CVE-2026-26931: Metricbeat Prometheus DoS Vulnerability

CVE-2026-26931 Overview

CVE-2026-26931 is a Memory Allocation with Excessive Size Value vulnerability (CWE-789) affecting the Prometheus remote_write HTTP handler in Metricbeat. This vulnerability enables attackers to trigger a Denial of Service condition through excessive memory allocation, potentially rendering the affected Metricbeat instance unresponsive and disrupting metrics collection and monitoring capabilities.

Critical Impact
Exploitation of this vulnerability allows adjacent network attackers with low privileges to cause resource exhaustion through crafted requests to the Prometheus remote_write endpoint, leading to service unavailability.

Affected Products

Metricbeat (versions prior to 8.19.13)
Metricbeat (versions prior to 9.2.5)

Discovery Timeline

2026-03-19 - CVE-2026-26931 published to NVD
2026-03-19 - Last updated in NVD database

Technical Details for CVE-2026-26931

Vulnerability Analysis

This vulnerability resides in the Prometheus remote_write HTTP handler within Metricbeat. The handler fails to properly validate or limit the size of memory allocations when processing incoming data, allowing an attacker to supply maliciously crafted input that triggers excessive memory allocation. This type of vulnerability falls under CWE-789 (Memory Allocation with Excessive Size Value), where user-controlled input directly influences the size of memory allocations without adequate bounds checking.

The attack requires adjacent network access, meaning the attacker must be on the same network segment or have network-level access to the vulnerable Metricbeat instance. While low-level privileges are required to exploit this vulnerability, no user interaction is necessary. The impact is limited to availability, with no effect on confidentiality or integrity.

Root Cause

The root cause of CVE-2026-26931 is insufficient validation of size parameters in the Prometheus remote_write HTTP handler. When the handler receives incoming metrics data, it allocates memory based on values derived from the request without implementing proper upper bounds or sanity checks. This allows an attacker to specify arbitrarily large allocation sizes, leading to memory exhaustion on the host system.

Attack Vector

An attacker with adjacent network access can exploit this vulnerability by sending specially crafted HTTP requests to the Metricbeat Prometheus remote_write endpoint. The attack follows the CAPEC-130 pattern (Excessive Allocation), where the attacker manipulates request parameters to force the application to allocate disproportionately large amounts of memory.

The attack requires the attacker to have network visibility to the Metricbeat instance and low-level authentication or access. Once these conditions are met, the attacker can send repeated requests that trigger excessive memory allocation, eventually exhausting available system memory and causing the Metricbeat service to become unresponsive or crash.

Detection Methods for CVE-2026-26931

Indicators of Compromise

Abnormally high memory consumption by Metricbeat processes
Unusual volume of HTTP requests to the Prometheus remote_write endpoint from adjacent network addresses
Metricbeat service crashes or restarts correlated with spikes in memory usage
System-level out-of-memory (OOM) events affecting the Metricbeat process

Detection Strategies

Monitor Metricbeat process memory usage and alert on anomalous growth patterns
Implement network-level monitoring for unusual request patterns to the remote_write endpoint
Configure host-based detection for memory exhaustion events targeting Metricbeat
Review HTTP access logs for repeated large or malformed requests to the Prometheus handler

Monitoring Recommendations

Set up resource utilization alerts for Metricbeat memory consumption thresholds
Enable detailed logging for the Prometheus remote_write HTTP handler to capture request metadata
Deploy network traffic analysis on segments where Metricbeat is accessible
Implement application-level health checks to detect service degradation

How to Mitigate CVE-2026-26931

Immediate Actions Required

Upgrade Metricbeat to version 8.19.13 or 9.2.5 or later, which contain the security fix
If immediate patching is not possible, restrict network access to the Metricbeat Prometheus remote_write endpoint
Review and limit which hosts can communicate with Metricbeat on the affected network segment
Monitor for signs of exploitation while preparing to deploy patches

Patch Information

Elastic has released security updates to address this vulnerability. Users should upgrade to Metricbeat version 8.19.13 or 9.2.5 (depending on their version branch) to remediate CVE-2026-26931. Detailed patch information is available in the Elastic Security Update Announcement.

Workarounds

Restrict network access to the Metricbeat Prometheus remote_write endpoint using firewall rules
Implement reverse proxy rate limiting and request size restrictions in front of Metricbeat
Disable the Prometheus remote_write module if not required for your monitoring use case
Deploy network segmentation to limit adjacent network access to Metricbeat instances

bash

# Example: Restrict access to Metricbeat port using iptables
# Allow only trusted monitoring hosts to reach the remote_write endpoint
iptables -A INPUT -p tcp --dport 5066 -s 192.168.1.100 -j ACCEPT
iptables -A INPUT -p tcp --dport 5066 -j DROP