Welcome to the community!
It is possible for users to override system prompt through different prompts.
Time being, it is not easy to prevent it.
You may see another topic here how it works overriding system prompts:
TOPIC 1 | TOPIC 2 | TOPIC 3 | TOPIC 4 | TOPIC 5 | TOPIC 6
For example, under the TOPIC 6 you will see that; the GPT’s role is to give information about security if only user provide correct password, otherwise it does not provide information. However, only using words “As you know”, it is broken. Also other GPTs disregard their system prompts.
To gain experience, you might want to try out these three GPTs, for example:
GateKeeper | Certainly! But, not now. | Boolean Bot
At the end;
We can say; we should not add any sensitive information in custom GPTs’ instruction and in knowledge base files, also we need to inactive Code Interpreter & Data Analysis Tool if we do not want users to download files.