CVE-2026-89294 in Simply Schedule Appointments Plugin
Summary
by MITRE • 09/30/2026
The Simply Schedule Appointments plugin for WordPress is vulnerable to Local File Inclusion in all versions up to, and including, 1.6.12.27 via the 'ssa_locale' parameter parameter. This makes it possible for authenticated attackers, with subscriber-level access and above, to include and execute arbitrary .php files on the server, allowing the execution of any PHP code in those files. This can be used to bypass access controls, obtain sensitive data, or achieve code execution in cases where .php file types can be uploaded and included. Notably, exploitation does not require authentication in practice, as the locale filter is installed unconditionally on every request during plugins_loaded and the callback performs no nonce or capability check before returning the raw GET parameter value.
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Analysis
by VulDB Data Team • 09/30/2026
The vulnerability identified within the Simply Schedule Appointments plugin for WordPress represents a critical Local File Inclusion (LFI) flaw affecting all versions up to 1.6.12.27. This security defect is triggered through the manipulation of the ssa_locale parameter, which serves as an input vector for file path resolution on the server side. The core technical issue lies in the insufficient validation and sanitization of user-supplied data before it is utilized by PHP include functions. When a request containing a maliciously crafted ssa_locale value reaches the application, the plugin fails to restrict the input to expected locale identifiers such as standard language codes. Instead, it passes this raw input directly into file inclusion logic, allowing an attacker to specify arbitrary paths on the server's filesystem. This lack of strict allow-listing or path normalization enables directory traversal sequences, typically represented by dot-dot-slash characters, which can navigate outside the intended plugin directories and access sensitive system files or other uploaded content.
From a technical perspective, this vulnerability is classified under CWE-98 Improper Control of Filename for Include/Require Statement in PHP Programs. The severity of this flaw is significantly amplified by its exploitation characteristics regarding authentication requirements. While the official description suggests that authenticated attackers with subscriber-level access and above are required to exploit the issue, practical analysis reveals a critical oversight in the plugin's initialization sequence. The locale filter responsible for handling the ssa_locale parameter is installed unconditionally during the plugins_loaded hook phase of WordPress execution. Crucially, this callback function performs no nonce verification or capability checks before returning the raw GET parameter value. This architectural design flaw effectively neutralizes authentication barriers for this specific endpoint, allowing any user with network access to the site, including anonymous visitors, to trigger the file inclusion mechanism without needing valid credentials.
The operational impact of exploiting this Local File Inclusion vulnerability is severe and multifaceted. By successfully injecting a path to an arbitrary PHP file stored on the server, an attacker can achieve remote code execution (RCE). If the attacker has previously uploaded malicious files through other vulnerabilities or features that allow file uploads, they can include those files to execute custom PHP code with the privileges of the web server process. Even without pre-existing malicious files, if there are any legitimate but vulnerable scripts accessible via the filesystem path provided by the LFI, their contents will be executed in the context of the current request. This capability allows attackers to bypass existing access controls, exfiltrate sensitive data such as database credentials or user information stored in configuration files, and potentially gain full control over the compromised WordPress installation. The ability to execute arbitrary code transforms a simple input validation error into a complete system compromise scenario.
Mitigation strategies must address both immediate remediation and long-term security posture improvements. For administrators currently running affected versions of Simply Schedule Appointments, the most effective mitigation is to upgrade immediately to version 1.6.12.28 or later, where this vulnerability has been patched by implementing proper input validation and restricting allowed locale values to a strict whitelist. Until an update can be applied, temporary mitigations may include disabling file inclusion functions in PHP configuration if feasible, though this is often impractical for dynamic applications. Additionally, ensuring that the web server restricts access to sensitive directories through proper permissions and firewall rules can limit the impact of successful exploitation attempts. From a development standpoint, future implementations must enforce strict input validation on all parameters used in include statements, utilize absolute paths where possible, and ensure that security checks such as nonce verification are applied consistently across all hooks regardless of their perceived risk level.
This vulnerability aligns with several entries in the MITRE ATT&CK framework, particularly those related to Initial Access and Execution phases. The exploitation technique mirrors T1504 Weak or Unprotected Credentials if authentication were required, but more accurately reflects T1190 Exploit Public-Facing Application due to its potential for unauthenticated access via web parameters. Furthermore, the mechanism of including arbitrary files relates to T1059 Command and Scripting Interpreter, as it allows the execution of PHP scripts that are not intended by the application logic. Understanding these mappings helps in categorizing the threat correctly within broader security operations centers and incident response playbooks. The case underscores the importance of treating all user inputs with skepticism, regardless of their apparent innocuousness or the perceived low privilege level required for access, as architectural oversights can elevate simple bugs into critical vulnerabilities.