CVE-2026-25541

Date: February 4, 2026

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.