CVE-2026-77021 Checkmk GmbH CVE debrief

Who should care

Defenders responsible for Checkmk instances, especially those with push mode enabled, should assess potential exposure and impact. They should verify instance versions and configurations, and consider upgrading to patched versions. Defenders should also monitor for potential exploitation attempts and review compensating controls for exposed systems. Checkmk instances with push mode enabled are particularly vulnerable, and defenders responsible for these

Why it matters

CVE-2026-77021 is a medium-severity vulnerability in Checkmk that allows an attacker to exhaust the memory of the agent receiver, potentially disrupting monitoring capabilities. Defenders responsible for Checkmk instances, especially those with push mode enabled, should assess potential exposure and impact, verify instance versions and configurations, and consider upgrading to patched versions.

• Potential memory exhaustion of agent receiver
• Possible disruption of monitoring capabilities
• Need for verification of Checkmk instance versions and configurations
• Potential for denial-of-service (DoS) attacks

Technical summary

The Checkmk monitoring tool is vulnerable to improper handling of highly compressed data, allowing 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. This vulnerability has a medium severity and can potentially disrupt monitoring capabilities. The vulnerability is caused by improper handling of highly compressed data, which can lead to memory exhaustion of the agent receiver. Checkmk instances with push mode enabled are particularly vulnerable.

Defensive priority

Defenders should prioritize verifying exposure and assessing potential impact, especially for Checkmk instances with push mode enabled.

Recommended defensive actions

• Verify Checkmk instance versions and configurations to determine exposure
• Assess potential impact of memory exhaustion on agent receiver
• Consider upgrading to Checkmk version 2.5.0p14 or later, 2.4.0p37 or later, or 2.3.0p51 or later
• Monitor for potential exploitation attempts
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record and NVD entry provide details on the vulnerability, but additional information from Checkmk's official sources may be necessary for comprehensive assessment. Checkmk's official documentation and security advisories should be consulted for further details on affected versions, patches, and potential mitigations. The vulnerability is caused by improper handling of highly compressed data, which can lead to memory exhaustion. The CVE record and NVD entry provide details on the vulnerability, but additional information from

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.