Arelle Unauthenticated Remote Code Execution Vulnerability

Arelle versions prior to 2.39.10 are susceptible to an unauthenticated remote code execution (RCE) vulnerability. The vulnerability resides in the /rest/configure REST endpoint, which improperly handles the plugins query parameter. This parameter is forwarded to the plugin manager without proper authentication or authorization checks. An attacker can exploit this flaw by providing a URL pointing to a malicious Python file via the plugins parameter. Upon receiving this request, the Arelle webserver downloads and executes the attacker-supplied Python code within the context of the Arelle process. This grants the attacker control over the Arelle server with the same privileges as the Arelle process. This vulnerability poses a significant risk, especially in environments where Arelle servers are exposed to the internet or untrusted networks.

Attack Chain

1. An unauthenticated attacker sends a crafted HTTP GET request to the /rest/configure endpoint of the Arelle web server.
2. The request includes the plugins query parameter, which contains a URL pointing to a malicious Python file hosted on an attacker-controlled server.
3. The Arelle web server receives the request and, without proper authentication or authorization, forwards the plugins parameter to the plugin manager.
4. The plugin manager downloads the Python file from the attacker-supplied URL using standard HTTP(S) protocols.
5. The Arelle process executes the downloaded Python code using the Python interpreter.
6. The malicious Python code executes arbitrary commands on the Arelle server, potentially installing malware, creating reverse shells, or exfiltrating sensitive data.
7. The attacker gains control of the Arelle server and can perform further actions, such as accessing internal network resources.

Impact

Successful exploitation of this vulnerability allows an unauthenticated attacker to achieve remote code execution on the Arelle server. This could lead to complete compromise of the server, including sensitive data theft, malware deployment, and further lateral movement within the network. The potential impact includes data breaches, service disruption, and reputational damage. Given the severity and ease of exploitation, any Arelle instance running a version prior to 2.39.10 is at critical risk.

Recommendation

• Immediately upgrade Arelle to version 2.39.10 or later to patch CVE-2026-42796.
• Deploy the Sigma rule “Detect Arelle Plugin Download via REST Endpoint” to identify exploitation attempts targeting the vulnerable /rest/configure endpoint.
• Monitor web server logs for suspicious requests to the /rest/configure endpoint containing the plugins parameter.
• Implement network segmentation to limit the potential impact of a compromised Arelle server.