pkcs12/crypto_test.go
+++ b/pkcs12/crypto_test.go
@@ -13,6 +13,46 @@ import (
 
 var sha1WithTripleDES = asn1.ObjectIdentifier([]int{1, 2, 840, 113549, 1, 12, 1, 3})
 
+func TestPbDecrypterForIterationLimit(t *testing.T) {
+	pass, _ := bmpString("Sesame open")
+
+	tests := []struct {
+		name       string
+		iterations int
+		wantErr    bool
+	}{
+		{"at limit", maxIterations, false},
+		{"over limit", maxIterations + 1, true},
+		{"negative", -1, true},
+		{"max int", 1<<31 - 1, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			params, _ := asn1.Marshal(pbeParams{
+				Salt:       []byte{1, 2, 3, 4, 5, 6, 7, 8},
+				Iterations: tt.iterations,
+			})
+			alg := pkix.AlgorithmIdentifier{
+				Algorithm: sha1WithTripleDES,
+				Parameters: asn1.RawValue{
+					FullBytes: params,
+				},
+			}
+			_, _, err := pbDecrypterFor(alg, pass)
+			if tt.wantErr {
+				if _, ok := err.(NotImplementedError); !ok {
+					t.Errorf("iterations=%d: got %v, want NotImplementedError", tt.iterations, err)
+				}
+			} else {
+				if err != nil {
+					t.Errorf("iterations=%d: unexpected error: %v", tt.iterations, err)
+				}
+			}
+		})
+	}
+}
+
 func TestPbDecrypterFor(t *testing.T) {
 	params, _ := asn1.Marshal(pbeParams{
 		Salt:       []byte{1, 2, 3, 4, 5, 6, 7, 8},
diff --git a/pkcs12/mac.go b/pkcs12/mac.go
index 5f38aa7..84039ce 100644
--- a/pkcs12/mac.go
+++ b/pkcs12/mac.go
@@ -27,11 +27,19 @@ var (
 	oidSHA1 = asn1.ObjectIdentifier([]int{1, 3, 14, 3, 2, 26})
 )
 
+// maxIterations is a safety limit to prevent CPU exhaustion from
+// crafted PKCS#12 files with unreasonable iteration counts.
+const maxIterations = 1 << 20 // ~1 million
+
 func verifyMac(macData *macData, message, password []byte) error {
 	if !macData.Mac.Algorithm.Algorithm.Equal(oidSHA1) {
 		return NotImplementedError("unknown digest algorithm: " + macData.Mac.Algorithm.Algorithm.String())
 	}
 
+	if macData.Iterations < 0 || macData.Iterations > maxIterations {
+		return NotImplementedError("iteration count is invalid or too high")
+	}
+
 	key := pbkdf(sha1Sum, 20, 64, macData.MacSalt, password, macData.Iterations, 3, 20)
 
 	mac := hmac.New(sha1.New, key)
diff --git a/pkcs12/mac_test.go b/pkcs12/mac_test.go
index 1ed4ff2..5d4a7ec 100644
--- a/pkcs12/mac_test.go
+++ b/pkcs12/mac_test.go
@@ -5,10 +5,52 @@
 package pkcs12
 
 import (
+	"crypto/x509/pkix"
 	"encoding/asn1"
 	"testing"
 )
 
+func TestVerifyMacIterationLimit(t *testing.T) {
+	password, _ := bmpString("Sesame open")
+	message := []byte{11, 12, 13, 14, 15}
+
+	tests := []struct {
+		name       string
+		iterations int
+		wantErr    bool
+	}{
+		{"at limit", maxIterations, false},
+		{"over limit", maxIterations + 1, true},
+		{"negative", -1, true},
+		{"max int", 1<<31 - 1, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			td := macData{
+				Mac: digestInfo{
+					Algorithm: pkix.AlgorithmIdentifier{
+						Algorithm: oidSHA1,
+					},
+					Digest: nil, // will fail MAC check, but iteration check comes first
+				},
+				MacSalt:    []byte{1, 2, 3, 4, 5, 6, 7, 8},
+				Iterations: tt.iterations,
+			}
+			err := verifyMac(&td, message, password)
+			if tt.wantErr {
+				if _, ok := err.(NotImplementedError); !ok {
+					t.Errorf("iterations=%d: got %v, want NotImplementedError", tt.iterations, err)
+				}
+			} else {
+				if _, ok := err.(NotImplementedError); ok {
+					t.Errorf("iterations=%d: got unexpected NotImplementedError", tt.iterations)
+				}
+			}
+		})
+	}
+}
+
 func TestVerifyMac(t *testing.T) {
 	td := macData{
 		Mac: digestInfo{