diff --git a/CHANGELOG.md b/CHANGELOG.md
index 236666af..f35a3b43 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -55,6 +55,7 @@
 
 ## Changes since v5.1.1
 
+- [#601](https://github.com/oauth2-proxy/oauth2-proxy/pull/601) Ensure decrypted user/email are valid UTF8 (@JoelSpeed)
 - [#560](https://github.com/oauth2-proxy/oauth2-proxy/pull/560) Fallback to UserInfo is User ID claim not present (@JoelSpeed)
 - [#598](https://github.com/oauth2-proxy/oauth2-proxy/pull/598) acr_values no longer sent to IdP when empty (@ScottGuymer)
 - [#548](https://github.com/oauth2-proxy/oauth2-proxy/pull/548) Separate logging options out of main options structure (@JoelSpeed)
diff --git a/pkg/apis/sessions/session_state.go b/pkg/apis/sessions/session_state.go
index 1bb8ff03..f2e6633e 100644
--- a/pkg/apis/sessions/session_state.go
+++ b/pkg/apis/sessions/session_state.go
@@ -2,8 +2,10 @@ package sessions
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"time"
+	"unicode/utf8"
 
 	"github.com/oauth2-proxy/oauth2-proxy/pkg/encryption"
 )
@@ -106,6 +108,9 @@ func DecodeSessionState(v string, c *encryption.Cipher) (*SessionState, error) {
 		if ss.Email != "" {
 			decryptedEmail, errEmail := c.Decrypt(ss.Email)
 			if errEmail == nil {
+				if !utf8.ValidString(decryptedEmail) {
+					return nil, errors.New("invalid value for decrypted email")
+				}
 				ss.Email = decryptedEmail
 			}
 		}
@@ -113,6 +118,9 @@ func DecodeSessionState(v string, c *encryption.Cipher) (*SessionState, error) {
 		if ss.User != "" {
 			decryptedUser, errUser := c.Decrypt(ss.User)
 			if errUser == nil {
+				if !utf8.ValidString(decryptedUser) {
+					return nil, errors.New("invalid value for decrypted user")
+				}
 				ss.User = decryptedUser
 			}
 		}
diff --git a/pkg/apis/sessions/session_state_test.go b/pkg/apis/sessions/session_state_test.go
index 529656fe..150e9c9d 100644
--- a/pkg/apis/sessions/session_state_test.go
+++ b/pkg/apis/sessions/session_state_test.go
@@ -49,15 +49,7 @@ func TestSessionStateSerialization(t *testing.T) {
 	// ensure a different cipher can't decode properly (ie: it gets gibberish)
 	ss, err = sessions.DecodeSessionState(encoded, c2)
 	t.Logf("%#v", ss)
-	assert.Equal(t, nil, err)
-	assert.NotEqual(t, "user@domain.com", ss.User)
-	assert.NotEqual(t, s.Email, ss.Email)
-	assert.NotEqual(t, s.PreferredUsername, ss.PreferredUsername)
-	assert.Equal(t, s.CreatedAt.Unix(), ss.CreatedAt.Unix())
-	assert.Equal(t, s.ExpiresOn.Unix(), ss.ExpiresOn.Unix())
-	assert.NotEqual(t, s.AccessToken, ss.AccessToken)
-	assert.NotEqual(t, s.IDToken, ss.IDToken)
-	assert.NotEqual(t, s.RefreshToken, ss.RefreshToken)
+	assert.NotEqual(t, nil, err)
 }
 
 func TestSessionStateSerializationWithUser(t *testing.T) {
@@ -91,14 +83,7 @@ func TestSessionStateSerializationWithUser(t *testing.T) {
 	// ensure a different cipher can't decode properly (ie: it gets gibberish)
 	ss, err = sessions.DecodeSessionState(encoded, c2)
 	t.Logf("%#v", ss)
-	assert.Equal(t, nil, err)
-	assert.NotEqual(t, s.User, ss.User)
-	assert.NotEqual(t, s.Email, ss.Email)
-	assert.NotEqual(t, s.PreferredUsername, ss.PreferredUsername)
-	assert.Equal(t, s.CreatedAt.Unix(), ss.CreatedAt.Unix())
-	assert.Equal(t, s.ExpiresOn.Unix(), ss.ExpiresOn.Unix())
-	assert.NotEqual(t, s.AccessToken, ss.AccessToken)
-	assert.NotEqual(t, s.RefreshToken, ss.RefreshToken)
+	assert.NotEqual(t, nil, err)
 }
 
 func TestSessionStateSerializationNoCipher(t *testing.T) {
@@ -278,6 +263,14 @@ func TestDecodeSessionState(t *testing.T) {
 			Cipher:  c,
 			Error:   true,
 		},
+		{
+			SessionState: sessions.SessionState{
+				Email: "user@domain.com",
+				User:  "YmFzZTY0LWVuY29kZWQtdXNlcgo=", // Base64 encoding of base64-encoded-user
+			},
+			Error:  true,
+			Cipher: c,
+		},
 	}
 
 	for i, tc := range testCases {