Protecting RS-232 from overvoltages
RS-232 transceivers such as MAX232 are guaranteed to survive connection (inputs or outputs) to +/-25V without damage. That's what they are designed for.

If  you're using CMOS levels direct from the MCU chip, the limits are more like Vss - 0.3 to Vdd + 0.3V. You may be able to protect them by adding some series resistance (a few K) and operating at a relatively low baud rate to compensate for the resulting more sluggish waveforms. The added resistance limits the current, but does not clamp the voltage (except via the on-chip protection networks, so it will exceed the voltages but current is limited), which may be acceptable. You could add unidirectional TVS diodes to the pins and a bit of series resistance for more protection.