PT-2026-6355 · Crates.Io · Bytes

Published

2026-02-03

·

Updated

2026-02-03

CVSS v4.0

5.4

Medium

VectorAV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:P

Details

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.
This vulnerability is also known as RUSTSEC-2026-0007.

Fix

Weakness Enumeration

CWE-680

Related Identifiers

GHSA-434X-W66G-QW3R

Affected Products

Bytes