Attack Chain

1. Initial Access / Authentication Bypass: An attacker, potentially unauthenticated if self-registration is enabled, crafts a POST request to /egroupware/json.php targeting the SmallPartMediaRecorder::ajax_upload() function.
2. Authorization Bypass: The crafted request includes a manipulated JSON body with participant_role set to 3 (ROLE_TEACHER), bypassing the isTeacher check intended to restrict file uploads.
3. Arbitrary File Write: Following the authorization bypass, the attacker exploits a path traversal vulnerability via the video_type parameter to write an arbitrary file to a controlled path, such as ./header.inc.php.
4. Arbitrary File Read: A separate vulnerability is exploited by making a GET request to /egroupware/index.php?menuaction=importexport.importexport_export_ui.download with a malicious _filename parameter (e.g., ../../../usr/share/egroupware/header.inc.php) to read the original content of the target file.
5. Code Injection Preparation: The attacker combines the original header.inc.php content with injected PHP code (e.g., a webshell or administrative credential modification logic) to preserve file validity and ensure continued server operation.
6. Remote Code Execution: The attacker overwrites header.inc.php using the arbitrary file write vulnerability with the crafted content, leading to Remote Code Execution when the file is subsequently loaded by the web server.

Impact

Successful exploitation of CVE-2026-27823 leads to critical consequences, including full system compromise and complete takeover of the EGroupware instance. Attackers can gain arbitrary remote code execution on the server, allowing them to install backdoors, exfiltrate sensitive data, manipulate or destroy data, or pivot to other systems within the network. This could result in significant data breaches, operational disruption, and reputational damage for affected organizations. The ability to modify administrative setup passwords further enables persistent control over the EGroupware application.

Recommendation

• Patch CVE-2026-27823 immediately: Upgrade EGroupware composer/egroupware/egroupware to version 23.1.20260224 or 26.2.20260224 to mitigate CVE-2026-27823.
• Disable user self-registration: If not strictly necessary, disable user self-registration in EGroupware to reduce the attack surface for unauthenticated exploitation of CVE-2026-27823.
• Deploy the Sigma rules in this brief: Integrate the provided Sigma rules into your SIEM to detect attempts at exploiting the authorization bypass, arbitrary file write, and arbitrary file read components of CVE-2026-27823.
• Enable webserver logging: Ensure comprehensive logging for your EGroupware web server (access logs, error logs) to capture HTTP requests including method, URI, query parameters, and POST body details, which are critical for activating the detection rules.