Integer overflow in `BytesMut::reserve`

{
  "id": "90cb0b4e5f511cd9d7684a30c949bec38d650b8b1e6c7e7e32f1d20e5a4ede64",
  "url": "https://osv.dev/vulnerability/RUSTSEC-2026-0007",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Integer overflow in `BytesMut::reserve`",
    "summary": "In the unique reclaim path of `BytesMut::reserve`, the condition ```rs 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 ```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.",
    "evidence": "osv:RUSTSEC-2026-0007",
    "products": [
      {
        "name": {
          "value": "bytes",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "1.11.1",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "unknown",
    "structured": {
      "id": "RUSTSEC-2026-0007",
      "aliases": [
        "CVE-2026-25541",
        "GHSA-434x-w66g-qw3r",
        "RUSTSEC-2026-0007"
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "1.2.1"
                },
                {
                  "fixed": "1.11.1"
                }
              ]
            }
          ],
          "package": {
            "name": "bytes",
            "purl": "pkg:cargo/bytes",
            "ecosystem": "crates.io"
          },
          "database_specific": {
            "cvss": null,
            "source": "https://github.com/rustsec/advisory-db/blob/osv/crates/RUSTSEC-2026-0007.json",
            "categories": [
              "memory-corruption"
            ],
            "informational": null
          },
          "ecosystem_specific": {
            "affects": {
              "os": [],
              "arch": [],
              "functions": []
            },
            "affected_functions": null
          }
        }
      ],
      "modified": "2026-10-12T18:00:09.239418978Z",
      "provider": "osv",
      "references": [
        "https://crates.io/crates/bytes",
        "https://rustsec.org/advisories/RUSTSEC-2026-0007.html",
        "https://github.com/tokio-rs/bytes/commit/d0293b0e35838123c51ca5dfdf468ecafee4398f"
      ],
      "provider_metadata": {
        "license": "CC-BY-4.0"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-25541",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": null
  },
  "source_id": "core-osv",
  "document_id": 306398,
  "published_at": "2026-02-03T12:00:00+00:00"
}