From a145022c28d1a04d620bcdd2b55ff48673f83080 Mon Sep 17 00:00:00 2001
From: 94xhn <87560781+94xhn@users.noreply.github.com>
Date: Wed, 15 Jul 2026 02:08:33 +0800
Subject: [PATCH] Fix unsigned underflow in SimpleString::subString() on empty
 strings

subString(beginPos, amount) rejected out-of-range beginPos with
`if (beginPos > size()-1) return "";`. Since size() returns size_t,
calling this on an empty string (size() == 0) makes size()-1 wrap
around to SIZE_MAX, so the bounds check is defeated for any beginPos
and the function falls through to an out-of-bounds read of the
string's internal buffer.

Use `beginPos >= size()` instead, which is mathematically equivalent
to the original check for every size() >= 1 and additionally handles
size() == 0 correctly.
---
 src/CppUTest/SimpleString.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/CppUTest/SimpleString.cpp b/src/CppUTest/SimpleString.cpp
index a62491e32..924dea256 100644
--- a/src/CppUTest/SimpleString.cpp
+++ b/src/CppUTest/SimpleString.cpp
@@ -580,7 +580,7 @@ void SimpleString::padStringsToSameLength(SimpleString& str1, SimpleString& str2
 
 SimpleString SimpleString::subString(size_t beginPos, size_t amount) const
 {
-    if (beginPos > size()-1) return "";
+    if (beginPos >= size()) return "";
 
     SimpleString newString = getBuffer() + beginPos;