Supabase MCP can leak your entire SQL database

Supabase engineer here working on MCP. A few weeks ago we added the following mitigations to help with prompt injections:

- Encourage folks to use read-only by default in our docs [1]

- Wrap all SQL responses with prompting that discourages the LLM from following instructions/commands injected within user data [2]

- Write E2E tests to confirm that even less capable LLMs don't fall for the attack [2]

We noticed that this significantly lowered the chances of LLMs falling for attacks - even less capable models like Haiku 3.5. The attacks mentioned in the posts stopped working after this. Despite this, it's important to call out that these are mitigations. Like Simon mentions in his previous posts, prompt injection is generally an unsolved problem, even with added guardrails, and any database or information source with private data is at risk.

Here are some more things we're working on to help:

- Fine-grain permissions at the token level. We want to give folks the ability to choose exactly which Supabase services the LLM will have access to, and at what level (read vs. write)

- More documentation. We're adding disclaimers to help bring awareness to these types of attacks before folks connect LLMs to their database

- More guardrails (e.g. model to detect prompt injection attempts). Despite guardrails not being a perfect solution, lowering the risk is still important

Sadly General Analysis did not follow our responsible disclosure processes [3] or respond to our messages to help work together on this.

[1] https://github.com/supabase-community/supabase-mcp/pull/94

[2] https://github.com/supabase-community/supabase-mcp/pull/96

[3] https://supabase.com/.well-known/security.txt

While I'm far from an expert in security, the time I've spent studying cryptography and cryptosystem design has made me extremely wary of words like "encourage" and "discourage", and "significantly lowered the chances" as a means of achieving security.

I'm honestly a bit surprised this is a the public response to actions being taken to increase security around attacks like these. Cryptosystems are not built around "being really hopeful" but making mathematical guarantees about the properties of the system (and of course, even then no system is perfect nor should be treated as such).

This reads more like "engineering optimism" than the "professional paranoia" encouraged by Schneier et al in Cryptography Engineering.

Yeah this is insane, and it highlights the fact that fundamental strength of LLMs is also its fundamental weakness: it’s a probabilistic black box, not a deterministic algorithm. By its very nature, you cannot secure a probabilistic black box, and you certainly can’t give it permissions that allow it access to sensitive data. The people working on this have got to realize this, but they’re doing it anyway.

I was recently part of a team at work that was taking a look at a product that uses LLMs to prepare corporate taxes. I have nothing to do with accounting, but I was on the demo because of my technical knowledge. The guys on the other end of the call were hyping this thing to no end, thinking we were all accountants. As expected, the accountants I work with were eating it up until I started asking about a word they were not even aware of in the context of these systems: hallucination. I asked what the hallucination rate was and whether they’ve had issues with the system just making up numbers. They responded with “it happens but I would say it’s accurate 98% of the time.” They said that with a straight face. The number told me they don’t actually know the hallucination rate, and this is not the kind of work where you want to fuck it up any percent of the time. Hallucinations are incompatible with corporate finance.

Again - using a probabilistic tool where only a deterministic tool will do.