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

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Needs clarification
Activity status
Active
Tech stack
rust
Domain
security

Research direction

Start by reading the advisory details and checking how event-listener 5.4.1 enters this repository. Identify the dependency manifest or lockfile involved and confirm the affected version's usage. Done means the unsound dependency is addressed and the repository no longer resolves the affected version.

Written by the indexing model from the issue text.

Description

event-listener allows !Send tags to cross thread boundaries via StackSlot

Details
Status unsound
Package event-listener
Version 5.4.1
URL https://github.com/smol-rs/event-listener/pull/163
Date 2026-07-13

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.

See advisory page for additional details.

Dominant language
Rust
Stars
10
Forks
5
Avg merge
4h 41m
Merged PRs (30d)
4

Contributor guide

No contributing guide indexed for this repository

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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