How can endothermic reaction be spontaneous?

Under normal conditions if constant temperature and pressure, the spontaneity of reaction is determined by 2 factors: changes in enthalpy, $\Delta$H and entropy, $\Delta$S. The overall change in Gibbs Free Energy is
$\Delta$G = $\Delta$H - T$\Delta$S
If $\Delta$G is negative the reaction is spontaneous. Usually this equation is dominated by enthalpy changes but in rare cases entropy can determine spontaneity.