diff --git a/backend/internal/audit/record.go b/backend/internal/audit/record.go
index 0f353fe..1b02543 100644
--- a/backend/internal/audit/record.go
+++ b/backend/internal/audit/record.go
@@ -11,6 +11,7 @@ const (
 	StatusUpstreamRejected    Status = "upstream_rejected"
 	StatusUpstreamTimeout     Status = "upstream_timeout"
 	StatusUpstreamUnavailable Status = "upstream_unavailable"
+	StatusUpstreamInvalid     Status = "upstream_invalid_response"
 	StatusInternalError       Status = "internal_error"
 )
 
diff --git a/backend/internal/audit/sanitize.go b/backend/internal/audit/sanitize.go
new file mode 100644
index 0000000..a26a9fe
--- /dev/null
+++ b/backend/internal/audit/sanitize.go
@@ -0,0 +1,48 @@
+package audit
+
+import (
+	"encoding/json"
+	"strings"
+)
+
+func SanitizeResponse(raw []byte, maskedCard string) []byte {
+	if len(raw) == 0 {
+		return nil
+	}
+
+	var value any
+	if err := json.Unmarshal(raw, &value); err != nil {
+		return nil
+	}
+
+	if maskedCard == "" {
+		maskedCard = "[redacted]"
+	}
+
+	redactCardFields(value, maskedCard)
+
+	sanitized, err := json.Marshal(value)
+	if err != nil {
+		return nil
+	}
+
+	return sanitized
+}
+
+func redactCardFields(value any, maskedCard string) {
+	switch current := value.(type) {
+	case map[string]any:
+		for key, child := range current {
+			if strings.EqualFold(key, "card") {
+				current[key] = maskedCard
+				continue
+			}
+
+			redactCardFields(child, maskedCard)
+		}
+	case []any:
+		for _, child := range current {
+			redactCardFields(child, maskedCard)
+		}
+	}
+}
diff --git a/backend/internal/audit/sanitize_test.go b/backend/internal/audit/sanitize_test.go
new file mode 100644
index 0000000..7bec67d
--- /dev/null
+++ b/backend/internal/audit/sanitize_test.go
@@ -0,0 +1,56 @@
+package audit
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestSanitizeResponse(t *testing.T) {
+	raw := []byte(`{
+		"data": {
+			"card": "6037997512345678",
+			"iban": "IR820540102680020817909002"
+		},
+		"meta": {"isSuccess": true}
+	}`)
+
+	got := SanitizeResponse(raw, "603799******5678")
+	if len(got) == 0 {
+		t.Fatal("SanitizeResponse() returned an empty response")
+	}
+
+	var decoded map[string]any
+	if err := json.Unmarshal(got, &decoded); err != nil {
+		t.Fatalf("unmarshal sanitized response: %v", err)
+	}
+
+	data, ok := decoded["data"].(map[string]any)
+	if !ok {
+		t.Fatal("sanitized response does not contain data object")
+	}
+
+	if gotCard := data["card"]; gotCard != "603799******5678" {
+		t.Errorf("sanitized card = %v, want masked card", gotCard)
+	}
+
+	if gotIBAN := data["iban"]; gotIBAN != "IR820540102680020817909002" {
+		t.Errorf("sanitized iban = %v, want original iban", gotIBAN)
+	}
+}
+
+func TestSanitizeResponseRejectsInvalidJSON(t *testing.T) {
+	if got := SanitizeResponse([]byte(`not-json`), "603799******5678"); got != nil {
+		t.Errorf("SanitizeResponse() = %q, want nil", got)
+	}
+}
+
+func TestSanitizeResponseRedactsCardWhenMaskIsEmpty(t *testing.T) {
+	got := SanitizeResponse(
+		[]byte(`{"card":"6037997512345678"}`),
+		"",
+	)
+
+	if string(got) != `{"card":"[redacted]"}` {
+		t.Errorf("SanitizeResponse() = %s, want redacted card", got)
+	}
+}
diff --git a/backend/internal/card/validation.go b/backend/internal/card/validation.go
index a750bc6..c97e3bf 100644
--- a/backend/internal/card/validation.go
+++ b/backend/internal/card/validation.go
@@ -25,3 +25,11 @@ func Validate(number string) error {
 
 	return nil
 }
+
+func Mask(number string) string {
+	if Validate(number) != nil {
+		return ""
+	}
+
+	return number[:6] + "******" + number[12:]
+}
diff --git a/backend/internal/card/validation_test.go b/backend/internal/card/validation_test.go
index 418eb2e..97ebf06 100644
--- a/backend/internal/card/validation_test.go
+++ b/backend/internal/card/validation_test.go
@@ -53,3 +53,35 @@ func TestValidate(t *testing.T) {
 		})
 	}
 }
+
+func TestMask(t *testing.T) {
+	tests := []struct {
+		name   string
+		number string
+		want   string
+	}{
+		{
+			name:   "valid card number",
+			number: "6037997512345678",
+			want:   "603799******5678",
+		},
+		{
+			name:   "invalid card number",
+			number: "603799",
+			want:   "",
+		},
+		{
+			name:   "non numeric card number",
+			number: "603799ABC2345678",
+			want:   "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := Mask(tt.number); got != tt.want {
+				t.Errorf("Mask() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}