It was the away side who started the better and they might have taken the lead after nine minutes with an effort from Antoine Griezmann, but he fired wide.
Santiago Cazorla might have doubled Malaga's lead just before the 30-minute mark, but he fired wide.
Carlos Vela came close for the away side late in the half, but his powerful effort was blocked.
It got worse for Sociedad late in the half, however, when Mikel Gonzalez was dismissed for a second bookable offence.
Sociedad levelled the match moments into the second period, however, when Prieto found the bottom corner following a free-kick.
Monreal was next to come close for the home side after 67 minutes, but his effort from outside the box was blocked.
It was the home side who continued to dominate in the latter stages, but they could not make the breakthrough, as both shared the points.