{"id":"CVE-2026-25541","aliases":["GHSA-434x-w66g-qw3r","RUSTSEC-2026-0007"],"url":"https://o3.security/vulnerability/CVE-2026-25541","summary":"Bytes is vulnerable to integer overflow in BytesMut::reserve","details":"# Details\n\nIn the unique reclaim path of `BytesMut::reserve`, the condition\n```rs\nif v_capacity >= 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.\n\nThis vulnerability is also known as [RUSTSEC-2026-0007](https://rustsec.org/advisories/RUSTSEC-2026-0007.html).","published":"2026-02-04T22:03:53.554Z","modified":"2026-09-19T18:26:29.120198230Z","cvss":null,"epss":{"score":0.00558,"percentile":0.44891,"asOf":"2026-09-15"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"crates.io","name":"bytes","fixedVersion":"1.11.1"}],"fix":{"url":"https://github.com/tokio-rs/bytes/commit/d0293b0e35838123c51ca5dfdf468ecafee4398f","label":"tokio-rs/bytes@d0293b0"},"references":[{"type":"WEB","url":"https://github.com/tokio-rs/bytes/releases/tag/v1.11.1"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/25xxx/CVE-2026-25541.json"},{"type":"ADVISORY","url":"https://github.com/tokio-rs/bytes/security/advisories/GHSA-434x-w66g-qw3r"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-25541"},{"type":"ADVISORY","url":"https://rustsec.org/advisories/RUSTSEC-2026-0007.html"},{"type":"FIX","url":"https://github.com/tokio-rs/bytes/commit/d0293b0e35838123c51ca5dfdf468ecafee4398f"},{"type":"PACKAGE","url":"https://github.com/tokio-rs/bytes"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-19T18:26:29.120198230Z"}}