CVE-2017-7803 in Firefox
Summary
by MITRE
When a page's content security policy (CSP) header contains a "sandbox" directive, other directives are ignored. This results in the incorrect enforcement of CSP. This vulnerability affects Thunderbird < 52.3, Firefox ESR < 52.3, and Firefox < 55.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability described in CVE-2017-7803 represents a critical flaw in the implementation of Content Security Policy (CSP) enforcement mechanisms within Mozilla Firefox and Thunderbird browsers. This issue stems from a fundamental misinterpretation of CSP directive processing logic where the presence of a sandbox directive causes the browser to completely disregard all other CSP directives in the header. The vulnerability affects versions prior to Firefox 55 and Firefox ESR 52.3, as well as Thunderbird versions before 52.3, indicating a widespread impact across the Mozilla ecosystem. The flaw demonstrates a classic case of improper input validation and security policy enforcement that directly compromises the intended security posture of web applications.
The technical root cause of this vulnerability lies in the browser's CSP parser and enforcement engine which incorrectly handles the interaction between different CSP directives. When a web server sends a CSP header containing both a sandbox directive and other directives such as script-src, style-src, or frame-src, the browser's processing logic fails to properly evaluate all directives. Instead, the presence of the sandbox directive triggers a bypass mechanism that effectively nullifies the enforcement of other security directives, creating a scenario where malicious content can execute despite the presence of seemingly robust CSP policies. This behavior violates the fundamental principle of least privilege that CSP is designed to enforce, allowing attackers to circumvent security controls that should prevent unauthorized script execution, frame loading, or other potentially harmful activities.
The operational impact of this vulnerability is severe as it fundamentally undermines the security model that web applications rely upon for protection against cross-site scripting attacks, malicious frame injection, and other code execution threats. Attackers can exploit this flaw by crafting CSP headers that include sandbox directives while containing malicious script sources or frame ancestors in other directives. The vulnerability creates a false sense of security for web developers and administrators who may believe their CSP policies are properly enforced, when in reality the sandbox directive effectively disables the protection provided by other CSP mechanisms. This allows for bypassing of script-src directives that should prevent unauthorized JavaScript execution, frame-src directives that should restrict frame loading, and other security controls that form the backbone of modern web application security. The vulnerability aligns with CWE-693, which covers protection mechanism failures, and represents a specific instance of improper enforcement of security policies.
Mitigation strategies for this vulnerability require immediate updates to affected browser versions, as the flaw cannot be effectively addressed through client-side configuration changes alone. Organizations should prioritize patching all affected systems to versions 52.3 or later, which contain the necessary fixes to properly enforce CSP directives regardless of the presence of sandbox directives. Security teams should also conduct comprehensive reviews of existing CSP policies to identify any that may be relying on the incorrect behavior of the sandbox directive, ensuring that security controls are properly implemented and validated. Additionally, the vulnerability highlights the importance of thorough security testing and validation of security policy implementations, as demonstrated by the ATT&CK technique T1068 which covers privilege escalation through exploitation of system vulnerabilities. Network administrators should consider implementing additional monitoring and detection mechanisms to identify potential exploitation attempts targeting this specific CSP bypass mechanism, while developers should ensure their CSP policies are tested against various directive combinations to verify proper enforcement across different browser implementations.