diff --git a/pkg/teeattestation/hash.go b/pkg/teeattestation/hash.go
index fb38dce1e0..830139b952 100644
--- a/pkg/teeattestation/hash.go
+++ b/pkg/teeattestation/hash.go
@@ -6,6 +6,7 @@ package teeattestation
 import (
 	"crypto/sha256"
 	"encoding/binary"
+	"fmt"
 	"hash"
 )
 
@@ -14,12 +15,26 @@ const DomainSeparator = "CONFIDENTIAL_COMPUTE_PAYLOAD"
 
 // DomainHash computes SHA-256 over the DomainSeparator and the length-prefixed
 // tag and data, so distinct (tag, data) pairs can never share a pre-image. [CL112-14]
-func DomainHash(tag string, data []byte) []byte {
+// It returns an error if tag is empty or contains non-ASCII alphanumeric characters.
+func DomainHash(tag string, data []byte) ([]byte, error) {
+	if tag == "" || !isAlphanumeric(tag) {
+		return nil, fmt.Errorf("invalid tag: must be non-empty and contain only alphanumeric characters")
+	}
 	h := sha256.New()
 	h.Write([]byte(DomainSeparator))
 	writeWithLength(h, []byte(tag))
 	writeWithLength(h, data)
-	return h.Sum(nil)
+	return h.Sum(nil), nil
+}
+
+// isAlphanumeric reports whether s consists only of ASCII letters and digits.
+func isAlphanumeric(s string) bool {
+	for _, r := range s {
+		if !(r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z' || r >= '0' && r <= '9') {
+			return false
+		}
+	}
+	return true
 }
 
 // writeWithLength writes an 8-byte big-endian length prefix followed by data.
diff --git a/pkg/teeattestation/hash_test.go b/pkg/teeattestation/hash_test.go
index 43ffe8c76b..d5065f364b 100644
--- a/pkg/teeattestation/hash_test.go
+++ b/pkg/teeattestation/hash_test.go
@@ -1,16 +1,18 @@
 package teeattestation
 
 import (
-	"bytes"
 	"crypto/sha256"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestDomainHash(t *testing.T) {
 	tag := "TestTag"
 	data := []byte(`{"key":"value"}`)
 
-	got := DomainHash(tag, data)
+	got, err := DomainHash(tag, data)
+	require.NoError(t, err)
 
 	h := sha256.New()
 	h.Write([]byte(DomainSeparator))
@@ -30,8 +32,10 @@ func TestDomainHash(t *testing.T) {
 
 func TestDomainHash_DifferentTags(t *testing.T) {
 	data := []byte("same-data")
-	h1 := DomainHash("Tag1", data)
-	h2 := DomainHash("Tag2", data)
+	h1, err := DomainHash("Tag1", data)
+	require.NoError(t, err)
+	h2, err := DomainHash("Tag2", data)
+	require.NoError(t, err)
 
 	for i := range h1 {
 		if h1[i] != h2[i] {
@@ -43,8 +47,10 @@ func TestDomainHash_DifferentTags(t *testing.T) {
 
 func TestDomainHash_DifferentData(t *testing.T) {
 	tag := "SameTag"
-	h1 := DomainHash(tag, []byte("data-a"))
-	h2 := DomainHash(tag, []byte("data-b"))
+	h1, err := DomainHash(tag, []byte("data-a"))
+	require.NoError(t, err)
+	h2, err := DomainHash(tag, []byte("data-b"))
+	require.NoError(t, err)
 
 	for i := range h1 {
 		if h1[i] != h2[i] {
@@ -54,9 +60,18 @@ func TestDomainHash_DifferentData(t *testing.T) {
 	t.Fatal("different data should produce different hashes")
 }
 
-// CL112-14 regression: the old newline scheme let ("A\nB","C") and ("A","B\nC") collide.
-func TestDomainHash_NoBoundaryCollision(t *testing.T) {
-	if bytes.Equal(DomainHash("A\nB", []byte("C")), DomainHash("A", []byte("B\nC"))) {
-		t.Fatal("tag/data boundary collision: distinct pairs must not share a hash")
+func TestDomainHash_InvalidTag(t *testing.T) {
+	for _, tag := range []string{"", "A\nB", "tag-1"} {
+		_, err := DomainHash(tag, []byte("C"))
+		require.EqualError(t, err, "invalid tag: must be non-empty and contain only alphanumeric characters")
 	}
 }
+
+// CL112-14 regression: without length-prefixing, ("AB","C") and ("A","BC") could collide.
+func TestDomainHash_NoBoundaryCollision(t *testing.T) {
+	h1, err := DomainHash("AB", []byte("C"))
+	require.NoError(t, err)
+	h2, err := DomainHash("A", []byte("BC"))
+	require.NoError(t, err)
+	require.NotEqual(t, h1, h2)
+}
diff --git a/pkg/teeattestation/nitro/validate_test.go b/pkg/teeattestation/nitro/validate_test.go
index a3b93caca6..0cbde2cc76 100644
--- a/pkg/teeattestation/nitro/validate_test.go
+++ b/pkg/teeattestation/nitro/validate_test.go
@@ -15,7 +15,8 @@ func TestValidateAttestation_Attestor(t *testing.T) {
 	fa, err := nitrofake.NewAttestor()
 	require.NoError(t, err)
 
-	userData := teeattestation.DomainHash("test-tag", []byte(`{"key":"value"}`))
+	userData, err := teeattestation.DomainHash("testtag", []byte(`{"key":"value"}`))
+	require.NoError(t, err)
 	doc, err := fa.CreateAttestation(userData)
 	require.NoError(t, err)
 
@@ -50,11 +51,13 @@ func TestValidateAttestation_WrongUserData(t *testing.T) {
 	fa, err := nitrofake.NewAttestor()
 	require.NoError(t, err)
 
-	userData := teeattestation.DomainHash("test-tag", []byte(`{"key":"value"}`))
+	userData, err := teeattestation.DomainHash("testtag", []byte(`{"key":"value"}`))
+	require.NoError(t, err)
 	doc, err := fa.CreateAttestation(userData)
 	require.NoError(t, err)
 
-	wrongData := teeattestation.DomainHash("wrong-tag", []byte(`{"key":"value"}`))
+	wrongData, err := teeattestation.DomainHash("wrongtag", []byte(`{"key":"value"}`))
+	require.NoError(t, err)
 	err = ValidateAttestationWithRoots(doc, wrongData, fa.TrustedPCRsJSON(), fa.CARootsPEM())
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "expected user data")
@@ -64,7 +67,7 @@ func TestValidateAttestation_WrongPCRs(t *testing.T) {
 	fa, err := nitrofake.NewAttestor()
 	require.NoError(t, err)
 
-	userData := []byte("test-data")
+	userData := []byte("testdata")
 	doc, err := fa.CreateAttestation(userData)
 	require.NoError(t, err)
 
@@ -78,7 +81,8 @@ func TestValidateAndParse_SurfacesIdentityFields(t *testing.T) {
 	fa, err := nitrofake.NewAttestor()
 	require.NoError(t, err)
 
-	userData := teeattestation.DomainHash("test-tag", []byte(`{"key":"value"}`))
+	userData, err := teeattestation.DomainHash("testtag", []byte(`{"key":"value"}`))
+	require.NoError(t, err)
 	nonce := []byte("request-id-as-nonce")
 	identityKey := []byte("enclave-long-lived-identity-pubkey")
 
@@ -109,7 +113,8 @@ func TestValidateAndParse_FailsOnWrongUserData(t *testing.T) {
 	fa, err := nitrofake.NewAttestor()
 	require.NoError(t, err)
 
-	userData := teeattestation.DomainHash("test-tag", []byte(`{"key":"value"}`))
+	userData, err := teeattestation.DomainHash("testtag", []byte(`{"key":"value"}`))
+	require.NoError(t, err)
 	doc, err := fa.CreateAttestation(userData, nitrofake.WithNonce([]byte("n")))
 	require.NoError(t, err)