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
25/100
Issue type
Bug
Clarity
Needs clarification
Activity status
Stale
Tech stack
rust
Domain
security

Research direction

The report names event-listener 5.4.1 and links upstream pull request 163; start by checking whether this repository depends on that version and reviewing the advisory. Done means the affected dependency is no longer present at the vulnerable 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
2.7k
Forks
162
Avg merge
1d 14h
Merged PRs (30d)
7

Contributor guide

Open the contributing guide

First steps

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  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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