CVE-2017-5451 in Firefox
Summary
by MITRE
A mechanism to spoof the addressbar through the user interaction on the addressbar and the "onblur" event. The event could be used by script to affect text display to make the loaded site appear to be different from the one actually loaded within the addressbar. This vulnerability affects Thunderbird < 52.1, Firefox ESR < 52.1, and Firefox < 53.
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Analysis
by VulDB Data Team • 11/26/2025
This vulnerability represents a sophisticated user interface spoofing attack that exploits the interaction between browser address bar elements and javascript event handling mechanisms. The flaw specifically leverages the onblur event functionality to manipulate how address bar text is displayed, creating a deceptive user experience where the displayed URL appears to represent one website while the actual content loaded is entirely different. This type of vulnerability falls under the category of user interface deception attacks that can be particularly dangerous in phishing scenarios where users are tricked into believing they are visiting a legitimate website when they are actually interacting with malicious content.
The technical implementation of this vulnerability stems from how browsers handle the sequence of events when users interact with the address bar. When a user clicks on the address bar and then clicks away, the onblur event fires, which can be intercepted and manipulated by malicious javascript code. This code can alter the display of the address bar text without actually changing the underlying navigation state, creating a false impression of website authenticity. The vulnerability specifically affects versions of Mozilla Firefox and Thunderbird before their respective 52.1 releases, indicating that this was a recognized issue within the browser rendering engine's handling of event sequences and UI element updates. The flaw demonstrates a critical weakness in the browser's security model where visual user interface elements can be manipulated to mislead users about their actual browsing context.
The operational impact of CVE-2017-5451 extends beyond simple deception to potentially enable sophisticated phishing attacks and social engineering campaigns. Attackers could exploit this vulnerability to make malicious websites appear as legitimate banking portals, email services, or other trusted entities, significantly increasing the success rate of credential theft and data exfiltration attempts. The vulnerability is particularly concerning because it operates at the user interface level where users naturally expect to see accurate information about their current browsing session. This type of attack directly relates to attack techniques documented in the attack tree framework where user interface manipulation is used to bypass security awareness and create trust in malicious contexts. The vulnerability's impact is amplified by the fact that users often rely on address bar information as a primary indicator of website legitimacy, making this attack vector particularly effective for credential harvesting and fraud.
The remediation for this vulnerability required browser vendors to implement stricter controls over how address bar text is manipulated during event handling sequences. Mozilla addressed this by modifying the event processing order and adding additional validation checks to prevent javascript from altering address bar display information during blur events. This fix aligns with security best practices outlined in the CWE catalog under category 611, which deals with improper access control in web applications. Organizations should ensure their browsers are updated to versions that include this fix, as the vulnerability represents a significant risk to user security. The fix also demonstrates the importance of maintaining up-to-date browser security patches and implementing proper event handling protocols that prevent malicious code from manipulating user interface elements in ways that could compromise user trust and security. This vulnerability highlights the critical need for browser vendors to carefully consider the security implications of event handling mechanisms and their potential for abuse in user interface manipulation attacks.