Affected versions of this crate did not properly detect invalid requests that could allow HTTP/1 request smuggling (HRS) attacks when running alongside a vulnerable front-end proxy server. This can result in leaked internal and/or user data, including credentials, when the front-end proxy is also vulnerable.

Popular front-end proxies and load balancers already mitigate HRS attacks so it is recommended that they are also kept up to date; check your specific set up. You should upgrade even if the front-end proxy receives exclusively HTTP/2 traffic and connects to the back-end using HTTP/1; several downgrade attacks are known that can also expose HRS vulnerabilities.

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.