Islam does not permit the killing of civilians.
No classical Muslim scholar has ever condoned the killing of civilians, even in the midst of a justified war, just as they generally understood the prohibition of suicide to include killing one’s self in order to kill the enemy. Indiscriminate acts of violence against civilians, which are often attributed to Islam, therefore have zero precedent in Islamic teachings. It is for this reason that car bombings, suicide bombings, mass shootings, and other crimes committed by self-identified Muslims are so strongly condemned by Muslim leaders and communities around the world.
In Islam, the deliberate targeting of civilians in war is categorically unjustified, going against both the higher aims of Islamic law to protect all life, as well as the universal Islamic maxim which criminalizes any and all avoidable harm.