「カンニングをしているところを [見]{み}つかる。」= "I am found cheating (on the test)."
This sentence is 100% grammatical. If you analyzed it using the grammar of another language, however, it might look as though it were ungrammatical.
「見つかる」 , as you stated, is an intransitive verb, but it happens to fall into a group of intransitive verbs that hold the transitive-verb-like characteristics. In particular, these intransitive verbs are used just like transitive verbs in the passive voice with 「を」 attached to the direct object.
Japanese-learners would need to know that it is far more natural for us to say the sentence above than to say:
「カンニングをしているところを見つけられる。」
using a real transitive verb 「見つける」 in its passive voice form.
This group of intransitive verbs include: 「[教]{おそ}わる = "to be taught"」、「[授]{さず}かる = "to be blessed with"」、「ことづかる = "to be told to convey a message or give a present"」, etc.