Submeter #544230: Open Source STB Project (https://github.com/nothings/stb) Latest (<= commit f056911) stb_dupreplace Integer Overflow -> Under Allocation

The function allocates memory for the resulting string using the expression:

p = (char *) malloc(strlen(src) + count * (len_replace - len_find) + 1);

The allocation size is calculated based on the original string length and the net difference from replacing occurrences of `find` with `replace`. However, if `len_replace` is smaller than `len_find`, the subtraction (`len_replace - len_find`) becomes negative. This can result in an allocation that is too small to hold the resulting string when the total decrease is subtracted from `strlen(src)`. Consequently, during the subsequent copying and replacement operations, the function may write past the end of the allocated buffer, leading to a buffer overflow and potential memory corruption.

char *stb_dupreplace(char *src, char *find, char *replace)
{
size_t len_find = strlen(find);
size_t len_replace = strlen(replace);
int count = 0;
char *s, *p;

// Count occurrences of 'find' in 'src'
s = strstr(src, find);
if (s == NULL)
return stb_p_strdup(src);
do {
++count;
s = strstr(s + len_find, find);
} while (s != NULL);

// Vulnerable allocation: may under-allocate if len_replace < len_find
p = (char *) malloc(strlen(src) + count * (len_replace - len_find) + 1);
if (p == NULL)
return NULL;

// ... (remaining replacement logic)
return p;
}

**Reproduction Steps:**
1. **Prepare Malicious Input:**
Craft a source string (`src`) containing one or more occurrences of the substring `find` and choose a `replace` string such that `len_replace` is strictly less than `len_find`. This ensures that the term `count * (len_replace - len_find)` subtracts from `strlen(src)`.
2. **Invoke the Function:**
Call `stb_dupreplace(src, find, replace)` with the crafted input.
3. **Trigger the Overflow:**
As the function performs the string replacement, the copying operations may exceed the bounds of the allocated buffer.

• Utilizador: ninpwn (UID 82253)
• Submissão: 27/03/2025 15h53 (há 1 Ano)
• Moderação: 07/04/2025 12h56 (11 days later)
• Estado: Aceite
• Entrada VulDB: 303686 [Nothings stb até f056911 stb_dupreplace Excesso de tampão]
• Pontos: 17