CVE-2021-26642: 
PHP vulnerability analysis and mitigation

A vulnerability was identified in XpressEngine's bulletin board system, tracked as CVE-2021-26642. The vulnerability allows for arbitrary file upload due to insufficient file verification when uploading image files. This security flaw was discovered and reported by KrCERT/CC (NIST NVD, MITRE CVE).

The vulnerability has been assigned a CVSS v3.1 base score of 9.8 (CRITICAL) by NIST with a vector string of CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. KrCERT/CC assigned a slightly lower score of 8.8 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H. The vulnerability is classified as CWE-434 (Unrestricted Upload of File with Dangerous Type) (NIST NVD).

When successfully exploited, this vulnerability allows remote attackers to execute arbitrary code on the server where the bulletin board is running. This could potentially lead to complete system compromise with the ability to execute commands, access sensitive data, and potentially gain full control of the affected server (NIST NVD).

The vulnerability is remotely exploitable and requires low attack complexity with no user interaction. The attack vector is network-based, making it accessible to remote attackers (NIST NVD).

The vulnerability affects XpressEngine versions up to (excluding) 3.0.14. Users should upgrade to version 3.0.14 or later to address this security issue (NIST NVD).