Details

In the unique reclaim path of BytesMut::reserve, the condition rs if vcapacity >= newcap + offset uses an unchecked addition. When newcap + 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 sparecapacitymut() 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.splitoff(5);

// Ensure b becomes the unique owner of the backing storage drop(a);

// Trigger overflow in newcap + offset inside reserve b.reserve(usize::MAX - 6);

// This call relies on the corrupted cap and may cause UB & HBO b.putu8(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.