CVE-2018-5478 in Contao
Summary
by MITRE • 09/21/2023
Contao 3.x before 3.5.32 allows XSS via the unsubscribe module in the frontend newsletter extension.
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Analysis
by VulDB Data Team • 09/15/2025
The vulnerability identified as CVE-2018-5478 affects Contao content management system versions 3.x prior to 3.5.32, specifically within the frontend newsletter extension's unsubscribe module. This represents a cross-site scripting vulnerability that could be exploited by malicious actors to inject arbitrary script code into web pages viewed by other users. The flaw exists in the handling of user input within the newsletter unsubscribe functionality, which fails to properly sanitize or validate data before rendering it in the browser context.
The technical implementation of this vulnerability stems from insufficient input validation and output encoding practices within the newsletter extension's unsubscribe component. When users attempt to unsubscribe from newsletters, the system processes their input without adequate sanitization measures, allowing attackers to craft malicious payloads that execute in the context of other users' browsers. This vulnerability falls under CWE-79 which specifically addresses cross-site scripting flaws, where web applications fail to properly validate or encode user-controllable data before incorporating it into dynamically generated web pages.
The operational impact of this vulnerability extends beyond simple script injection, as it could enable attackers to perform session hijacking, deface websites, steal sensitive user information, or redirect victims to malicious domains. An attacker could exploit this vulnerability by crafting a specially formatted unsubscribe request containing malicious javascript code that would execute whenever other users view the affected newsletter pages. This threat model aligns with ATT&CK technique T1566 which covers social engineering attacks through malicious links or content, and T1213 which addresses data from information repositories.
The risk is particularly elevated in environments where newsletter subscribers include administrators or privileged users who might inadvertently click on malicious links. The vulnerability demonstrates poor secure coding practices in input sanitization and output encoding, which are fundamental requirements for preventing XSS attacks according to OWASP top ten security principles. Organizations using Contao 3.x versions should immediately implement patches to address this vulnerability, as the timeframe between vulnerability disclosure and potential exploitation in the wild typically ranges from weeks to months. The recommended mitigation involves upgrading to Contao 3.5.32 or later versions, which contain proper input validation and output encoding mechanisms that prevent malicious script execution in the unsubscribe module's frontend components.