CVE-2017-7788 in Firefox
Summary
by MITRE
When an "iframe" has a "sandbox" attribute and its content is specified using "srcdoc", that content does not inherit the containing page's Content Security Policy (CSP) as it should unless the sandbox attribute included "allow-same-origin". This vulnerability affects Firefox < 55.
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Analysis
by VulDB Data Team • 01/09/2021
The vulnerability described in CVE-2017-7788 represents a significant bypass of web security mechanisms within Firefox browsers version 55 and earlier. This issue specifically targets the implementation of sandboxed iframe content and how it interacts with Content Security Policy enforcement. The flaw occurs when an iframe element utilizes both the sandbox attribute and the srcdoc attribute to load content, creating a scenario where the security protections that should be inherited from the parent page are not properly applied. This represents a direct violation of expected web security practices and demonstrates a failure in the browser's security model implementation.
The technical root cause of this vulnerability stems from improper handling of Content Security Policy inheritance within sandboxed contexts. When an iframe element contains both the sandbox attribute and srcdoc attribute, the browser should ensure that the content loaded through srcdoc inherits the parent page's CSP directives. However, in affected versions of Firefox, this inheritance only occurs when the sandbox attribute explicitly includes the allow-same-origin flag. Without this specific flag, the sandboxed content operates with reduced security protections, effectively creating a bypass of the expected CSP enforcement mechanism. This behavior creates a security gap where malicious content within sandboxed iframes could potentially exploit the absence of proper CSP enforcement.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it fundamentally undermines the security model that web applications rely upon for protection against cross-site scripting and other code injection attacks. Attackers could potentially leverage this vulnerability to bypass security controls that would normally prevent execution of malicious scripts or access to sensitive resources. The vulnerability particularly affects web applications that depend on CSP to protect against script injection attacks, as the compromised iframe content could potentially execute code that would otherwise be blocked by the parent page's CSP directives. This creates a dangerous scenario where security boundaries are weakened in a way that could be exploited in targeted attacks against vulnerable web applications.
Security researchers have classified this vulnerability under CWE-284, which deals with improper access control mechanisms, and it aligns with ATT&CK technique T1059.007 for script-based attacks and T1211 for privilege escalation through browser vulnerabilities. The mitigation strategy involves ensuring that Firefox browsers are updated to version 55 or later, where the proper CSP inheritance behavior has been implemented. Organizations should also review their web application security policies to ensure that they do not rely on CSP enforcement within sandboxed contexts, as the vulnerability demonstrates that such reliance may be unsafe in older browser versions. Additionally, security teams should consider implementing additional monitoring for potentially malicious iframe content and ensure that CSP policies are properly configured even when using sandboxed elements.
This vulnerability highlights the complexity of modern web security implementations and the critical importance of proper inheritance mechanisms in browser security models. The issue demonstrates how seemingly minor implementation details in browser security can create significant vulnerabilities that affect the broader web ecosystem. The fix implemented in Firefox 55 demonstrates the importance of proper security testing and validation of security model implementations, particularly in areas where multiple security mechanisms interact with each other. Organizations should prioritize updating affected browsers and consider the broader implications of browser security model inconsistencies when designing web application security strategies. The vulnerability also underscores the need for continuous security assessment and monitoring of browser implementations to identify and address potential security gaps before they can be exploited by malicious actors.