From c62b9e0480372c4c42986df1450f139e0ebb6a55 Mon Sep 17 00:00:00 2001
From: iceman1001 <iceman@iuse.se>
Date: Tue, 24 Mar 2026 14:21:09 +0700
Subject: [PATCH] add guard for size_t underflow

---
 armsrc/desfire_crypto.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/armsrc/desfire_crypto.c b/armsrc/desfire_crypto.c
index b27d19595..afe66345e 100644
--- a/armsrc/desfire_crypto.c
+++ b/armsrc/desfire_crypto.c
@@ -600,6 +600,15 @@ void *mifare_cryto_postprocess_data(desfiretag_t tag, void *data, size_t *nbytes
 
                     if ((communication_settings & MAC_VERIFY) == MAC_VERIFY) {
 
+                        if (*nbytes < key_macing_length(key)) {
+                            *nbytes = -1;
+                            res = NULL;
+#ifdef WITH_DEBUG
+                            Dbprintf("No room for MAC!");
+#endif
+                            break;
+                        }
+
                         *nbytes -= key_macing_length(key);
 
                         if (*nbytes == 0) {