CVE-2026-25541 — Bytes is vulnerable to integer overflow in BytesMut::reserve

Affected: tokio-rs bytes

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.

GHSA-434x-w66g-qw3r — bytes has integer overflow in BytesMut::reserve

Affected: crates.io: bytes

# 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).

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CVE-2026-25541 — Bytes is vulnerable to integer overflow in BytesMut::reserve

GHSA-434x-w66g-qw3r — bytes has integer overflow in BytesMut::reserve