Payload CMS is an open-source headless content management system that prior to version 3.75.0 contained a Server-Side Request Forgery (SSRF) vulnerability identified as CVE-2026-27567 (CWE-918). This vulnerability exists in the external file upload functionality, where the system insufficiently validates HTTP redirects when processing URLs for file uploads. Specifically, if an authenticated user has create permissions on at least one collection with upload enabled, they can supply crafted URLs that cause the server to make HTTP requests to internal network resources. Because the server follows redirects without proper validation, an attacker can leverage this to access internal services that are otherwise inaccessible externally. The attacker can then retrieve and view response content from these internal services via the Payload CMS interface, potentially exposing sensitive internal information or services. Exploitation requires authentication and specific permissions, limiting the attack surface to authorized users with upload create rights. The vulnerability does not affect availability but impacts confidentiality and integrity by exposing internal data and potentially allowing further attacks. The issue was addressed in Payload CMS version 3.75.0 by improving validation and handling of external URLs in file uploads. As a temporary mitigation, administrators can disable external file uploads using the `disableExternalFile` option or restrict create access on upload-enabled collections to trusted users only. The CVSS v3.1 score of 6.5 reflects a medium severity, with network attack vector, low attack complexity, high privileges required, no user interaction, and high confidentiality and integrity impact. No public exploits have been reported so far.

The primary impact of this SSRF vulnerability is unauthorized access to internal network resources through the Payload CMS server, which acts as a proxy for HTTP requests. This can lead to exposure of sensitive internal services, data leakage, and potential reconnaissance for further attacks within an organization's internal network. Confidentiality is significantly affected as attackers can retrieve response content from internal endpoints. Integrity could also be impacted if internal services are manipulated or exploited further after discovery. Availability is not directly affected by this vulnerability. Since exploitation requires authenticated users with specific permissions, the risk is higher in environments with multiple users or where user access controls are lax. Organizations using vulnerable versions of Payload CMS with upload-enabled collections and multiple users with create permissions are at risk. This could affect internal services that are not otherwise exposed to the internet, increasing the attack surface. The vulnerability could be leveraged by malicious insiders or compromised accounts to pivot into internal networks. Although no known exploits are reported in the wild, the medium CVSS score and the nature of SSRF vulnerabilities suggest that attackers could weaponize this flaw if discovered. The impact is particularly critical for organizations relying on Payload CMS for content management in sensitive environments, such as enterprises, government agencies, and service providers.

1. Upgrade Payload CMS to version 3.75.0 or later immediately to apply the official patch that fixes the SSRF vulnerability. 2. If upgrading is not immediately possible, disable external file uploads by setting the `disableExternalFile` option in upload-enabled collections to prevent processing of external URLs. 3. Restrict 'create' permissions on upload-enabled collections strictly to trusted and verified users to minimize the risk of exploitation by unauthorized or compromised accounts. 4. Implement network segmentation and firewall rules to limit the Payload CMS server's ability to access sensitive internal services, reducing the impact of potential SSRF exploitation. 5. Monitor logs for unusual or unexpected outbound HTTP requests originating from the Payload CMS server, which could indicate attempted exploitation. 6. Conduct regular access reviews and enforce the principle of least privilege for users with upload permissions. 7. Employ web application firewalls (WAFs) with rules to detect and block SSRF patterns targeting the Payload CMS endpoints. 8. Educate administrators and developers about the risks of SSRF and the importance of validating and sanitizing user-supplied URLs in file upload features. These targeted mitigations go beyond generic advice by focusing on configuration options, access control, network restrictions, and monitoring specific to Payload CMS environments.