# RUSTSEC-2026-0386: `AsyncFd::new` and related constructors violate I/O safety


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 13 Oct 2026

## What is RUSTSEC-2026-0386?

RUSTSEC-2026-0386 is a vulnerability in tokio, published on 13 Oct 2026. 

### Official description

`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<T>` 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 remains valid, open, and unchanged for the lifetime of the `AsyncFd`.

## Which versions are affected by RUSTSEC-2026-0386?

1 open source package ship an affected version, including tokio.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `tokio` | crates.io | From 0.3.2 | 1.51.6, 1.53.3 |

## How do I fix RUSTSEC-2026-0386?

Upgrade tokio to a fixed version for your release line: 1.51.6 or 1.53.3.

## Is RUSTSEC-2026-0386 being exploited?

There is no known exploitation of RUSTSEC-2026-0386 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0386 and fix it if it is affected.

What it is: RUSTSEC-2026-0386 is a vulnerability in tokio, published on 13 Oct 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0386 in the wild and no public exploit code.

Affected packages and fixed versions:
- tokio (crates.io): affected from 0.3.2; fixed in 1.51.6, 1.53.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/RUSTSEC-2026-0386/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0386/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0386

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0386/
Summaries are generated and may contain mistakes; the official description above is the record.