# RUSTSEC-2026-0403: Uninitialized memory exposed when streaming decompression of a legacy frame fails


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

## What is RUSTSEC-2026-0403?

RUSTSEC-2026-0403 is a vulnerability in zstd-safe, published on 16 Oct 2026. 

### Official description

When decompression fails inside a legacy (zstd v0.4 to v0.7) frame, the
bundled zstd library reports the output buffer as fully written although
nothing was written. `DCtx::decompress_stream` committed that position even
on error, and for a `Vec<u8>` destination this extended the vector's length
over uninitialized memory.

Legacy support is enabled by default. A 5- or 6-byte corrupt legacy frame is
enough: through `zstd::stream::write::Decoder`, up to 32 KiB of stale heap
memory is then passed to the wrapped writer on the next `write()` or
`flush()`, or when an `auto_flush()`/`on_flush()` decoder is dropped. Whether
this reaches an attacker depends on where the application sends the
decompressed output. `zstd::stream::read::Decoder`, `zstd::decode_all` and
the bulk API are not affected.

Before zstd-safe 4.0.0, output buffers could only be initialized slices.
The zstd crate's own writer had the same flaw from 0.4.22 to 0.7.x, which is
covered by a separate advisory.

The flaw was corrected by only committing the buffer positions of a
successful call.

## Which versions are affected by RUSTSEC-2026-0403?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `zstd-safe` | crates.io | From 4.0.0 | 8.2.0 |

## How do I fix RUSTSEC-2026-0403?

Upgrade zstd-safe to 8.2.0 or later.

## Is RUSTSEC-2026-0403 being exploited?

There is no known exploitation of RUSTSEC-2026-0403 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0403 and fix it if it is affected.

What it is: RUSTSEC-2026-0403 is a vulnerability in zstd-safe, published on 16 Oct 2026. 
Exploitation: There is no known exploitation of RUSTSEC-2026-0403 in the wild and no public exploit code.

Affected packages and fixed versions:
- zstd-safe (crates.io): affected from 4.0.0; fixed in 8.2.0

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-2026-0403/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0403/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0403

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0403/
Summaries are generated and may contain mistakes; the official description above is the record.