External risk intelligence

Alluxio S3 Proxy Authentication Bypass via Unverified Signatures

CVE advisorySeverity: CRITICAL (CVSS 9.3)

CVE-2026-79787

The vulnerability exists in an S3 REST proxy, which is a network service designed to act as an interface for data access. Such services are commonly deployed as API endpoints or gateways to facilitate communication with data storage systems, making them frequently accessible via the network to serve requests.

Authentication Bypass

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

Alluxio's S3 REST proxy has a configuration issue that allows unauthenticated attackers to impersonate any user. This could enable unauthorized access, modification, or deletion of data. The main concern is confirming if this technology is in use and exposed.

Unauthenticated users can impersonate any user.
Affects data access and integrity in Alluxio.
Confirm Alluxio usage and network exposure.

Attack Path

How an attacker could exploit the issue

Attackers can exploit this vulnerability by sending unauthenticated requests to the S3 REST proxy. Because the proxy does not properly verify AWS Signature Version 4, attackers can craft requests that impersonate any user, including service accounts. This allows them to access, modify, or delete any data stored within the system.

Unauthenticated network access required.
Spoofed user identity via unsigned headers.
Read, write, delete arbitrary data.

Live Threat

Current exploitation, exposure, and threat context

The Alluxio S3 REST proxy, when using its default configuration, does not properly verify AWS Signature Version 4, enabling unauthenticated attackers to impersonate users. This could allow unauthorized access to read, write, or delete any data stored within Alluxio.

Arbitrary data stored in Alluxio.
Spoofing user identity via unsigned headers.
Unauthorized data access and modification.

Operational Fix

Recommended remediation, mitigation, and detection steps

This critical vulnerability in Alluxio's S3 REST proxy, which allows unauthenticated attackers to impersonate users and access arbitrary data, requires immediate attention from teams managing data storage and access control. The first practical step is to identify all instances of the affected Alluxio S3 REST proxy, confirm their network exposure and business criticality, identify the accountable owner, and then prioritize remediation based on assessed risk.

Data platform and storage teams should own this.
Verify S3 proxy network exposure.
Plan remediation based on identified risk.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is Alluxio and why does it use an S3 REST proxy?

Alluxio is an open-source data orchestration layer that bridges compute engines and various storage systems. It provides a unified data access interface. The S3 REST proxy is a specific component that allows applications or tools designed to communicate with Amazon S3 to interact with data stored in Alluxio using the standard S3 API, effectively making Alluxio appear as an S3-compatible storage service.

How does CVE-2026-79787 allow unauthorized access?

This vulnerability is classified as CWE-287, or improper authentication. Specifically, the S3 REST proxy fails to perform mandatory cryptographic checks on AWS Signature Version 4 headers. Because these signatures are not validated, the system cannot confirm the identity of the requestor. An attacker can supply a username in the header, and the proxy will treat that request as if it were legitimately authenticated, granting the attacker the target user's permissions.

Do I need to be authenticated to trigger CVE-2026-79787?

No. The vulnerability exists specifically because the system fails to require or verify valid credentials. An attacker does not need an existing account or password to interact with the proxy. Sending an unsigned request is sufficient to impersonate other users. Note that the bug is triggered by the lack of signature verification in the proxy; simply accessing the proxy without attempting to spoof a header does not necessarily indicate a successful compromise of data.

Why is Halo Surface Signal labeling this as external?

Halo Surface Signal flags this as external because the affected S3 REST proxy is a network-facing service intended to act as an API gateway. Since this interface is designed to facilitate communication between different systems, it is frequently deployed where it can be reached over a network. If your proxy is accessible from outside your internal perimeter, the risk of unauthorized impersonation by remote actors increases significantly.

How should I respond to this Alluxio security issue?

Begin by inventorying your environment to locate all instances of the Alluxio S3 REST proxy. Once identified, evaluate whether these services are exposed to untrusted networks. Coordinate with your data and infrastructure teams to determine which proxies handle sensitive data. Prioritize these high-value instances for remediation by updating or modifying configurations as recommended by the vendor to enforce proper request authentication.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.