# RUSTSEC-2026-0301: Double free in `StackVec::retain` when a predicate or element `Drop` panics

> A double-free vulnerability in StackVec::retain allows attackers to cause memory corruption or crashes if a panic occurs during element removal.

- **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:** 22 Sep 2026

## What is RUSTSEC-2026-0301?

RUSTSEC-2026-0301 is a memory corruption vulnerability in stack_collections, published on 22 Sep 2026. 

### Official description

## Summary

`StackVec::retain` committed its new length to `self.len` only after
its internal loop completed. If the retain predicate or a removed
element's `Drop` implementation panicked before the loop finished,
unwinding proceeded with `self.len` still equal to the original,
pre-retain length, leaving either a duplicated or already-destroyed
element inside `0..len`. `StackVec`'s own `Drop` then revisited that
slot, causing a double-drop (and for heap-owning types, a double-free).

## Impact

Affects `StackVec<T, CAP>::retain` for `T: Drop` types where the
predicate or the removed element's destructor can panic, on builds
with unwinding enabled (`panic = "unwind"`). `no_std`/`panic = "abort"`
builds cannot trigger this, since unwinding never occurs.

## Patch

Fixed in 0.3.3 using an unwind-safe backshift guard, matching the
approach `alloc::vec::Vec::retain` uses.

## Which versions are affected by RUSTSEC-2026-0301?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `stack_collections` | crates.io | From 0.3.0 | 0.3.3 |

## How do I fix RUSTSEC-2026-0301?

Upgrade stack_collections to 0.3.3 or later.

## Is RUSTSEC-2026-0301 being exploited?

There is no known exploitation of RUSTSEC-2026-0301 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0301 and fix it if it is affected.

What it is: RUSTSEC-2026-0301 is a memory corruption vulnerability in stack_collections, published on 22 Sep 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0301 in the wild and no public exploit code.

Affected packages and fixed versions:
- stack_collections (crates.io): affected from 0.3.0; fixed in 0.3.3

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-2026-0301/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0301/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0301

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0301/
Summaries are generated and may contain mistakes; the official description above is the record.