{"id":"RUSTSEC-2026-0007","summary":"Integer overflow in `BytesMut::reserve`","details":"In the unique reclaim path of `BytesMut::reserve`, the condition\n```rs\nif v_capacity \u003e= new_cap + offset\n```\nuses 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.\n\nThis behavior is observable in release builds (integer overflow wraps), whereas debug builds panic due to overflow checks.\n\n## PoC\n\n```rs\nuse bytes::*;\n\nfn main() {\n    let mut a = BytesMut::from(&b\"hello world\"[..]);\n    let mut b = a.split_off(5);\n\n    // Ensure b becomes the unique owner of the backing storage\n    drop(a);\n\n    // Trigger overflow in new_cap + offset inside reserve\n    b.reserve(usize::MAX - 6);\n\n    // This call relies on the corrupted cap and may cause UB & HBO\n    b.put_u8(b'h');\n}\n```\n\n# Workarounds\n\nUsers 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.","aliases":["CVE-2026-25541","GHSA-434x-w66g-qw3r"],"modified":"2026-02-04T06:56:11Z","published":"2026-02-03T12:00:00Z","database_specific":{"license":"CC-BY-4.0"},"references":[{"type":"PACKAGE","url":"https://crates.io/crates/bytes"},{"type":"ADVISORY","url":"https://rustsec.org/advisories/RUSTSEC-2026-0007.html"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-434x-w66g-qw3r"}],"affected":[{"package":{"name":"bytes","ecosystem":"crates.io","purl":"pkg:cargo/bytes"},"ranges":[{"type":"SEMVER","events":[{"introduced":"1.2.1"},{"fixed":"1.11.1"}]}],"ecosystem_specific":{"affects":{"arch":[],"os":[],"functions":[]},"affected_functions":null},"database_specific":{"categories":["memory-corruption"],"cvss":null,"source":"https://github.com/rustsec/advisory-db/blob/osv/crates/RUSTSEC-2026-0007.json","informational":null}}],"schema_version":"1.7.3"}