diff --git a/internal/outpost/radius/request.go b/internal/outpost/radius/request.go
index 962c929832..bdf20cbcf6 100644
--- a/internal/outpost/radius/request.go
+++ b/internal/outpost/radius/request.go
@@ -1,7 +1,6 @@
 package radius
 
 import (
-	"bytes"
 	"crypto/hmac"
 	"crypto/md5"
 	"errors"
@@ -40,13 +39,18 @@ func (r *RadiusRequest) ID() string {
 
 func (r *RadiusRequest) validateMessageAuthenticator() error {
 	mauth := rfc2869.MessageAuthenticator_Get(r.Packet)
-	hash := hmac.New(md5.New, r.Secret)
+	// Per RFC 2869 §5.14, the Message-Authenticator field must be treated as
+	// 16 zero bytes when computing the HMAC-MD5 for verification.
+	_ = rfc2869.MessageAuthenticator_Set(r.Packet, make([]byte, 16))
+	hash := hmac.New(md5.New, r.pi.SharedSecret)
 	encode, err := r.MarshalBinary()
+	// Restore the original value regardless of whether marshaling succeeded.
+	_ = rfc2869.MessageAuthenticator_Set(r.Packet, mauth)
 	if err != nil {
 		return err
 	}
 	hash.Write(encode)
-	if bytes.Equal(mauth, hash.Sum(nil)) {
+	if !hmac.Equal(mauth, hash.Sum(nil)) {
 		return ErrInvalidMessageAuthenticator
 	}
 	return nil
@@ -54,7 +58,7 @@ func (r *RadiusRequest) validateMessageAuthenticator() error {
 
 func (r *RadiusRequest) setMessageAuthenticator(rp *radius.Packet) error {
 	_ = rfc2869.MessageAuthenticator_Set(rp, make([]byte, 16))
-	hash := hmac.New(md5.New, rp.Secret)
+	hash := hmac.New(md5.New, r.pi.SharedSecret)
 	encode, err := rp.MarshalBinary()
 	if err != nil {
 		return err
diff --git a/internal/outpost/radius/request_test.go b/internal/outpost/radius/request_test.go
new file mode 100644
index 0000000000..744da04e9c
--- /dev/null
+++ b/internal/outpost/radius/request_test.go
@@ -0,0 +1,63 @@
+package radius
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"layeh.com/radius"
+)
+
+var (
+	radiusPacketAccReq = []byte{0x1, 0x8f, 0x0, 0x4d, 0x4a, 0xd5, 0x47, 0x98, 0xbf, 0x18, 0xe, 0x4b, 0x6a, 0xdd, 0x0, 0xc7, 0x99, 0xb4, 0xa6, 0x57, 0x50, 0x12, 0xa5, 0xf7, 0x16, 0x88, 0xc5, 0xd8, 0xd9, 0xec, 0x19, 0xc8, 0x51, 0x47, 0x9, 0x5f, 0xe5, 0x60, 0x1, 0x9, 0x61, 0x6b, 0x61, 0x64, 0x6d, 0x69, 0x6e, 0x2, 0x12, 0x37, 0x36, 0x8, 0xa3, 0x72, 0x20, 0xf, 0xf4, 0xc0, 0xc, 0xd2, 0x40, 0xc1, 0xc3, 0x3f, 0xef, 0x4, 0x6, 0xa, 0x78, 0x14, 0x4c, 0x5, 0x6, 0x0, 0x0, 0x0, 0xa}
+)
+
+func Test_Request_validateMessageAuthenticator_valid(t *testing.T) {
+	p, err := radius.Parse(radiusPacketAccReq, []byte("foo"))
+	assert.NoError(t, err)
+	req := RadiusRequest{
+		Request: &radius.Request{
+			Packet: p,
+		},
+		pi: &ProviderInstance{
+			SharedSecret: []byte("foo"),
+		},
+	}
+	assert.NoError(t, req.validateMessageAuthenticator())
+}
+
+func Test_Request_validateMessageAuthenticator_invalid(t *testing.T) {
+	p, err := radius.Parse(radiusPacketAccReq, []byte("bar"))
+	assert.NoError(t, err)
+	req := RadiusRequest{
+		Request: &radius.Request{
+			Packet: p,
+		},
+		pi: &ProviderInstance{
+			SharedSecret: []byte("bar"),
+		},
+	}
+	assert.Error(t, req.validateMessageAuthenticator(), ErrInvalidMessageAuthenticator)
+}
+
+func Test_Request_setMessageAuthenticator(t *testing.T) {
+	p, err := radius.Parse(radiusPacketAccReq, []byte("foo"))
+	assert.NoError(t, err)
+	req := RadiusRequest{
+		Request: &radius.Request{
+			Packet: p,
+		},
+		pi: &ProviderInstance{
+			SharedSecret: []byte("foo"),
+		},
+	}
+	res := p.Response(radius.CodeAccessAccept)
+	assert.NoError(t, req.setMessageAuthenticator(res))
+
+	nr := RadiusRequest{
+		Request: &radius.Request{
+			Packet: res,
+		},
+		pi: req.pi,
+	}
+	assert.NoError(t, nr.validateMessageAuthenticator())
+}