CVE-2026-75003 in Roundcube
Summary
by MITRE • 08/17/2026
In Roundcube Webmail before 1.6.18 and 1.7.x before 1.7.3, an unclosed url() in a FuncIRI attribute of an SVG image could evade the remote image blocking, which may lead to information disclosure or privilege escalation.
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Analysis
by VulDB Data Team • 08/17/2026
The vulnerability identified in Roundcube Webmail versions prior to 1.6.18 and 1.7.x before 1.7.3 represents a critical security flaw rooted in improper handling of Cascading Style Sheets (CSS) properties within SVG image contexts. Specifically, the issue arises from an unclosed url() function call located within a FuncIRI attribute associated with an SVG element. This technical deficiency allows for the bypassing of remote image blocking mechanisms that are typically employed by webmail clients to prevent automatic loading of external resources. By manipulating this CSS property, an attacker can construct malicious content that circumvents these protective filters, leading to significant security implications including information disclosure and potential privilege escalation.
From a technical perspective, Roundcube utilizes various sanitization layers to process user-supplied data, particularly when rendering HTML or SVG content in the email viewing pane. The remote image blocking feature is designed to prevent the automatic fetching of external images, which serves as a primary defense against tracking pixels and certain types of cross-site scripting attacks that rely on resource loading side effects. However, the parsing logic for CSS properties within SVG elements failed to correctly validate or close url() declarations in specific edge cases involving FuncIRI attributes. This oversight means that when an attacker crafts an email containing such malformed SVG content, the client's sanitizer does not recognize it as a blocked remote image reference. Instead, the browser engine interprets the unclosed URL function and proceeds with resource resolution based on the context provided by the malicious payload.
The operational impact of this vulnerability is severe due to its potential for information disclosure. An attacker can leverage this flaw to force the victim's mail client to send HTTP requests containing sensitive session cookies or authentication tokens to a server controlled by the adversary. This constitutes a form of Cross-Site Request Forgery (CSRF) combined with data exfiltration, where the victim acts as an unwitting proxy for stealing their own credentials. Furthermore, if the application environment allows for further interaction based on these loaded resources or subsequent state changes triggered by the request, it could lead to privilege escalation. For instance, if the successful loading of a resource triggers specific client-side scripts that modify user settings or execute administrative functions without proper verification, an attacker with limited privileges might elevate their access level within the application.
This vulnerability aligns closely with CWE-79, which covers Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting (XSS). Although it manifests through CSS and SVG manipulation rather than direct script injection, the core issue remains the failure to properly neutralize user-supplied input that affects client-side execution context. Additionally, from a threat intelligence perspective, this behavior maps to MITRE ATT&CK technique T1059.007, which involves Command and Scripting Interpreter via JavaScript or similar scripting languages used for data exfiltration over HTTP/S channels. The ability to bypass remote image blocking is also indicative of weaknesses in Content Security Policy (CSP) enforcement or sanitization logic failures often categorized under CWE-20 Improper Input Validation.
Mitigation strategies must focus on immediate patching and enhanced input validation. Administrators should upgrade Roundcube Webmail to version 1.6.18 or later, or if using the 1.7 branch, update to version 1.7.3 or higher where this specific parsing flaw has been addressed by developers. In environments where upgrading is not immediately feasible, implementing strict Content Security Policy headers can help mitigate some risks by restricting source origins for images and scripts. Additionally, deploying web application firewalls with rules specifically targeting malformed SVG content and unclosed CSS functions may provide a layer of defense against exploitation attempts. Regular security audits focusing on the sanitization logic for rich text editors and email rendering engines are recommended to identify similar vulnerabilities in other components that handle complex markup structures.