diff --git a/hw/uefi/var-service-auth.c b/hw/uefi/var-service-auth.c
index 795f2f54e4..f3dc9c6ca6 100644
--- a/hw/uefi/var-service-auth.c
+++ b/hw/uefi/var-service-auth.c
@@ -194,7 +194,7 @@ static efi_status uefi_vars_check_auth_2_sb(uefi_vars_state *uv,
         return EFI_SUCCESS;
     }
 
-    if (auth.hdr_length == 24) {
+    if (auth.hdr_length == (sizeof(auth) - sizeof(auth.timestamp))) {
         /* no signature (auth->cert_data is empty) */
         return EFI_SECURITY_VIOLATION;
     }
@@ -228,6 +228,9 @@ efi_status uefi_vars_check_auth_2(uefi_vars_state *uv, uefi_variable *var,
     }
     memcpy(&auth, data, sizeof(auth));
 
+    if (auth.hdr_length < (sizeof(auth) - sizeof(auth.timestamp))) {
+        return EFI_SECURITY_VIOLATION;
+    }
     if (uadd64_overflow(sizeof(efi_time), auth.hdr_length, &data_offset)) {
         return EFI_SECURITY_VIOLATION;
     }
diff --git a/hw/uefi/var-service-pkcs7.c b/hw/uefi/var-service-pkcs7.c
index c859743e86..8a1f1395a2 100644
--- a/hw/uefi/var-service-pkcs7.c
+++ b/hw/uefi/var-service-pkcs7.c
@@ -113,9 +113,9 @@ static gnutls_datum_t *build_pkcs7(void *data)
 
     memcpy(&auth, data, sizeof(auth));
     pkcs7 = g_new(gnutls_datum_t, 1);
-    pkcs7->size = auth.hdr_length - 24;
+    pkcs7->size = auth.hdr_length - (sizeof(auth) - sizeof(auth.timestamp));
     pkcs7->data = g_malloc(pkcs7->size);
-    memcpy(pkcs7->data, data + 16 + 24, pkcs7->size);
+    memcpy(pkcs7->data, data + sizeof(auth), pkcs7->size);
 
     wrap_pkcs7(pkcs7);