CVE-2026-53641 in FOSSBilling
Summary
by MITRE • 07/07/2026
FOSSBilling is a free, open-source billing and client management system. Versions 0.6.0 through 0.7.2 have a stored cross-site scripting (XSS) vulnerability in the client-facing email history views of FOSSBilling. Email HTML content (`content_html`) is rendered into a JavaScript template literal using the `|raw` filter, bypassing all output escaping. An attacker with admin access can inject malicious JavaScript payloads into email content that execute in the browser of any client who views their email history. Version 0.8.0 contains a fix. Some workarounds are available. Restrict admin account access, audit email content in the database for suspicious payloads, and/or monitor client accounts for unusual activity.
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Analysis
by VulDB Data Team • 07/07/2026
The FOSSBilling system presents a critical stored cross-site scripting vulnerability that affects versions ranging from 0.6.0 through 0.7.2, specifically within the client-facing email history views. This vulnerability stems from improper output escaping mechanisms where HTML content submitted through email templates is rendered directly into JavaScript template literals without adequate sanitization. The use of the `|raw` filter in the templating system bypasses all standard output escaping protocols that would normally prevent malicious code execution, creating an environment where attackers can inject persistent JavaScript payloads into email content that will execute whenever clients view their email history.
The technical flaw operates through a combination of template injection and insufficient input validation mechanisms. When administrators process emails containing HTML content, the system treats this content as trusted data and directly incorporates it into JavaScript contexts without proper sanitization. This creates a persistent XSS vector where malicious scripts can be stored in the database and executed in the browsers of any user who accesses their email history. The vulnerability is particularly dangerous because it leverages administrative privileges to compromise client sessions, potentially enabling attackers to steal session cookies, perform unauthorized actions on behalf of clients, or redirect users to malicious domains.
The operational impact of this vulnerability extends beyond simple script execution capabilities and represents a significant threat to client data integrity and system security. Attackers with administrative access can craft malicious email content that persists in the database and executes automatically when clients view their email history, potentially leading to session hijacking, credential theft, or data exfiltration. The stored nature of this vulnerability means that once exploited, the malicious payloads remain active until manually removed from the database, creating ongoing security risks for all affected users.
Mitigation strategies for this vulnerability include implementing the official fix available in version 0.8.0 which addresses the improper output escaping and template handling mechanisms. Organizations should immediately upgrade to the patched version and implement multiple defensive measures including restricting administrative account access to minimize potential attack vectors. Database auditing procedures should be established to monitor email content for suspicious payloads, particularly looking for JavaScript execution patterns or known malicious indicators. Additionally, monitoring client accounts for unusual activity patterns can help detect potential exploitation attempts, while network-level monitoring may reveal attempts to redirect users to malicious domains. This vulnerability aligns with CWE-79 (Cross-site Scripting) and represents a classic example of how improper output escaping in web applications creates persistent security risks that can be exploited through administrative privileges.
The attack surface for this vulnerability demonstrates the importance of proper input validation and output sanitization across all user-supplied content processing pathways. Organizations using FOSSBilling should also consider implementing additional security controls such as content security policies, regular security assessments, and comprehensive monitoring of administrative activities to prevent similar vulnerabilities from being exploited in other components of the system. The incident highlights the critical need for maintaining up-to-date software versions and implementing robust access control measures to protect against privilege escalation attacks that can lead to widespread client compromise.