# CVE-2026-77021: Checkmk agent receiver memory exhaustion via compressed data amplification

Source: https://junglewise.ai/threats/cve-2026-77021-improper-handling-of-highly-compressed-data-data-amplification-in  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-21. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-77021 · Severity: info · Published 2026-09-21

Checkmk's agent receiver processes monitoring data from managed hosts using compression to reduce network traffic. An attacker controlling a host registered for push mode can send a small compressed payload that decompresses to an extremely large size, exhausting the receiver's memory and causing a denial of service. The vulnerability requires the attacker to have a valid host certificate for the push mode protocol.

Vendors: [Checkmk](https://junglewise.ai/threats/vendors/checkmk).

## Quick answers

**Is CVE-2026-77021 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-77021 being exploited in the wild.

**How severe is CVE-2026-77021?** Informational severity, published 21 September 2026.

**Which products and versions are affected?** Checkmk Cloud before 2.5.0p14, before 2.4.0p37, before 2.3.0p51; Checkmk Ultimate before 2.5.0p14, before 2.4.0p37, before 2.3.0p51, 2.2.0; Checkmk Ultimate MT before 2.5.0p14, before 2.4.0p37, before 2.3.0p51.

**Is there a fix for CVE-2026-77021?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-77021 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-77021?** Check whether you run Checkmk Cloud, Checkmk Ultimate or Checkmk Ultimate MT at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The agent receiver decompresses zlib-compressed agent data without enforcing a limit on the decompressed output size, allowing a zip bomb style attack where small payloads expand orders of magnitude. An authenticated attacker (one with a valid host certificate for push mode) can send specially crafted compressed data that causes memory exhaustion. The fix limits decompressed data to 512 MiB and rejects oversized payloads.

## Affected products

- Checkmk Checkmk Ultimate before 2.5.0p14, before 2.4.0p37, before 2.3.0p51, 2.2.0
- Checkmk Checkmk Cloud before 2.5.0p14, before 2.4.0p37, before 2.3.0p51
- Checkmk Checkmk Ultimate MT before 2.5.0p14, before 2.4.0p37, before 2.3.0p51

## Timeline

- 2026-09-21: disclosed: Public vulnerability disclosure

## References

- [https://checkmk.com/werk/22116](https://checkmk.com/werk/22116)

## Related threats

- [Checkmk agent receiver mTLS authentication bypass](https://junglewise.ai/threats/cve-2026-15576-checkmk-agent-receiver-mtls-authentication-bypass)