CVE-2026-78587 Overview
CVE-2026-78587 is an incorrect authorization vulnerability in Elastic Fleet Server. The flaw is tracked under CWE-863: Incorrect Authorization and maps to CAPEC-122: Privilege Abuse. Fleet Server does not correctly verify session ownership during multi-part data upload operations. Any authenticated agent can interfere with active upload sessions belonging to other enrolled agents, producing a denial of service against agent upload operations.
Critical Impact
An authenticated agent can disrupt in-progress multi-part uploads from other enrolled agents, degrading the availability of telemetry and diagnostic data collected through Fleet Server.
Affected Products
- Elastic Fleet Server versions prior to 8.19.16
- Elastic Fleet Server 9.x versions prior to 9.3.5
- Elastic Fleet Server 9.4.x versions prior to 9.4.2
Discovery Timeline
- 2026-09-02 - CVE-2026-78587 published to NVD
- 2026-09-02 - Last updated in NVD database
Technical Details for CVE-2026-78587
Vulnerability Analysis
Fleet Server supports multi-part data uploads from enrolled Elastic Agents. These uploads are used to transfer files such as agent diagnostics and other artifacts through the control plane. The vulnerability stems from missing session ownership validation during the multi-part upload workflow. The server authenticates the caller as an enrolled agent but does not confirm that the agent making a request to a given upload session is the same agent that initiated it.
Root Cause
The root cause is an authorization gap classified as [CWE-863]. Fleet Server verifies that the caller holds a valid agent credential and that the referenced upload session exists. It does not verify that the session identifier belongs to the caller. Because upload session state is shared across enrolled agents, any authenticated agent that learns or guesses a session identifier can act on that session.
Attack Vector
Exploitation requires network access to Fleet Server and valid agent enrollment credentials, which limits the attack surface to insiders or compromised agents. The attack does not require user interaction. A malicious or compromised agent enumerates or brute forces active upload session identifiers and issues multi-part upload operations against those sessions. This interference corrupts or terminates the target upload, resulting in loss of availability for that operation. No confidentiality or integrity impact is documented for this issue.
No verified exploit code is publicly available. See the Elastic Security Update ESA-2026-164 for the vendor description.
Detection Methods for CVE-2026-78587
Indicators of Compromise
- Multiple Fleet Server upload session interactions from a single agent identifier targeting sessions attributed to different agents.
- Elevated rates of failed, aborted, or corrupted multi-part upload sessions in Fleet Server logs.
- Unexpected agent diagnostic bundles that are truncated or fail integrity checks on ingest.
Detection Strategies
- Correlate Fleet Server access logs by agent identifier and upload session identifier to detect cross-agent session access.
- Alert on agents interacting with upload session identifiers they did not initiate, using request path and payload metadata.
- Baseline normal per-agent upload volume and flag agents that touch a disproportionate number of distinct sessions.
Monitoring Recommendations
- Forward Fleet Server audit and access logs to a centralized analytics platform for retention and correlation.
- Track failure rates for agent diagnostic uploads and open a review when rates deviate from baseline.
- Monitor agent enrollment events and revoke tokens associated with agents exhibiting anomalous upload behavior.
How to Mitigate CVE-2026-78587
Immediate Actions Required
- Upgrade Fleet Server to version 8.19.16, 9.3.5, or 9.4.2, matching the deployed release train.
- Rotate Elastic Agent enrollment tokens if agent compromise is suspected in the environment.
- Restrict network access to Fleet Server so that only authorized agent networks can reach the upload endpoints.
Patch Information
Elastic addressed CVE-2026-78587 in Fleet Server 8.19.16, 9.3.5, and 9.4.2. Refer to the Elastic Security Update ESA-2026-164 for release notes and upgrade guidance.
Workarounds
- No vendor supplied workaround is documented; upgrading Fleet Server is the supported remediation.
- Reduce exposure by segmenting Fleet Server behind network controls that limit which hosts can enroll as agents.
- Increase monitoring of agent behavior until the upgrade is completed across the fleet.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.