Arelle unauthenticated RCE (CVE-2026-42796)

Patch now - CVE-2026-42796 is a critical unauthenticated remote code execution vulnerability in Arelle before 2.39.10 that lets an attacker execute arbitrary Python code on the server. Patched in version 2.39.10; update immediately.

Overview

CVE-2026-42796 affects the Arelle XBRL taxonomy processing server. The /rest/configure REST endpoint accepts a plugins query parameter and forwards it to the plugin manager without any authentication or authorization checks. An attacker can supply a URL pointing to a malicious Python file through this parameter. Arelle’s webserver downloads that file and executes it with all of Arelle’s process privileges.

The vulnerability is rated CRITICAL (CVSS 9.8) because it requires no authentication, no user interaction, and can be triggered over the network with low complexity. The attacker does not need to be on the same network or have any prior access to the server.

Impact

An attacker who successfully exploits CVE-2026-42796 gains the ability to run arbitrary code on the Arelle server. This can lead to:

• Full compromise of the Arelle application and its data.
• Lateral movement within the internal network if the server has network access.
• Installation of persistent backdoors or malicious scripts.
• Theft, modification, or destruction of financial and XBRL reporting data processed by Arelle.

Affected Versions

Arelle releases before version 2.39.10 are vulnerable. Users should confirm their current version and plan an upgrade immediately.

Remediation

Patch: Upgrade to Arelle 2.39.10 or later. The fix requires proper authentication checks on the /rest/configure endpoint.

Mitigation: If an immediate upgrade is not possible, block external network access to the /rest/configure endpoint at the firewall or reverse proxy level. Restrict access to only trusted internal IP addresses as a temporary measure.

Detection: Review web server logs for suspicious requests to /rest/configure containing URL parameters pointing to external file hosts. Monitor for unexpected outbound connections from the Arelle server to unknown hosts.

Security Insight

This vulnerability follows a pattern common in data-processing middleware: treating a configuration endpoint as an internal utility without authentication. Vendors building REST APIs that accept remote resource URIs should always treat those parameters as potential attack vectors and enforce strict access controls. For more on similar configuration-based RCE risks, see our security news coverage. Data breach reports related to unauthenticated API endpoints are available at breach reports.

Further Reading

• All Critical Vulnerabilities
• Recent Critical Vulnerabilities
• Top Critical CVEs