From 4ea135dd8bfd7be74c7c4721a27e0c37014b5a24 Mon Sep 17 00:00:00 2001
From: Robert Ekl <ekl.rob@gmail.com>
Date: Tue, 24 Feb 2026 00:05:23 -0600
Subject: [PATCH] fix(utils): reject invalid hex characters in fromHex

Make hexVal return -1 for invalid input and validate each nibble in Utils::fromHex before decoding.

Also simplify isHexChar to rely on the same validation path, preventing invalid strings from being accepted as zero bytes.
---
 src/Utils.cpp | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/Utils.cpp b/src/Utils.cpp
index 186c8720a..7942739d5 100644
--- a/src/Utils.cpp
+++ b/src/Utils.cpp
@@ -108,15 +108,15 @@ void Utils::printHex(Stream& s, const uint8_t* src, size_t len) {
   }
 }
 
-static uint8_t hexVal(char c) {
+static int8_t hexVal(char c) {
   if (c >= 'A' && c <= 'F') return c - 'A' + 10;
   if (c >= 'a' && c <= 'f') return c - 'a' + 10;
   if (c >= '0' && c <= '9') return c - '0';
-  return 0;
+  return -1;
 }
 
 bool Utils::isHexChar(char c) {
-  return c == '0' || hexVal(c) > 0;
+  return hexVal(c) >= 0;
 }
 
 bool Utils::fromHex(uint8_t* dest, int dest_size, const char *src_hex) {
@@ -127,7 +127,10 @@ bool Utils::fromHex(uint8_t* dest, int dest_size, const char *src_hex) {
   while (dp - dest < dest_size) {
     char ch = *src_hex++;
     char cl = *src_hex++;
-    *dp++ = (hexVal(ch) << 4) | hexVal(cl);
+    int8_t hi = hexVal(ch);
+    int8_t lo = hexVal(cl);
+    if (hi < 0 || lo < 0) return false;  // invalid chars
+    *dp++ = (hi << 4) | lo;
   }
   return true;
 }
@@ -150,4 +153,4 @@ int Utils::parseTextParts(char* text, const char* parts[], int max_num, char sep
   return num;
 }
 
-}
\ No newline at end of file
+}