The following presentation at this year's DEF CON was brought to our attention on the SQLx Discord:
SQL Injection isn't Dead: Smuggling Queries at the Protocol Level
http://web.archive.org/web/20240812130923/https://media.defcon.org/DEF%20CON%2032/DEF%20CON%2032%20presentations/DEF%20CON%2032%20-%20Paul%20Gerste%20-%20SQL%20Injection%20Isn't%20Dead%20Smuggling%20Queries%20at%20the%20Protocol%20Level.pdf
(Archive link for posterity.)
Essentially, encoding a value larger than 4GiB can cause the length prefix in the protocol to overflow, causing the server to interpret the rest of the string as binary protocol commands or other data.
It appears SQLx does perform truncating casts in a way that could be problematic, for example: https://github.com/launchbadge/sqlx/blob/6f2905695b9606b5f51b40ce10af63ac9e696bb8/sqlx-postgres/src/arguments.rs#L163
This code has existed essentially since the beginning,
so it is reasonable to assume that all published versions <= 0.8.0 are affected.
Mitigation
As always, you should make sure your application is validating untrustworthy user input. Reject any input over 4 GiB, or any input that could encode to a string longer than 4 GiB. Dynamically built queries are also potentially problematic if it pushes the message size over this 4 GiB bound.
Encode::size_hint()
can be used for sanity checks, but do not assume that the size returned is accurate.
For example, the Json<T> and Text<T> adapters have no reasonable way to predict or estimate the final encoded size,
so they just return size_of::<T>() instead.
For web application backends, consider adding some middleware that limits the size of request bodies by default.
Resolution
sqlx 0.8.1 has been released with the fix: https://github.com/launchbadge/sqlx/blob/main/CHANGELOG.md#081---2024-08-23
Postgres users are advised to upgrade ASAP as a possible exploit has been demonstrated: https://github.com/launchbadge/sqlx/issues/3440#issuecomment-2307956901
MySQL and SQLite do not appear to be exploitable, but upgrading is recommended nonetheless.