PT-2026-6538 · Crates.Io · Bytes
Published
2026-02-03
·
Updated
2026-02-03
None
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In the unique reclaim path of
BytesMut::reserve, the conditionrs
if v capacity >= new cap + offsetuses 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. Found an issue in the description? Have something to add? Feel free to write us 👾
Related Identifiers
Affected Products
Bytes