RUSTSEC-2026-0007: Integer overflow in `BytesMut::reserve`

What to do

1. See which version you have: cargo tree -i bytes (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p bytes. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example bytes = "1.11.1") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

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.

References

• Package: crates.io/crates/bytes
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0007.html
• Web: github.com/tokio-rs/bytes/commit/d0293b0e35838123c51ca5dfdf468ecafee4398f

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.