CVE-2017-7839 in Firefox
Summary
by MITRE
Control characters prepended before "javascript:" URLs pasted in the addressbar can cause the leading characters to be ignored and the pasted JavaScript to be executed instead of being blocked. This could be used in social engineering and self-cross-site-scripting (self-XSS) attacks where users are convinced to copy and paste text into the addressbar. This vulnerability affects Firefox < 57.
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Analysis
by VulDB Data Team • 03/22/2023
This vulnerability represents a sophisticated browser security flaw that exploits user behavior and input validation weaknesses in the Firefox browser's URL handling mechanism. The issue specifically targets the address bar parsing functionality where control characters prepended to javascript: URLs can bypass security restrictions, allowing malicious code execution. This represents a critical bypass of the browser's security model that typically prevents execution of javascript code from user-supplied input sources.
The technical implementation of this vulnerability stems from improper handling of control characters in URL parsing routines within Firefox's address bar validation system. When users paste URLs containing javascript: protocols with prepended control characters such as tabs, newlines, or other non-printable characters, the browser's parsing logic fails to properly sanitize these inputs. The leading control characters are stripped during parsing, leaving the javascript: protocol intact and executable, while the security mechanisms designed to block such content are effectively circumvented. This flaw operates at the intersection of input sanitization and protocol handling, creating a pathway for malicious code injection.
The operational impact of this vulnerability extends beyond simple code execution to encompass sophisticated social engineering attacks and self-XSS exploitation techniques. Attackers can craft malicious payloads that appear legitimate to users, encouraging them to copy and paste seemingly harmless text into the browser address bar. The vulnerability enables attackers to exploit user trust and cognitive biases, particularly when users encounter what appears to be a legitimate URL or command but actually contains hidden malicious code. This creates a particularly dangerous attack vector because it leverages user behavior rather than requiring complex exploitation techniques. The vulnerability affects Firefox versions prior to 57, representing a significant window of exposure for users who had not yet updated their browsers.
The security implications of this vulnerability align with common attack patterns documented in the attack framework, particularly those involving user interaction and input manipulation. This issue demonstrates how seemingly minor parsing inconsistencies can create substantial security risks, especially when combined with social engineering tactics. The vulnerability can be classified under CWE-170, which addresses improper handling of input that can lead to unexpected behavior in software systems. The attack vector specifically relates to CWE-79, which covers cross-site scripting vulnerabilities, though this instance operates through self-XSS mechanisms where users themselves become the conduit for the attack. Organizations and users should prioritize updating to Firefox 57 or later versions to remediate this vulnerability, as the patch addresses the core URL parsing logic that was allowing control character manipulation.
Mitigation strategies should focus on both immediate browser updates and user education regarding address bar input validation. Security teams should implement monitoring for unusual address bar activity and establish clear protocols for verifying URL legitimacy before execution. The vulnerability highlights the importance of comprehensive input sanitization and the need for robust protocol validation mechanisms that can handle various character encodings and control sequences. Organizations should consider implementing additional security layers such as content security policies and browser security extensions to provide defense-in-depth against similar vulnerabilities. Regular security assessments of browser parsing logic and user interaction points should be conducted to identify potential edge cases that might create similar bypass opportunities.