CVE-2026-78587 in Fleet Server
Summary
by MITRE • 09/02/2026
Incorrect Authorization (CWE-863) in Fleet Server can lead to a denial of service of agent upload operations via Privilege Abuse (CAPEC-122). Fleet Server does not correctly verify session ownership during multi-part data upload operations, allowing any authenticated agent to interfere with the active upload sessions belonging to other enrolled agents.
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Analysis
by VulDB Data Team • 09/02/2026
The vulnerability identified as CWE-863 represents a critical failure in access control mechanisms within the Fleet Server component of Elastic Agent deployments. This flaw specifically manifests during the handling of multi-part data uploads, which are commonly used for transmitting large configuration files, logs, or binary payloads from enrolled agents to the central server. In a properly secured system, each upload session must be strictly bound to the specific authenticated identity that initiated it. However, due to an incorrect authorization logic error, Fleet Server fails to adequately verify that the user context associated with incoming data chunks matches the owner of the active upload session. This oversight creates a significant gap in the security boundary between different agent identities operating within the same infrastructure.
The operational impact of this vulnerability is severe and directly aligns with CAPEC-122, which describes Privilege Abuse as an attack pattern where an attacker exploits improperly implemented privilege checks to perform actions beyond their authorized scope. In this context, any authenticated agent that has valid credentials for the Fleet Server can exploit this flaw by intercepting or manipulating upload requests intended for other agents. By sending data chunks with session identifiers belonging to different enrolled agents, a malicious actor can disrupt ongoing uploads from legitimate peers. This interference does not necessarily require elevated privileges beyond standard agent access; rather, it relies on the ability to forge or reuse valid session tokens associated with active transfers of other entities within the system.
The primary consequence of this exploitation is a denial of service affecting data ingestion and configuration management processes. When an attacker successfully injects malicious or malformed chunks into another agent's upload stream, that agent’s transfer will likely fail due to checksum mismatches, protocol errors, or server-side validation failures triggered by corrupted payloads. This results in the loss of critical telemetry data, such as security events or system metrics, which are essential for monitoring and incident response capabilities. Furthermore, if the injected payload contains malicious configurations or binaries intended to be executed upon successful upload completion, it could lead to further compromise scenarios including remote code execution or lateral movement within the network environment dependent on how Fleet Server processes these uploads post-completion.
Mitigation strategies must focus on reinforcing session binding and input validation at both the application logic level and infrastructure layer. Developers should implement strict checks ensuring that every chunk of a multi-part upload is validated against the original requestor’s identity before processing or storage. This includes verifying token ownership, checking sequence numbers for continuity within the correct user context, and rejecting any packets where the session ID does not correspond to an active session initiated by the authenticating principal. Additionally, implementing rate limiting per agent identity can help mitigate bulk injection attempts while detection systems are updated. Security operations teams should monitor for anomalous patterns in upload failures or unexpected termination of transfer sessions across multiple agents simultaneously, as these may indicate ongoing exploitation of this authorization bypass flaw until a patch is applied by the vendor.