Just seen this on another thread:
Tie-breaking rules
1. Number of points
2. Higher number of points obtained in the group matches played among the teams in question
3. Superior goal difference from the group matches played among the teams in question
4. Higher number of goals scored in the group matches played among the teams in question
5. Higher number of goals scored away from home in the group matches played among the teams in question
6. If, after applying criteria 2) to 5) to several teams, two teams still have an equal ranking, criteria 2) to 5) are reapplied exclusively to the matches between the two teams in question to determine their final rankings. If this procedure does not lead to a decision, criteria 7) and 8) apply
7. Superior goal difference from all group matches played
8. Higher number of goals scored from all group matches played