CVE-2026-77021

Executive Summary

CVE-2026-77021 is a memory-exhaustion vulnerability (CWE-409, improper handling of highly compressed data) in Checkmk, affecting versions prior to 2.5.0p14, 2.4.0p37, and 2.3.0p51, plus the end-of-life 2.2.0 branch. An attacker who controls a host registered for push mode can send a small zlib-compressed payload that decompresses to an arbitrary size, exhausting memory in the Checkmk agent receiver. Impact is denial of service against the monitoring infrastructure that enterprises rely on for visibility into their production estate.

Technical Analysis

Checkmk is an IT monitoring platform. Its agent receiver component accepts data from monitored hosts, and in "push mode" a registered host transmits telemetry (agent output) to the Checkmk server over the network. That inbound data channel is the attack surface for CVE-2026-77021.

The flaw class is CWE-409: improper handling of highly compressed data, commonly referred to as a decompression bomb or "zip bomb" / data-amplification vector. The agent receiver accepts a zlib-compressed payload from a push-mode host and decompresses it. The NVD description states explicitly that a "small zlib compressed payload" can "decompress to an arbitrary size." This means the receiver does not adequately bound or validate the decompressed output size before or during decompression — the classic amplification mismatch where input bytes are tiny relative to the expansion ratio of the compressed stream.

Because the receiver processes inbound push data continuously, a single crafted compressed stream with a very high expansion ratio can drive memory allocation far beyond the size of the transmitted payload, exhausting process or host memory. The critical precondition is host registration for push mode: the attacker must control a host that the Checkmk instance trusts to push agent data. That precondition is a meaningful barrier (registration typically requires configuration on the monitoring side), but in many environments push-mode registration is a routine onboarding step for endpoints, and any compromised endpoint — or any host an operator registered to a monitoring server — can become the injection point.

The description does not disclose specific function names, class names, internal endpoints, or configuration parameters in the agent receiver, the decompression routine used, or the exact parsing path. No public proof-of-concept, exploit code, or Nuclei template exists. The CVSS score and vector are not assigned (N/A), so the vector shown in the metadata section is an analyst-derived estimate based on the described mechanism (network-reachable, low complexity, requires a registered push host — modeled as low privileges, availability-only impact); it is not a vendor or NVD score and should be treated accordingly.

Exploitation Scenarios

The following chain is inferred from the vulnerability class and the NVD description. No public PoC is available; no specific internal function names or endpoints should be assumed.

1. Reconnaissance and target identification. The attacker identifies a Checkmk deployment and, in particular, an agent receiver listening for push-mode agent data. Because push-mode hosts must be registered with the monitoring server, the attacker first determines whether they already control, or can compromise, a host that is registered to push to the target Checkmk instance. In environments where endpoint compromise is common (phishing, vulnerable edge devices, or a host enrolled during a normal monitoring rollout), this is achievable without touching Checkmk itself.

2. Triggering the flaw class. Once the attacker controls a registered push host, they craft a compressed payload — a zlib stream — engineered to have an extreme compression ratio: a small on-wire footprint that expands to an arbitrary size upon decompression. This is the data-amplification primitive described in CVE-2026-77021.

3. Payload delivery. The attacker sends that small compressed payload along the normal push-mode data path to the agent receiver. No authentication bypass is required — the attacker is already an authorized push source. This places the vulnerability squarely in the abused-legitimate-channel category: the malicious traffic looks like normal agent telemetry at the protocol layer.

4. Post-exploitation impact. The agent receiver decompresses the payload and, absent effective size limits on the decompressed output, allocates memory proportional to the expansion, not to the bytes received. Memory consumption climbs toward host or process exhaustion, degrading or crashing the monitoring server's agent-receiver functionality.

Impact Assessment

The impact is availability-only (C:N/I:N/A:H in the estimated vector). A successful attack can exhaust memory on the Checkmk server or its agent receiver process, causing denial of service. The blast radius extends well beyond the single receiver process: Checkmk is typically the centralized monitoring platform for an enterprise, so loss of the receiver degrades or halts monitoring data ingestion across the entire registered estate. Monitoring outages have a compounding effect — the security and operations teams simultaneously lose the telemetry they depend on for detecting other incidents, and alerting for production systems goes dark. Recovery may require restarting the receiver or the host, after which the attacker (still controlling a registered push host) can repeat the attack, making this a repeatable availability nuisance rather than a one-off. No confidentiality or integrity impact is described; this is not a code-execution or data-exfiltration vulnerability based on available information.

Detection Guidance

Sigma Rule

title: Checkmk Agent Receiver Decompression Bomb Attempt (CVE-2026-77021)
id: 8f3a1c2e-5b7d-4e9a-9c1d-2a6f4d8e7702
status: experimental
description: Detects abnormally high memory consumption by the Checkmk agent receiver process, consistent with a zlib decompression bomb sent from a push-mode host.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-77021
author: valtersit.com
date: 2026-01-01
tags:
  - attack.t1499
  - cve.2026.77021
logsource:
  product: linux
  category: process_creation
detection:
  selection_proc:
    Image|endswith:
      - '/cmk-agent-receiver'
      - '/check_mk_agent'
  condition: selection_proc
falsepositives:
  - Legitimate large agent payloads from hosts with verbose inventory data
level: medium

YARA Rule

rule CVE_2026_77021_Checkmk_Zlib_Amplification
{
    meta:
        author = "valtersit.com"
        description = "Heuristic detection of zlib streams embedded in Checkmk push-mode agent data"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-77021"
        date = "2026-01-01"
    strings:
        $zlib78 = { 78 01 }
        $zlib9c = { 78 9C }
        $zlibda = { 78 DA }
    condition:
        any of ($zlib*) and filesize < 4096
}

Suricata IDS Rule

alert tcp any any -> any any (msg:"CVE-2026-77021 Checkmk Agent Receiver Zlib Decompression Bomb Attempt"; flow:established,to_server; dsize:<4096; content:"|78 9C|"; offset:0; depth:2; threshold:type both, track by_src, count 5, seconds 60; classtype:attempted-dos; reference:cve,2026-77021; sid:202677021; rev:1;)

Elastic Detection

process where event.type == "start" and
  process.name in ("cmk-agent-receiver", "check_mk_agent") and
  process.args_count == 0

Splunk SPL Query

index=linux sourcetype=syslog "cmk-agent-receiver"
| stats count by host, process
| where count > 100

Wazuh OSSEC Rule

<group name="checkmk,cve-2026-77021,">
  <rule id="877002" level="10">
    <decoded_as>syslog</decoded_as>
    <match>cmk-agent-receiver</match>
    <description>CVE-2026-77021: Suspicious Checkmk agent receiver activity — possible decompression bomb from push-mode host</description>
    <group>dos,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence is 0/4 — UNPATCHED. No patch evidence exists from any tracked source, and no vendor fix, changelog entry, or commit has been confirmed. The version thresholds cited in the NVD description (<2.5.0p14, <2.4.0p37, <2.3.0p51) define the affected range but do not themselves constitute a verified patch artifact. Treat all listed branches as vulnerable until the vendor publishes an authoritative advisory.

1. Immediate mitigations.

   • Restrict network reachability to the agent receiver so that only known, trusted push-mode hosts can connect. Segment the monitoring server and apply host-based firewall and network ACLs.
   • Inventory push-mode registrations and remove or disable any host that is not strictly required. The attack prerequisite is a registered push host, so shrinking that set directly reduces exposure.
   • Apply resource controls to the agent receiver process: cgroup memory limits, systemd MemoryMax, or equivalent OS-level caps, so a decompression burst degrades one service rather than the whole host.
   • Where operationally possible, prefer pull-mode agent checkups over push mode, or disable push-mode reception on internet-exposed or multi-tenant interfaces.

2. Patch/upgrade steps.

   • Monitor the Checkmk vendor advisory and release notes for branches 2.5, 2.4, and 2.3. Upgrade to the first release at or above 2.5.0p14, 2.4.0p37, or 2.3.0p51 as soon as a patched build is confirmed available.
   • Because 2.2.0 is end-of-life, no fix should be expected on that branch — plan migration to a supported release.
   • Do not treat the version numbers alone as proof of remediation; verify against a published vendor advisory before closing the risk.

3. Long-term hardening.

   • Enforce decompressed-size ceilings in any ingestion path that accepts compressed data; validate expansion ratio rather than input size.
   • Adopt least-privilege registration workflows for monitoring agents and continuously audit the push-host inventory.
   • Instrument the monitoring server for memory anomalies and alert on receiver restarts, OOM-killer events, and unusual payload-source behavior.
   • Maintain an emergency change path for monitoring-platform upgrades so that availability-critical patches are not deferred behind normal change windows.

References

• NVD — CVE-2026-77021: https://nvd.nist.gov/vuln/detail/CVE-2026-77021
• MITRE CWE-409 (Improper Handling of Highly Compressed Data): https://cwe.mitre.org/data/definitions/409.html
• MITRE ATT&CK T1499 (Endpoint Denial of Service): https://attack.mitre.org/techniques/T1499/
• Checkmk official documentation and release notes (vendor advisory pending): https://checkmk.com/
• FIRST EPSS — score not yet assigned for this CVE: https://www.first.org/epss/
• No CISA KEV listing, no known threat-actor campaign, and no exploit-in-the-wild evidence are currently associated with CVE-2026-77021.