`AsyncFd::new` and related constructors violate I/O safety

{
  "id": "6aa202b95f86b1ac7ad346d75f1ab8231ad5e2c6ef7e88f1db59bb197dfe9ef2",
  "url": "https://osv.dev/vulnerability/RUSTSEC-2026-0386",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "`AsyncFd::new` and related constructors violate I/O safety",
    "summary": "`tokio::io::unix::AsyncFd` provides safe constructors (`new`, `with_interest`, `try_new`, and `try_with_interest`) that accept any `T: AsRawFd` rather than requiring proof of ownership or validity of the file descriptor. Because `RawFd` implements `AsRawFd`, safe code can construct an `AsyncFd` from an arbitrary or closed file descriptor, or close/replace the underlying file descriptor while the `AsyncFd` is still alive. This violates I/O safety in two ways: 1. `AsyncFd` implements `AsFd` for all `T: AsRawFd` by calling `BorrowedFd::borrow_raw(self.as_raw_fd())`, allowing safe code to obtain an arbitrary `BorrowedFd<'_>` without owning the file descriptor. 2. Tokio's I/O driver stores a pointer to an internal allocation in the `epoll`/`kqueue` user data for registered file descriptors. If the underlying file descriptor is duplicated and then closed while an `AsyncFd` is alive, and the OS reuses that file descriptor number for a another IO resource on the same runtime, dropping the first `AsyncFd` deregisters the reused file descriptor number and frees the first while the original file description remains registered in `epoll`, leading to a use-after-free when a readiness event fires. Starting in Tokio 1.51.6, 1.53.3, and 1.54.0, `AsyncFd::new`, `AsyncFd::with_interest`, `AsyncFd::try_new`, and `AsyncFd::try_with_interest` are deprecated in favor of the `unsafe` constructors `AsyncFd::register` and `AsyncFd::register_with_interest`. Callers must ensure that the file descriptor",
    "evidence": "osv:RUSTSEC-2026-0386",
    "products": [
      {
        "name": {
          "value": "tokio",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "1.51.6",
            "evidence": "/affected/0/ranges"
          },
          {
            "value": "1.53.3",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "unknown",
    "structured": {
      "id": "RUSTSEC-2026-0386",
      "aliases": [
        "RUSTSEC-2026-0386"
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "0.3.2"
                },
                {
                  "fixed": "1.51.6"
                },
                {
                  "introduced": "1.52.0"
                },
                {
                  "fixed": "1.53.3"
                }
              ]
            }
          ],
          "package": {
            "name": "tokio",
            "purl": "pkg:cargo/tokio",
            "ecosystem": "crates.io"
          },
          "database_specific": {
            "cvss": null,
            "source": "https://github.com/rustsec/advisory-db/blob/osv/crates/RUSTSEC-2026-0386.json",
            "categories": [
              "memory-corruption"
            ],
            "informational": "unsound"
          },
          "ecosystem_specific": {
            "affects": {
              "os": [],
              "arch": [],
              "functions": [
                "tokio::io::unix::AsyncFd::new",
                "tokio::io::unix::AsyncFd::try_new",
                "tokio::io::unix::AsyncFd::try_with_interest",
                "tokio::io::unix::AsyncFd::with_interest"
              ]
            },
            "affected_functions": null
          }
        }
      ],
      "modified": "2026-10-23T14:45:03.847864944Z",
      "provider": "osv",
      "references": [
        "https://crates.io/crates/tokio",
        "https://rustsec.org/advisories/RUSTSEC-2026-0386.html",
        "https://github.com/tokio-rs/tokio/pull/8587",
        "https://github.com/tokio-rs/tokio/issues/8538",
        "https://github.com/tokio-rs/tokio/issues/8539"
      ],
      "provider_metadata": {
        "license": "CC0-1.0"
      }
    },
    "related_cves": [],
    "security_relevant": true,
    "severity_evidence": null
  },
  "source_id": "core-osv",
  "document_id": 413703,
  "published_at": "2026-10-13T12:00:00+00:00"
}