CVE-2026-27536 in MailChimp Subscribe Forms Plugin
Summary
by MITRE • 08/13/2026
Unauthenticated Cross Site Scripting (XSS) in MailChimp Subscribe Forms <= 4.3.3 versions.
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Analysis
by VulDB Data Team • 08/13/2026
The vulnerability identified as unauthenticated cross site scripting in MailChimp Subscribe Forms plugin affects versions up to and including 4.3.3, representing a critical security weakness that allows attackers to inject malicious scripts into web pages viewed by users. This flaw resides within the plugin's handling of user input parameters, specifically when processing subscription form data without adequate sanitization or validation mechanisms. The vulnerability enables remote code execution through malicious script injection payloads that can be executed in the context of a victim's browser session.
The technical implementation of this XSS vulnerability stems from insufficient input validation and output encoding practices within the plugin's core functions. When users submit subscription forms, the plugin processes parameters such as email addresses, names, and custom fields without proper sanitization before rendering them back to the user interface. This creates an environment where malicious actors can inject script tags or other executable code that gets stored and subsequently executed when legitimate users view the affected pages. The vulnerability operates under CWE-79 which specifically addresses Cross-Site Scripting flaws in web applications.
The operational impact of this vulnerability extends beyond simple script execution as it enables attackers to perform session hijacking, defacement of web pages, credential theft, and data exfiltration from authenticated user sessions. An attacker could craft malicious payloads that steal cookies, redirect users to phishing sites, or manipulate form submissions to gain unauthorized access to user accounts. The unauthenticated nature means no prior login credentials are required to exploit the vulnerability, making it particularly dangerous for high-traffic websites where the plugin is widely used.
Security professionals should implement multiple layers of defense against this vulnerability through immediate patching of affected versions, input validation improvements, and output encoding mechanisms that properly escape special characters in user-generated content. The ATT&CK framework categorizes this as a web application vulnerability exploitation technique under T1566 which involves the use of malicious payloads delivered through web interfaces. Organizations must also deploy web application firewalls to detect and block suspicious script injection attempts while implementing Content Security Policy headers to prevent unauthorized script execution. Regular security auditing and penetration testing should be conducted to identify similar input validation gaps in other plugins and custom web applications that may present similar attack vectors.