CVE-2018-5143 in Firefox
Summary
by MITRE
URLs using "javascript:" have the protocol removed when pasted into the addressbar to protect users from cross-site scripting (XSS) attacks, but if a tab character is embedded in the "javascript:" URL the protocol is not removed and the script will execute. This could allow users to be socially engineered to run an XSS attack against themselves. This vulnerability affects Firefox < 59.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 02/05/2021
The vulnerability described in CVE-2018-5143 represents a critical security flaw in Mozilla Firefox's URL handling mechanism that specifically targets the browser's protection against cross-site scripting attacks. This issue exploits a subtle inconsistency in Firefox's sanitization process for potentially dangerous URLs that begin with the javascript: protocol. The vulnerability demonstrates how seemingly minor implementation details can create significant security risks, particularly when dealing with user input that should be strictly controlled and sanitized. The flaw exists in Firefox versions prior to 59, indicating that this was a well-known issue that required immediate attention from the security community and browser vendors.
The technical nature of this vulnerability stems from Firefox's protocol removal mechanism that normally strips the javascript: prefix from URLs to prevent XSS attacks. However, the implementation contains a specific edge case where the presence of a tab character within the javascript: URL prevents the protocol removal from occurring properly. This allows malicious actors to craft URLs that appear benign but actually contain executable javascript code. The tab character serves as a delimiter that bypasses the sanitization logic, enabling the script to execute in the browser context. This behavior creates a dangerous scenario where users might inadvertently execute malicious code by simply pasting a specially crafted URL into the address bar, even when they believe they are following safe browsing practices.
The operational impact of this vulnerability extends beyond simple XSS attacks, as it leverages social engineering tactics to exploit user trust and behavior. Attackers can craft URLs that appear legitimate but contain embedded tab characters that bypass Firefox's security protections. This creates a sophisticated attack vector where users are tricked into executing code against themselves, making it particularly dangerous because the victim is essentially performing the action that triggers the exploit. The vulnerability affects the fundamental security model of the browser's address bar protection system, undermining user confidence in the browser's ability to protect against common web-based attacks. This type of vulnerability falls under the CWE-79 category of Cross-Site Scripting, specifically demonstrating how improper input validation can lead to code execution, and aligns with ATT&CK technique T1203 for Exploitation for Client Execution.
The mitigation for this vulnerability required Firefox developers to enhance their URL parsing and sanitization logic to properly handle all types of whitespace characters, including tab characters, within javascript: URLs. The fix involved updating the protocol removal mechanism to account for various whitespace characters that could be embedded within the javascript: prefix. This represents a classic case of security through defense in depth, where multiple layers of protection are necessary to prevent bypasses of security mechanisms. The vulnerability also highlights the importance of thorough testing of edge cases in security implementations, particularly when dealing with user input that contains various types of whitespace and special characters. Organizations should ensure that their Firefox installations are updated to version 59 or later, where this vulnerability has been properly addressed through improved URL sanitization routines that consistently remove the javascript: protocol regardless of embedded whitespace characters.