void append_stat(const char *name, ADD_STAT add_stats, conn *c,
                 const char *fmt, ...) {
    char val_str[STAT_VAL_LEN];
    int vlen;
    va_list ap;

    assert(name);
    assert(add_stats);
    assert(c);
    assert(fmt);

    va_start(ap, fmt);
    vlen = vsnprintf(val_str, sizeof(val_str) - 1, fmt, ap);
    va_end(ap);

    add_stats(name, strlen(name), val_str, vlen, c);
}

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.