CVE-2026-25541

Bytes is a utility library for working with bytes. From version 1.2.1 to before 1.11.1, Bytes is vulnerable to integer overflow in BytesMut::reserve. In the unique reclaim path of BytesMut::reserve, if the condition "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. This issue has been patched in version 1.11.1.

Bytes is a utility library for working with bytes. From version 1.2.1 to before 1.11.1, Bytes is vulnerable to integer overflow in BytesMut::reserve. In the unique reclaim path of BytesMut::reserve, if the condition "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. This issue has been patched in version 1.11.1.

In the unique reclaim path of `BytesMut::reserve`, the condition ```rs 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 ```rs 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.

# Details In the unique reclaim path of `BytesMut::reserve`, the condition ```rs 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 ```rs 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. This vulnerability is also known as [RUSTSEC-2026-0007](https://rustsec.org/advisories/RUSTSEC-2026-0007.html).

Bytes es una librería de utilidad para trabajar con bytes. Desde la versión 1.2.1 hasta antes de la 1.11.1, Bytes es vulnerable a desbordamiento de entero en BytesMut::reserve. En la ruta de recuperación única de BytesMut::reserve, si la condición 'v_capacity >= new_cap + offset' utiliza una adición sin verificar. Cuando new_cap + offset desborda usize en compilaciones de lanzamiento, esta condición puede pasar incorrectamente, haciendo que self.cap se establezca en un valor que excede la capacidad asignada real. Las API posteriores, como spare_capacity_mut(), confían entonces en este valor de cap corrupto y pueden crear segmentos fuera de límites, lo que lleva a UB. Este comportamiento es observable en compilaciones de lanzamiento (el desbordamiento de entero se ajusta), mientras que las compilaciones de depuración entran en pánico debido a las comprobaciones de desbordamiento. Este problema ha sido parcheado en la versión 1.11.1.