CVE-2017-9420 in Calendar Plugin
Summary
by MITRE
Cross site scripting (XSS) vulnerability in the Spiffy Calendar plugin before 3.3.0 for WordPress allows remote attackers to inject arbitrary JavaScript via the yr parameter.
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Analysis
by VulDB Data Team • 12/26/2020
The CVE-2017-9420 vulnerability represents a critical cross site scripting flaw in the Spiffy Calendar WordPress plugin affecting versions prior to 3.3.0. This vulnerability exposes WordPress installations to remote code execution risks through improper input validation mechanisms. The flaw specifically manifests when the plugin processes the yr parameter without adequate sanitization, allowing malicious actors to inject arbitrary JavaScript code into calendar displays. The vulnerability falls under the CWE-79 category of Cross-Site Scripting, which is classified as a common weakness in web application security. Attackers can exploit this weakness by crafting malicious URLs containing JavaScript payloads in the yr parameter, which then gets executed in the browsers of unsuspecting users who view the affected calendar pages.
The technical implementation of this vulnerability stems from inadequate input filtering within the Spiffy Calendar plugin's backend processing logic. When users access calendar views with manipulated yr parameters, the plugin fails to properly escape or validate the input before rendering it in HTML output contexts. This lack of proper sanitization creates an environment where attacker-controlled data can be interpreted as executable JavaScript code rather than mere text content. The vulnerability's impact is amplified because WordPress plugins often run with elevated privileges and can access sensitive user data, making this XSS flaw particularly dangerous for compromised installations.
Operationally, this vulnerability enables attackers to perform several malicious activities including session hijacking, defacement of calendar content, redirection to malicious sites, and data exfiltration from authenticated users. The remote nature of the attack means that threat actors can exploit this vulnerability without requiring local system access or user interaction beyond visiting a malicious calendar page. Users with administrative privileges who view affected calendar displays become potential victims of privilege escalation attacks, as the injected JavaScript can access and manipulate WordPress admin interfaces. The vulnerability affects the plugin's calendar rendering functionality, which is typically used for displaying event information, potentially compromising the integrity of all calendar-based content on affected WordPress sites.
Mitigation strategies for CVE-2017-9420 primarily focus on immediate plugin updates to version 3.3.0 or later, which includes proper input sanitization mechanisms. Administrators should implement comprehensive input validation and output encoding practices for all user-supplied data, following OWASP's secure coding guidelines. The implementation of Content Security Policy headers can provide additional defense-in-depth measures by restricting script execution sources and preventing unauthorized code injection. Regular security audits of WordPress plugins should include verification of input handling mechanisms and compliance with established security standards. Organizations should also consider implementing web application firewalls to detect and block suspicious parameter values before they reach the vulnerable plugin components. This vulnerability aligns with ATT&CK technique T1059.007 for Command and Scripting Interpreter, specifically focusing on JavaScript-based attacks, and demonstrates the importance of proper input validation as outlined in the OWASP Top 10 security framework.