# CVE-2026-77021: Missing decompression size limit in agent receiver allows memory exhaustion via push agent data

> An attacker controlling a push-mode host can exhaust agent receiver memory by sending highly compressed data, causing denial of service.

- **Severity:** Medium 5.3 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 21 Sep 2026
- **Updated:** 21 Sep 2026
- **Also known as:** GHSA-5q8h-fhcg-2395

## What is CVE-2026-77021?

CVE-2026-77021 is a medium-severity denial of service vulnerability in Checkmk, published on 21 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Improper handling of highly compressed data (data amplification) in Checkmk <2.5.0p14, <2.4.0p37, <2.3.0p51 and 2.2.0 (EOL) allows an attacker who controls a host registered for push mode to exhaust the memory of the agent receiver by sending a small zlib compressed payload that decompresses to an arbitrary size.

Weakness: CWE-409 (Improper Handling of Highly Compressed Data (Data Amplification))

## Which versions are affected by CVE-2026-77021?


## How do I fix CVE-2026-77021?

No fix has been published for CVE-2026-77021 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-77021 being exploited?

There is no known exploitation of CVE-2026-77021 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-77021 and fix it if it is affected.

What it is: CVE-2026-77021 is a medium-severity denial of service vulnerability in Checkmk, published on 21 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-77021 in the wild and no public exploit code.

Affected product: Checkmk. 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-2026-77021/ (Markdown: https://insights.sbomify.com/CVE-2026-77021/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77021
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77021
- OSV: https://osv.dev/vulnerability/CVE-2026-77021

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77021/
Summaries are generated and may contain mistakes; the official description above is the record.