static char *
mymemreplace(const char *str, Py_ssize_t len,           
         const char *pat, Py_ssize_t pat_len,            pattern string to find */
         const char *sub, Py_ssize_t sub_len,            substitution string */
         Py_ssize_t count,                               number of replacements */
         Py_ssize_t *out_len)
{
    [...]

    new_len = len + nfound*(sub_len - pat_len); <<<< Unchecked arithmetic can overflow here.
    if (new_len == 0) {
        /* Have to allocate something for the caller to free(). */
        out_s = (char *)PyMem_MALLOC(1);
        if (out_s == NULL)
            return NULL;
        out_s[0] = '\0';
    }
    else {
        assert(new_len > 0);
        new_s = (char *)PyMem_MALLOC(new_len); <<<< An allocation is performed using overflowed value.
        if (new_s == NULL)
            return NULL;
        out_s = new_s;

        for (; count > 0 && len > 0; --count) { <<<< Memory is copied to new_s using len, which can be greater than the overflowed new_len value.
            /* find index of next instance of pattern */
            offset = mymemfind(str, len, pat, pat_len);
            if (offset == -1)
                break;

            /* copy non matching part of input string */
            memcpy(new_s, str, offset);
            str += offset + pat_len;
            len -= offset + pat_len;

            /* copy substitute into the output string */
            new_s += offset;
            memcpy(new_s, sub, sub_len);
            new_s += sub_len;
        }
        /* copy any remaining values into output string */
        if (len > 0)
            memcpy(new_s, str, len);
    }