# RUSTSEC-2026-0221: `event-listener` allows `!Send` tags to cross thread boundaries via `StackSlot`

> The event-listener library allows unsafe thread boundary crossings for non-Send types, causing data races in safe code.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Not stated
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 13 Jul 2026

## What is RUSTSEC-2026-0221?

RUSTSEC-2026-0221 is a vulnerability in event-listener, published on 13 Jul 2026. 

### Official description

Affected versions of `event-listener` unconditionally implement `Send` and
`Sync` for `StackSlot<'_, T>`, the stack-allocated listener type created
by the `listener!` macro.

This allows a `!Send` tag type set via `Event::with_tag` to be moved to
another thread and accessed via `StackSlot::wait`, causing a data race in safe
code.

## Which versions are affected by RUSTSEC-2026-0221?

1 open source package ship an affected version, including event-listener.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `event-listener` | crates.io | From 5.1.0 | 5.4.2 |

## How do I fix RUSTSEC-2026-0221?

Upgrade event-listener to 5.4.2 or later.

## Is RUSTSEC-2026-0221 being exploited?

There is no known exploitation of RUSTSEC-2026-0221 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0221 and fix it if it is affected.

What it is: RUSTSEC-2026-0221 is a vulnerability in event-listener, published on 13 Jul 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0221 in the wild and no public exploit code.

Affected packages and fixed versions:
- event-listener (crates.io): affected from 5.1.0; fixed in 5.4.2

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-2026-0221/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0221/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0221

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0221/
Summaries are generated and may contain mistakes; the official description above is the record.