Question:
Is there any standard rule for firing AA against air units making a Harassment mission?

Answer:
Per Rule 20G2d, Harassment is a bombing mission. Per Rule 22B 1, the enemy player may fire AA against air units flying bombing missions. Harassment is not listed in Rule 22B1 as being one of the exceptions to the general rule on firing AA. Therefore, the standard rules apply, and any friendly units, except for naval units, may fire at any enemy air units flying a harassment mission in the hex the friendly units occupy. Advanced rules 43B1 and 43B2 simply modify the standard rules concerning AA fire versus this particular mission.

Source:
TEM 50