CVE-2023-5877 is a critical Server-Side Request Forgery (SSRF) vulnerability identified in the affiliate-toolkit WordPress plugin, specifically in versions prior to 3.4.3. The vulnerability arises due to a lack of authorization and authentication controls on the plugin's endpoint affiliate-toolkit-starter/tools/atkp_imagereceiver.php. This endpoint accepts requests from unauthenticated visitors and allows them to make arbitrary HTTP requests to any URL, including internal network addresses defined by RFC1918 (e.g., 10.x.x.x, 192.168.x.x, 172.16.x.x). SSRF vulnerabilities enable attackers to abuse the server as a proxy to access internal systems that are otherwise inaccessible from the internet. This can lead to unauthorized access to sensitive internal resources, data exfiltration, port scanning of internal networks, and potentially further exploitation of internal services. The CVSS 3.1 base score of 9.8 reflects the high severity, with the vector indicating network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), and full impact on confidentiality, integrity, and availability (C:H/I:H/A:H). Although no public exploits are currently known, the ease of exploitation and critical impact make this vulnerability a significant threat to WordPress sites using the affected plugin versions. The lack of vendor information and patch links suggests that mitigation may require manual updates or plugin replacement until an official fix is released.

For European organizations, this vulnerability poses a substantial risk, especially for those relying on WordPress sites with the affiliate-toolkit plugin installed. Exploitation could allow attackers to pivot from the web server into internal networks, potentially accessing sensitive corporate data, internal APIs, or administrative interfaces that are not exposed externally. This could lead to data breaches, disruption of business operations, and compromise of internal systems. Given the critical severity, attackers could also disrupt service availability or alter data integrity, impacting trust and compliance with data protection regulations such as GDPR. Organizations in sectors with high reliance on web presence and affiliate marketing, including e-commerce, media, and digital services, are particularly vulnerable. The ability to exploit this vulnerability without authentication or user interaction increases the risk of automated attacks and widespread exploitation attempts.

Immediate mitigation steps include: 1) Upgrading the affiliate-toolkit plugin to version 3.4.3 or later once available, as this version addresses the vulnerability by implementing proper authorization and authentication controls. 2) If an official patch is not yet available, temporarily disabling or removing the affiliate-toolkit plugin to eliminate exposure. 3) Implementing web application firewall (WAF) rules to block requests to the atkp_imagereceiver.php endpoint or to restrict outbound HTTP requests from the web server to internal IP ranges. 4) Restricting server-side HTTP requests to internal network addresses via network-level controls or application-level allowlists to prevent SSRF exploitation. 5) Monitoring web server logs for unusual requests targeting the vulnerable endpoint and for outbound connections to internal IPs. 6) Conducting internal network segmentation to limit the impact of SSRF by isolating critical systems from the web server. 7) Reviewing and hardening WordPress security configurations and ensuring minimal plugin usage to reduce attack surface.