Which issued Freedom of Information requests to 141 UK universities for documents setting out the institutions' right to vary courses after enrolment.
Of the 131 who responded, 40 were considered by Which to have policies that demonstrated bad practice while eight needed an improvement in terms.
Universities UK said it was engaging with new guidance for the sector.
With undergraduates now paying up to £9,000 a year on tuition fees, the higher education world is increasingly seen by students as a consumer market.
Which is calling on universities to address unfair terms "as a matter of urgency".
It suggests the higher education sector comes together to create a standard consumer-friendly format for student contracts.
It also calls on the Competition and Markets Authority to check which universities are complying with its new guidance on how consumer law applies to the sector.
The study by Which also found:
- Six in 10 (58%) students had experienced a change to their course such as changes to modules or location of teaching
- One in 10 (12%) had experienced an increase in tuition fees either part-way through the year or between years
- A third (35%) of students that had experienced one or more changes thought one or more of these was unfair.
