diff --git a/CHANGELOG.md b/CHANGELOG.md
index 426977b87..db9c7d0a7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,6 +33,10 @@
 - Pomerium and its services will gracefully shutdown on [interrupt signal](http://man7.org/linux/man-pages/man7/signal.7.html). [GH-230]
 - [Google](https://developers.google.com/identity/protocols/OpenIDConnect) now prompts the user to select a user account (by adding `select_account` to the sign in url). This allows a user who has multiple accounts at the authorization server to select amongst the multiple accounts that they may have current sessions for.
 
+### FIXED
+
+- Fixed potential race condition when signing requests. [GH-240]
+
 ## v0.1.0
 
 ### NEW
diff --git a/internal/cryptutil/encrypt.go b/internal/cryptutil/encrypt.go
index fbfca9a09..b974be7d7 100644
--- a/internal/cryptutil/encrypt.go
+++ b/internal/cryptutil/encrypt.go
@@ -77,7 +77,7 @@ func (c *XChaCha20Cipher) GenerateNonce() []byte {
 func (c *XChaCha20Cipher) Encrypt(plaintext []byte) (joined []byte, err error) {
 	defer func() {
 		if r := recover(); r != nil {
-			err = fmt.Errorf("internal/aead: error encrypting bytes: %v", r)
+			err = fmt.Errorf("cryptutil: error encrypting bytes: %v", r)
 		}
 	}()
 	nonce := c.GenerateNonce()
@@ -92,7 +92,7 @@ func (c *XChaCha20Cipher) Encrypt(plaintext []byte) (joined []byte, err error) {
 // Decrypt a value using XChaCha20-Poly1305
 func (c *XChaCha20Cipher) Decrypt(joined []byte) ([]byte, error) {
 	if len(joined) <= c.aead.NonceSize() {
-		return nil, fmt.Errorf("internal/aead: invalid input size: %d", len(joined))
+		return nil, fmt.Errorf("cryptutil: invalid input size: %d", len(joined))
 	}
 	// grab out the nonce
 	pivot := len(joined) - c.aead.NonceSize()
@@ -161,13 +161,13 @@ func compress(data []byte) ([]byte, error) {
 	var buf bytes.Buffer
 	writer, err := gzip.NewWriterLevel(&buf, gzip.DefaultCompression)
 	if err != nil {
-		return nil, fmt.Errorf("internal/aead: failed to create a gzip writer: %q", err)
+		return nil, fmt.Errorf("cryptutil: failed to create a gzip writer: %q", err)
 	}
 	if writer == nil {
-		return nil, fmt.Errorf("internal/aead: failed to create a gzip writer")
+		return nil, fmt.Errorf("cryptutil: failed to create a gzip writer")
 	}
 	if _, err = writer.Write(data); err != nil {
-		return nil, fmt.Errorf("internal/aead: failed to compress data with err: %q", err)
+		return nil, fmt.Errorf("cryptutil: failed to compress data with err: %q", err)
 	}
 	if err = writer.Close(); err != nil {
 		return nil, err
@@ -178,7 +178,7 @@ func compress(data []byte) ([]byte, error) {
 func decompress(data []byte) ([]byte, error) {
 	reader, err := gzip.NewReader(bytes.NewReader(data))
 	if err != nil {
-		return nil, fmt.Errorf("internal/aead: failed to create a gzip reader: %q", err)
+		return nil, fmt.Errorf("cryptutil: failed to create a gzip reader: %q", err)
 	}
 	defer reader.Close()
 	var buf bytes.Buffer
diff --git a/internal/cryptutil/marshal.go b/internal/cryptutil/marshal.go
index a2bfcef67..a906531ac 100644
--- a/internal/cryptutil/marshal.go
+++ b/internal/cryptutil/marshal.go
@@ -14,7 +14,7 @@ import (
 func DecodePublicKey(encodedKey []byte) (*ecdsa.PublicKey, error) {
 	block, _ := pem.Decode(encodedKey)
 	if block == nil {
-		return nil, fmt.Errorf("marshal: decoded nil PEM block")
+		return nil, fmt.Errorf("cryptutil: decoded nil PEM block")
 	}
 	pub, err := x509.ParsePKIXPublicKey(block.Bytes)
 	if err != nil {
@@ -23,7 +23,7 @@ func DecodePublicKey(encodedKey []byte) (*ecdsa.PublicKey, error) {
 
 	ecdsaPub, ok := pub.(*ecdsa.PublicKey)
 	if !ok {
-		return nil, errors.New("marshal: data was not an ECDSA public key")
+		return nil, errors.New("cryptutil: data was not an ECDSA public key")
 	}
 
 	return ecdsaPub, nil
@@ -53,7 +53,7 @@ func DecodePrivateKey(encodedKey []byte) (*ecdsa.PrivateKey, error) {
 		block, encodedKey = pem.Decode(encodedKey)
 
 		if block == nil {
-			return nil, fmt.Errorf("failed to find EC PRIVATE KEY in PEM data after skipping types %v", skippedTypes)
+			return nil, fmt.Errorf("cryptutil: failed to find EC PRIVATE KEY in PEM data after skipping types %v", skippedTypes)
 		}
 
 		if block.Type == "EC PRIVATE KEY" {
diff --git a/internal/cryptutil/mock_cipher_test.go b/internal/cryptutil/mock_cipher_test.go
new file mode 100644
index 000000000..037fc2dff
--- /dev/null
+++ b/internal/cryptutil/mock_cipher_test.go
@@ -0,0 +1,44 @@
+package cryptutil // import "github.com/pomerium/pomerium/internal/cryptutil"
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestMockCipher_Unmarshal(t *testing.T) {
+	e := errors.New("err")
+	mc := MockCipher{
+		EncryptResponse: []byte("EncryptResponse"),
+		EncryptError:    e,
+		DecryptResponse: []byte("DecryptResponse"),
+		DecryptError:    e,
+		MarshalResponse: "MarshalResponse",
+		MarshalError:    e,
+		UnmarshalError:  e,
+	}
+	b, err := mc.Encrypt([]byte("test"))
+	if string(b) != "EncryptResponse" {
+		t.Error("unexpected encrypt response")
+	}
+	if err != e {
+		t.Error("unexpected encrypt error")
+	}
+	b, err = mc.Decrypt([]byte("test"))
+	if string(b) != "DecryptResponse" {
+		t.Error("unexpected Decrypt response")
+	}
+	if err != e {
+		t.Error("unexpected Decrypt error")
+	}
+	s, err := mc.Marshal("test")
+	if err != e {
+		t.Error("unexpected Marshal error")
+	}
+	if s != "MarshalResponse" {
+		t.Error("unexpected MarshalResponse error")
+	}
+	err = mc.Unmarshal("s", "s")
+	if err != e {
+		t.Error("unexpected Unmarshal error")
+	}
+}
diff --git a/internal/cryptutil/sign.go b/internal/cryptutil/sign.go
index 6687e7610..aa7ce6ac3 100644
--- a/internal/cryptutil/sign.go
+++ b/internal/cryptutil/sign.go
@@ -1,6 +1,7 @@
 package cryptutil // import "github.com/pomerium/pomerium/internal/cryptutil"
 import (
 	"fmt"
+	"sync"
 	"time"
 
 	jose "gopkg.in/square/go-jose.v2"
@@ -15,6 +16,9 @@ type JWTSigner interface {
 
 // ES256Signer is struct containing the required fields to create a ES256 signed JSON Web Tokens
 type ES256Signer struct {
+	signer jose.Signer
+
+	mu sync.Mutex
 	// User (sub) is unique, stable identifier for the user.
 	// Use in place of the x-pomerium-authenticated-user-id header.
 	User string `json:"sub,omitempty"`
@@ -42,8 +46,6 @@ type ES256Signer struct {
 	// IssuedAt (nbf) is the time is measured in seconds since the UNIX epoch.
 	// Allow 1 minute for skew.
 	NotBefore jwt.NumericDate `json:"nbf,omitempty"`
-
-	signer jose.Signer
 }
 
 // NewES256Signer creates an Elliptic Curve, NIST P-256 (aka secp256r1 aka prime256v1) JWT signer.
@@ -56,7 +58,7 @@ type ES256Signer struct {
 func NewES256Signer(privKey []byte, audience string) (*ES256Signer, error) {
 	key, err := DecodePrivateKey(privKey)
 	if err != nil {
-		return nil, fmt.Errorf("internal/cryptutil parsing key failed %v", err)
+		return nil, fmt.Errorf("cryptutil: parsing key failed %v", err)
 	}
 	signer, err := jose.NewSigner(
 		jose.SigningKey{
@@ -65,7 +67,7 @@ func NewES256Signer(privKey []byte, audience string) (*ES256Signer, error) {
 		},
 		(&jose.SignerOptions{}).WithType("JWT"))
 	if err != nil {
-		return nil, fmt.Errorf("internal/cryptutil new signer failed %v", err)
+		return nil, fmt.Errorf("cryptutil: new signer failed %v", err)
 	}
 	return &ES256Signer{
 		Issuer:   "pomerium-proxy",
@@ -77,6 +79,8 @@ func NewES256Signer(privKey []byte, audience string) (*ES256Signer, error) {
 // SignJWT creates a signed JWT containing claims for the logged in
 // user id (`sub`), email (`email`) and groups (`groups`).
 func (s *ES256Signer) SignJWT(user, email, groups string) (string, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	s.User = user
 	s.Email = email
 	s.Groups = groups
@@ -86,7 +90,7 @@ func (s *ES256Signer) SignJWT(user, email, groups string) (string, error) {
 	s.NotBefore = *jwt.NewNumericDate(now.Add(-1 * jwt.DefaultLeeway))
 	rawJWT, err := jwt.Signed(s.signer).Claims(s).CompactSerialize()
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("cryptutil: sign failed %v", err)
 	}
 	return rawJWT, nil
 }