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.

In the unique reclaim path of BytesMut::reserve, the condition

if v_capacity >= new_cap + offset

uses an unchecked addition. When new_cap + offset overflows usize in release builds, this condition may incorrectly pass, causing self.cap to be set to a value that exceeds the actual allocated capacity. Subsequent APIs such as spare_capacity_mut() then trust this corrupted cap value and may create out-of-bounds slices, leading to UB.

This behavior is observable in release builds (integer overflow wraps), whereas debug builds panic due to overflow checks.

PoC

use bytes::*;

fn main() {
    let mut a = BytesMut::from(&b"hello world"[..]);
    let mut b = a.split_off(5);

    // Ensure b becomes the unique owner of the backing storage
    drop(a);

    // Trigger overflow in new_cap + offset inside reserve
    b.reserve(usize::MAX - 6);

    // This call relies on the corrupted cap and may cause UB & HBO
    b.put_u8(b'h');
}

Workarounds

Users of BytesMut::reserve are only affected if integer overflow checks are configured to wrap. When integer overflow is configured to panic, this issue does not apply.