Integer overflow in BytesMut::reserve

Reported February 3, 2026 Issued February 3, 2026 Package bytes (crates.io) Type Vulnerability Categories

• memory-corruption Keywords #integer-overflow Aliases

• GHSA-434x-w66g-qw3r References

• https://github.com/advisories/GHSA-434x-w66g-qw3r Patched

• >=1.11.1 Unaffected

• <1.2.1

Description

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.

Advisory available under CC-BY-4.0 license. Source: https://github.com/advisories/GHSA-434x-w66g-qw3r