# CVE-2026-70455: rsync 3.4.2 < 3.5.0 DoS via --zt Zstandard Compression Thread Exhaustion

> Remote attackers can exhaust system resources by specifying a large thread count via the --zt option, causing denial of service in rsync.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 13 Aug 2026
- **Updated:** 14 Aug 2026
- **Also known as:** ALPINE-CVE-2026-70455, AZL-95807, BELL-CVE-2026-70455, DEBIAN-CVE-2026-70455, ECHO-984f-9a90-ebe1, GHSA-rjvj-qgqg-cvx9, ROOT-OS-DEBIAN-11-CVE-2026-70455, UBUNTU-CVE-2026-70455

## What is CVE-2026-70455?

CVE-2026-70455 is a high-severity denial of service vulnerability in RsyncProject rsync, published on 13 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

rsync 3.4.2 before 3.5.0 contains a denial of service vulnerability that allows a remote sender to exhaust system resources by specifying the --zt short alias for --compress-threads, which bypasses the refuse options directive's string matching on long option names. Attackers can specify --zt=N with a large value to spawn an unbounded number of Zstandard worker threads on the receiver, exhausting available thread and memory resources.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2026-70455?

It affects 1 product.


Products: Samba Rsync.

## How do I fix CVE-2026-70455?

Upgrade to a fixed version.

## Is CVE-2026-70455 being exploited?

There is no known exploitation of CVE-2026-70455 in the wild and no public exploit code.

## Other vulnerabilities in RsyncProject rsync

- [CVE-2017-15994](https://insights.sbomify.com/CVE-2017-15994/): Security feature bypass in n/a (critical 9.8)
- [CVE-2026-53791](https://insights.sbomify.com/CVE-2026-53791/): rsync < 3.5.0 Daemon IP Spoofing via PROXY Protocol Header (critical 9.1)
- [CVE-2026-70461](https://insights.sbomify.com/CVE-2026-70461/): rsync 3.2.5 < 3.5.0 Heap Out-of-Bounds Write via files-from Entry (high 8.8)
- [CVE-2026-70463](https://insights.sbomify.com/CVE-2026-70463/): rsync 3.1.0 < 3.5.0 Authorization Bypass via auth users Directive Parsing (high 8.6)
- [CVE-2026-53803](https://insights.sbomify.com/CVE-2026-53803/): rsync < 3.5.0 Symlink Following Arbitrary File Overwrite (high 8.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-70455 and fix it if it is affected.

What it is: CVE-2026-70455 is a high-severity denial of service vulnerability in RsyncProject rsync, published on 13 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-70455 in the wild and no public exploit code.

Affected product: RsyncProject rsync. No package-level version data is available, so check the vendor advisory linked from the reference page.

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/CVE-2026-70455/ (Markdown: https://insights.sbomify.com/CVE-2026-70455/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-70455
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-70455
- OSV: https://osv.dev/vulnerability/CVE-2026-70455

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-70455/
Summaries are generated and may contain mistakes; the official description above is the record.