CVE-2017-7834 in Firefox
Summary
by MITRE
A "data:" URL loaded in a new tab did not inherit the Content Security Policy (CSP) of the original page, allowing for bypasses of the policy including the execution of JavaScript. In prior versions when "data:" documents also inherited the context of the original page this would allow for potential cross-site scripting (XSS) attacks. This vulnerability affects Firefox < 57.
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Analysis
by VulDB Data Team • 03/22/2023
The vulnerability described in CVE-2017-7834 represents a critical security flaw in Firefox's handling of data URLs within new browser tabs. This issue stems from the improper inheritance of Content Security Policy (CSP) directives when data URLs are opened in separate tabs, creating a significant bypass mechanism that undermines the security protections designed to prevent cross-site scripting attacks. The flaw specifically affects Firefox versions prior to 57, where the browser failed to maintain the CSP context when loading data URLs in new tab environments.
The technical implementation of this vulnerability occurs at the browser's URL processing layer where data URLs are treated differently from regular web addresses. When a user clicks on a link that resolves to a data URL, and this URL opens in a new tab, the browser should maintain the security context of the originating page's CSP policies. However, in affected versions, the data URL tab operates with a completely separate security context that does not inherit the parent page's CSP, effectively nullifying the security controls that would normally prevent script execution and other malicious activities.
This flaw creates a direct pathway for attackers to bypass CSP protections that are fundamental to modern web security architectures. The vulnerability enables what is classified as a cross-site scripting attack vector under CWE-79, where malicious scripts can be executed in contexts where they would normally be blocked by security policies. The bypass occurs because the data URL tab operates outside the security boundaries established by the originating page's CSP, allowing for unrestricted script execution that could include malicious payloads designed to steal session cookies, perform unauthorized actions, or redirect users to malicious sites.
The operational impact of this vulnerability extends beyond simple script execution, as it fundamentally undermines the trust model that web browsers implement to protect users from malicious content. When a user navigates to a page with strict CSP policies and then clicks on a data URL that opens in a new tab, the security boundary that should protect against XSS attacks is effectively removed. This creates a window of opportunity for attackers to exploit the inconsistency in security policy enforcement between different tab contexts. The vulnerability affects the browser's security model at a core level, specifically impacting how the browser handles the relationship between different security contexts and their respective CSP policies.
The mitigation for this vulnerability requires updating Firefox to version 57 or later, where the browser correctly implements CSP inheritance for data URLs opened in new tabs. Security researchers and system administrators should ensure that all Firefox installations are updated to prevent exploitation of this bypass mechanism. The fix implemented in Firefox 57 addresses the fundamental issue by ensuring that data URLs opened in new tabs properly inherit the CSP context of their originating pages, thereby maintaining the security boundaries that protect against cross-site scripting attacks. This vulnerability also highlights the importance of consistent security policy enforcement across all browser contexts and serves as a reminder of the critical need for proper CSP implementation in modern web browsers. Organizations should monitor their browser security configurations and ensure that all users are running patched versions to prevent exploitation of this type of policy bypass vulnerability. The ATT&CK framework categorizes this as a technique for privilege escalation through browser security model exploitation, where attackers leverage implementation flaws to bypass security controls that would normally protect against malicious script execution.