Uninitialized memory exposed when streaming decompression of a legacy frame fails

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

not scored

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

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 ` 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…

Published by the source

16 Oct 2026, 12:00 UTC

Last modified by the source

2 Nov 2026, 09:30 UTC

Source last checked

3 Nov 2026, 06:10 UTC

1. Revision 2 · amended · 3 Nov 2026

   Amended from osv record RUSTSEC-2026-0403

2. Revision 1 · created · 27 Oct 2026

   Published from osv record RUSTSEC-2026-0403

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# Patch Next advisory brief: rustsec-2026-0403

Advisory: Uninitialized memory exposed when streaming decompression of a legacy frame fails
IDs: rustsec-2026-0403
Record: https://patchnext.com/advisories/rustsec-2026-0403 (revision 2, source checked 3 Nov 2026, 06:10 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Unrated
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: zstd-safe 8.2.0, 7.4.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io zstd-safe: affected >= 4.0.0, < 7.4.0; >= 8.0.0-0, < 8.2.0; fixed in 7.4.0, 8.2.0

## Suggested inspection (read-only)
- crates.io zstd-safe: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on zstd-safe.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/RUSTSEC-2026-0403 (OSV record RUSTSEC-2026-0403 lists affected ranges: crates.io zstd-safe >=4.0.0 <8.2.0 (fixed))
- Verified: https://osv.dev/vulnerability/RUSTSEC-2026-0403 (OSV record RUSTSEC-2026-0403 lists affected ranges: crates.io zstd-safe >=4.0.0 =8.0.0-0 <8.2.0 (fixed))

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