June 4, 2024

Exploitation of Roundcube Webmail XSS Vulnerability (CVE-2023-43770)

U.S. Cybersecurity and Infrastructure Security Agency (CISA), recently issued a warning regarding the exploitation of CVE-2023-43770, a vulnerability discovered in the Roundcube webmail software. This vulnerability, which was addressed and fixed in September 2023, is currently being actively exploited by malicious attackers. As a result, CISA has included this vulnerability in its Known Exploited Vulnerabilities (KEV) catalog, signaling the importance of taking immediate action to safeguard systems and prevent unauthorized access.

About the Vulnerability

Roundcube is an open-source IMAP client that operates within a web browser and boasts an intuitive user interface akin to a standalone application. However, a significant security flaw in the form of CVE-2023-43770 has been identified. This flaw is categorized as a persistent cross-site scripting (XSS) bug, enabling attackers to gain unauthorized access to restricted information. Exploitation of this vulnerability occurs through the utilization of specially crafted links in plain/text messages, requiring user interaction but with relatively low complexity. Roundcube email servers running versions newer than 1.4.14, 1.5.x prior to 1.5.4, and 1.6.x before 1.6.3 are susceptible to this vulnerability.

The Impact of the Vulnerability

This vulnerability poses a substantial threat to Roundcube users, as it grants attackers the ability to execute arbitrary code within the victim's browser environment. Exploiting this vulnerability can result in a range of malicious outcomes, including:

Theft of User Credentials

Attackers can pilfer login credentials for Roundcube or other websites accessed through the same browser, compromising the victim's accounts and potentially exposing personal or sensitive information.

Session Hijacking

By exploiting this vulnerability, attackers can hijack the victim's active Roundcube session, gaining unauthorized access to their email messages, contacts, and other confidential data, leading to privacy breaches and unauthorized disclosure of sensitive information.

Phishing Attacks

The injected code can be utilized by attackers to display deceptive phishing forms that mimic legitimate Roundcube elements. Unsuspecting users may unknowingly provide their login credentials, falling victim to the attackers' phishing scheme.

Data Exfiltration

Exploiting this vulnerability grants attackers the ability to extract sensitive information stored within the victim's email account. This can include personal communications, attachments, and other private data, potentially leading to identity theft, blackmail, or unauthorized access to sensitive systems.


To mitigate the risks associated with this vulnerability, it is recommended to take the following actions:

Upgrade Roundcube

Ensure that your Roundcube installation is updated to the latest patched versions, such as 1.4.14, 1.5.4, or 1.6.3. These versions include fixes specifically addressing the identified vulnerability. Keeping your software up to date helps protect against known security flaws.

Implement Input Sanitization

Modify the replace function within Roundcube to include input sanitization mechanisms. This involves implementing measures to sanitize user-supplied patterns before performing replacements. Consider leveraging established libraries like HTMLPurifier or implementing custom filtering rules to sanitize and validate input effectively. This can help prevent the execution of arbitrary code and mitigate the impact of potential attacks.

Configure Content Security Policy (CSP)

Implement a robust Content Security Policy for your Roundcube webmail interface. CSP allows you to define and enforce restrictions on the types of scripts and content that can be executed within the browser. By configuring appropriate CSP directives, you can limit the potential risks associated with cross-site scripting (XSS) attacks and reduce the attack surface for malicious code injections.

CVE-2023-43770 is a critical vulnerability that demands immediate attention. Taking prompt action and implementing the mitigation steps mentioned above can effectively minimize the risk of exploitation for both users and administrators. By following these measures, the potential impact of the vulnerability can be significantly reduced.

Nexusguard and Customers Remain Unaffected by Vulnerability

Nexusguard's Application Protection is a formidable security solution designed to offer extensive defense against a wide range of attacks spanning L3-L4 and L7 layers. This robust solution ensures comprehensive protection, including safeguarding against potential zero-day attacks, thereby effectively fortifying your applications with unparalleled efficiency and effectiveness.

Leveraging its cloud-based, enterprise-grade Web Application Firewall (WAF), Nexusguard's Application Protection provides a reliable shield for public-facing websites and applications. With its advanced capabilities, it effectively guards against the Top 10 OWASP vulnerabilities, such as the critical Cross-Site Scripting vulnerability. Rest assured that your applications are secure and shielded against these common and high-risk threats with Nexusguard's Application Protection.

For more information, please read about Nexusguard’s Application Protection or reach out to our experts via our contact form.

Further Steps to Help Safeguard Your Organization

As part of our commitment to providing comprehensive web application security, Nexusguard Academy offers the Nexusguard Web Application Security course. This program is designed to equip individuals with invaluable knowledge on a range of web application security vulnerabilities, including the critical concern of Cross-Site Scripting (XSS). Through this course, participants will gain a deep understanding of these vulnerabilities and acquire essential skills to effectively safeguard web applications against potential threats. For more information, please read about Nexusguard Academy.

Nexusguard's Application Protection offers unparalleled defense against critical vulnerabilities like CVE-2023-43770. With our cloud-based WAF, your public-facing websites and applications remain secure from XSS attacks and other high-risk threats.

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