# RUSTSEC-2026-0393: Out-of-bounds read when training a dictionary from inconsistent sample sizes


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 16 Oct 2026

## What is RUSTSEC-2026-0393?

RUSTSEC-2026-0393 is a vulnerability in zstd, published on 16 Oct 2026. 

### Official description

`ZDICT_trainFromBuffer` is never told the length of the samples buffer.
Before 0.4.10, `zstd::dict::from_continuous` (initially `train_dict`) called
it directly: up to 0.2.1 without checking the sample sizes against the
buffer at all, and from 0.3.0 with a sum that wraps around when overflow
checks are off, as in release builds. Sizes that do not describe the buffer
made the trainer read past its end.

This requires the sample sizes to come from an untrusted source. From
0.4.10, training goes through zstd-safe, where the same flaw is covered by
the zstd-safe advisory for it, fixed in zstd-safe 8.2.0.

## Which versions are affected by RUSTSEC-2026-0393?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `zstd` | crates.io | From 0.1.1 | 0.4.10 |

## How do I fix RUSTSEC-2026-0393?

Upgrade zstd to 0.4.10 or later.

## Is RUSTSEC-2026-0393 being exploited?

There is no known exploitation of RUSTSEC-2026-0393 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0393 and fix it if it is affected.

What it is: RUSTSEC-2026-0393 is a vulnerability in zstd, published on 16 Oct 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0393 in the wild and no public exploit code.

Affected packages and fixed versions:
- zstd (crates.io): affected from 0.1.1; fixed in 0.4.10

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-2026-0393/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0393/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0393

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0393/
Summaries are generated and may contain mistakes; the official description above is the record.