Browser Attack Surface

Browser Attack surface

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In the vast landscape of cybersecurity, vulnerabilities often present themselves as a series of interconnected events, aptly termed the “vulnerability kill chain.” Each link in this chain provides insights into the cascading steps that can lead to unintended consequences for software applications. Today, we delve deep into an intriguing flaw that strikes at the heart of one of the world’s most popular web browsers: Firefox. For Linux users, something as innocuous as opening an empty .atom or .rss file from a local source can trigger an infinite tab opening loop, causing a complete breakdown of the browser’s functionality. Join us as we dissect this vulnerability, step by step, and navigate the intricate maze that leads to Firefox’s unexpected crash.

Browser Security Risks

Web browsers are more than just tools for accessing information; they are the gateways to our digital identities, the portals through which we engage with the vast universe of the Internet. As our dependency on these platforms grows, so too does the attention they receive from those with malicious intent. Recent years have seen a disturbing surge in browser-based vulnerabilities, a phenomenon not merely incidental but emblematic of the evolving cybersecurity landscape. This spike in potential threats isn’t just a matter of numbers but speaks to a deeper, more insidious trend. Cyber adversaries are no longer merely exploiting known vulnerabilities—they are crafting complex, adaptive strategies designed to probe, test, and breach browser defenses. As we peel back the layers on this issue, we delve into an environment marked by subterfuge, innovation, and a constant game of cat and mouse between defenders and attackers. This article seeks to illuminate the intricate dance between browser security mechanisms and the sophisticated techniques employed by cybercriminals, with a spotlight on the revealing world of adversary simulation operations.

The Ever-Present Threat Landscape

Browser vulnerabilities are not a novel threat; they’ve been a persistent concern since the dawn of the internet era. However, the surge by over 20% in just the last year, as highlighted in the 2021 report, emphasizes an acceleration of risk. This rapid increase raises immediate questions about the evolving nature of these vulnerabilities and the broader implications for digital security.

The Pandemic’s Digital Pivot

The COVID-19 pandemic dramatically altered the global digital landscape. With lockdowns and remote work becoming the new norm, there was a notable surge in online activity. Personal, professional, and educational interactions shifted to the web, leading to a more extensive and more varied user base. This uptick in traffic, combined with users perhaps less familiar with online best practices, created a vast playground for cybercriminals.

Cybercriminal Evolution

While the pandemic broadened the attack surface, it was the evolution in cybercriminal strategies that exploited it. Gone are the days of rudimentary phishing attacks. Today’s hackers employ sophisticated methods, combining multiple vulnerabilities, deploying zero-day exploits, and leveraging advanced persistent threats (APTs) to penetrate defenses.

Proliferation of Web Technologies

The web ecosystem has witnessed a diversification of technologies and platforms, such as WebAssembly, progressive web apps, and single-page applications. While these innovations enhance user experience and functionality, they also introduce novel vulnerabilities, broadening the attack spectrum.

Attack Sophistication and Economic Incentives

The dark web and cybercrime forums have become hotbeds for exchanging tools, strategies, and even zero-day exploits. With ransomware attacks becoming increasingly profitable, there’s a heightened economic incentive for hackers to identify and exploit browser-based vulnerabilities.

The Challenge of Patch Management

One key factor contributing to the vulnerability surge is the challenge of timely patch management. Software developers constantly race against time, striving to identify and rectify vulnerabilities. Yet, even when patches are available, a significant proportion of users delay or even neglect updates, leaving themselves exposed.

The Intersection with Other Technologies

Modern browsers integrate with a plethora of other applications and technologies, from plugins to cloud services. Each integration point potentially introduces new vulnerabilities, making browsers a nexus of multiple security concerns.

The Road Ahead

The current threat landscape, marked by an ever-increasing number of browser vulnerabilities, demands vigilance. As the web continues to evolve and become more complex, a multifaceted, proactive approach to browser security will be paramount in safeguarding users and organizations.

The Rise of Browser-Based Attacks in Adversary Simulations

In adversary simulations conducted over the last year, a significant 30% focused on exploiting browser-based vulnerabilities. This underscores the importance and susceptibility of browsers in the current threat landscape.

Scripting Attacks: A Key Concern

Cross-site scripting (XSS) remains a dominant concern. This type of vulnerability allows attackers to inject malicious scripts, with 40% of browser-related breaches in simulations attributed to XSS exploits.

Phishing and Credential Harvesting

Phishing attacks are another major player. Cybersecurity firms noted a 25% increase in simulated phishing operations targeting browser vulnerabilities, aiming to deceive users into providing sensitive information.

Third-party Plug-ins: A Double-edged Sword

One notable vulnerability stems from third-party plug-ins. While they enhance browser functionality, they also introduce potential weak points. An estimated 15% of browser-related breaches in adversary simulations can be traced back to third-party plug-in vulnerabilities.

The Impact of Browser Sandboxing

Modern browsers use sandboxing techniques to isolate web processes, preventing malicious code from accessing critical system resources. However, sandbox escape techniques have been employed in 10% of adversary simulations, revealing gaps in this security measure.

Browser Capabilities

  • User Interaction and Behavior
    • Whether it’s the websites a user visits, the bookmarks they save, or the search queries they perform, browsers have an extensive record of user interaction. Queries like SELECT url, title, visit_count FROM visits WHERE visit_count > 100; can provide insights into the most frequently visited websites, while SELECT keyword, COUNT(*) AS query_count FROM search_engines GROUP BY keyword ORDER BY query_count DESC LIMIT 10; would yield the most common search terms. This data, in the wrong hands, can be used to profile users, making it a potential privacy concern.
  • Sensitive Data Storage
    • Browsers often offer to save user inputs to enhance the user experience. This includes form submissions, credit card details, passwords, and even auto-fill data. However, if this data is compromised, it could have disastrous consequences for the user. For instance, SELECT formSubmitURL, encryptedUsername, encryptedPassword FROM moz_logins WHERE formSubmitURL IS NOT NULL; provides access to login details, and a query like SELECT name_on_card, card_number, expiration_month, expiration_year FROM credit_cards; can potentially expose saved credit card details.
  • Engagement Metrics and Preferences
    • Browsers not only store raw user data but also compile engagement metrics. Queries like SELECT origin, SUM(count) AS total_engagement_count, MAX(last_engagement_time_usec) AS last_engagement_time FROM media_engagement GROUP BY origin; offer insights into user engagement with media content. These metrics can be leveraged for targeted advertising or content recommendations, but they can also be a privacy intrusion if accessed without consent.
  • Potential Threat Vectors
    • The data that browsers hold can be a goldmine for malicious actors. Whether they’re targeting saved passwords, looking for patterns in visited URLs, or trying to exploit downloaded files, the potential threat vectors are vast. For instance, a query like SELECT url, target_path, start_time, end_time FROM downloads WHERE url LIKE ‘%malware%’; might expose downloaded files from suspicious URLs, and SELECT guid, manufacturer, product FROM usb_devices WHERE manufacturer LIKE ‘%unknown%’ ORDER BY connection_timestamp DESC LIMIT 5; could unveil potentially harmful USB device connections.
  • User Sessions and Interactions
    • Monitoring the length and frequency of user sessions can shed light on browsing habits.
SELECT session_id, start_time, end_time, total_duration FROM user_sessions ORDER BY total_duration DESC LIMIT 10;

Extensions and Plugins Data

  • Extensions can access a lot of user data. Understanding which extensions are frequently used and what permissions they have is essential.
SELECT extension_id, name, permissions, last_access_time FROM extensions_data WHERE permissions LIKE '%readData%';

Ad Interaction and Tracking

  • Advertisers heavily track user interactions with ads for targeted marketing.
SELECT ad_id, click_count, hover_duration FROM ads_interactions WHERE click_count > 10;

Accessed Files and Local Data

  • Some websites allow or require users to access local files, and this can be a potential vector for vulnerabilities.
SELECT file_path, last_access_time FROM local_files_access WHERE file_path LIKE '%.exe%';

Pop-up Interactions

  • Pop-ups can sometimes be a front for malicious activities.
SELECT pop_up_url, interaction_type, interaction_time FROM pop_ups_history WHERE interaction_type = 'allowed';

Browser Notifications and Permissions

  • Notifications require permissions, and tracking these can prevent potential misuse.
SELECT origin_url, notification_type, permission_status FROM notifications_data WHERE permission_status = 'granted';

=Camera and Microphone Access Logs

  • Unauthorized access to hardware components can be a serious breach.
SELECT origin_url, hardware_component, access_time FROM hardware_access_logs WHERE hardware_component IN ('camera', 'microphone');

Google Chrome:

Popular worldwide, Google Chrome’s data management has become a reference point for many. The browser stores various user-specific settings, bookmarks, extensions, and importantly, login credentials, in a “Profile” directory. These credentials are stored in a file named “Login Data”.

Location of Chrome’s Profile data:

  • Windows: C:\Users\<YourUsername>\AppData\Local\Google\Chrome\User Data\Default\Login Data
  • macOS: ~/Library/Application Support/Google/Chrome/Default/Login Data
  • Linux: ~/.config/google-chrome/Default/Login Data

Mozilla Firefox:

Mozilla Firefox, an open-source favorite, similarly organizes its data. Firefox segregates its user data into various profiles, each containing a unique set of user data. The “logins.json” file within each profile directory holds the login credentials.

Location of Firefox’s Profile data:

  • Windows: C:\Users\<YourUsername>\AppData\Roaming\Mozilla\Firefox\Profiles\<ProfileName>\logins.json
  • macOS: ~/Library/Application Support/Firefox/Profiles/<ProfileName>/logins.json
  • Linux: ~/.mozilla/firefox/<ProfileName>/logins.json


Brave Browser, recognized for its privacy-focused features, also keeps its user data in a profile directory. Like Chrome, it uses a “Login Data” file to store credentials, given that it’s built on the same Chromium platform.

Location of Brave’s Profile data:

  • Windows: C:\Users\<YourUsername>\AppData\Local\BraveSoftware\Brave-Browser\User Data\Default\Login Data
  • macOS: ~/Library/Application Support/BraveSoftware/Brave-Browser/Default/Login Data
  • Linux: ~/.config/BraveSoftware/Brave-Browser/Default/Login Data


Opera, while not as widely adopted as some others on this list, has been a long-standing player in the browser market. Its profile data storage, like the others, includes a specific file, “Login Data,” where credentials are stored.

Location of Opera’s Profile data:

  • Windows: C:\Users\<YourUsername>\AppData\Roaming\Opera Software\Opera Stable\Login Data
  • macOS: ~/Library/Application Support/com.operasoftware.Opera/Login Data
  • Linux: ~/.config/opera/Login Data

Paths and Profiles Across Platforms

Windows: A Haven of Browser Diversity

Windows, with its vast user base, naturally supports a multitude of browsers. Here’s where each browser stashes its user data:

  • Google Chrome & Its Siblings:
    • Chrome: homeDir + “/AppData/Local/Google/Chrome/User Data/Default/”
    • Chrome Beta: homeDir + “/AppData/Local/Google/Chrome Beta/User Data/Default/”
    • Chromium (Open-source variant of Chrome): homeDir + “/AppData/Local/Chromium/User Data/Default/”
  • Microsoft Edge:
    • homeDir + “/AppData/Local/Microsoft/Edge/User Data/Default/”
  • Brave (Privacy-focused, built on the same engine as Chrome):
    • homeDir + “/AppData/Local/BraveSoftware/Brave-Browser/User Data/Default/”
  • Asian Market Dominants:
    • 360 Speed Browser: homeDir + “/AppData/Local/360chrome/Chrome/User Data/Default/”
    • QQ Browser (Popular in China): homeDir + “/AppData/Local/Tencent/QQBrowser/User Data/Default/”
    • Sogou (A notable Chinese search engine’s browser): homeDir + “/AppData/Roaming/SogouExplorer/Webkit/Default/”
  • Opera & Its Gaming Variant:
    • Opera: homeDir + “/AppData/Roaming/Opera Software/Opera Stable/”
    • Opera GX (A version of Opera designed for gamers): homeDir + “/AppData/Roaming/Opera Software/Opera GX Stable/”
  • Others:
    • Vivaldi (A highly customizable browser): homeDir + “/AppData/Local/Vivaldi/User Data/Default/”
    • Coc Coc (Tailored for the Vietnamese audience): homeDir + “/AppData/Local/CocCoc/Browser/User Data/Default/”
    • Yandex (Russian multinational specializing in Internet-related products): homeDir + “/AppData/Local/Yandex/YandexBrowser/User Data/Default/”
    • DC Browser: homeDir + “/AppData/Local/DCBrowser/User Data/Default/”
  • Mozilla Firefox:
    • Unlike other browsers which have a single profile directory, Firefox organizes its data into various profiles. The profile root is located at homeDir + “/AppData/Roaming/Mozilla/Firefox/Profiles/”

Linux: The Open-Source Paradise

Linux, known for its customizability and open-source nature, also supports a myriad of browsers.

  • The Chrome Family:
    • Chrome: homeDir + “/.config/google-chrome/Default/”
    • Chrome Beta: homeDir + “/.config/google-chrome-beta/Default/”
    • Chromium: homeDir + “/.config/chromium/Default/”
  • Brave:
    • homeDir + “/.config/BraveSoftware/Brave-Browser/Default/”
  • Microsoft Edge:
    • homeDir + “/.config/microsoft-edge/Default/”
  • Opera:
    • homeDir + “/.config/opera/Default/”
  • Vivaldi:
    • homeDir + “/.config/vivaldi/Default/”
  • Mozilla Firefox:
    • homeDir + “/.mozilla/firefox/”

Darwin (macOS): Apple’s Unix-Based OS

macOS, with its unique blend of user-friendliness and Unix power, places user data within the “Library” directory.

  • The Chrome Lineage:
    • Chrome: homeDir + “/Library/Application Support/Google/Chrome/Default/”
    • Chrome Beta: homeDir + “/Library/Application Support/Google/Chrome Beta/Default/”
    • Chromium: homeDir + “/Library/Application Support/Chromium/Default/”
  • Brave & Edge:
    • Brave: homeDir + “/Library/Application Support/BraveSoftware/Brave-Browser/Default/”
    • Edge: homeDir + “/Library/Application Support/Microsoft Edge/Default/”
  • Opera Variants:
    • Opera: homeDir + “/Library/Application Support/com.operasoftware.Opera/Default/”
    • Opera GX: homeDir + “/Library/Application Support/com.operasoftware.OperaGX/Default/”
  • Others:
    • Vivaldi: homeDir + “/Library/Application Support/Vivaldi/Default/”
    • Coc Coc: homeDir + “/Library/Application Support/Coccoc/Default/”
    • Yandex: homeDir + “/Library/Application Support/Yandex/YandexBrowser/Default/”
    • Arc Browser: homeDir + “/Library/Application Support/Arc/User Data/Default”
  • Mozilla Firefox:
    • homeDir + “/Library/Application Support/Firefox/Profiles/”

Analyzing Important Tables and Columns

Google Chrome

Google Chrome, being the world’s most popular browser, has a myriad of tables that store user data. Here are the most pivotal ones:

  • Logins: This table stores saved website login credentials. Columns such as action_url, username_value, and password_value provide the website’s URL, the saved username, and the saved password, respectively.
  • Autofill: As the name suggests, this table contains data related to the browser’s autofill functionality. The name and value columns capture the autofill data for forms and fields.
  • Cookies: It captures stored browser cookies. The host_key, name, and value columns contain details about the cookies’ origin website, their names, and values.
  • Bookmarks: This table contains information on user bookmarks. url and title columns provide the URL and title of the bookmarked page.
  • History: Holds browsing history data. The url and title columns detail the websites visited and their respective titles.
  • Downloads: A repository of downloaded file records. The url and target_path columns shed light on the source URL of the download and the location it was saved to.
  • Extensions: Lists the browser extensions installed. The name and permissions columns describe the extension’s name and the permissions it has.
  • Media Engagement: Stores data regarding media engagement. The origin and last_engagement_time_usec columns highlight the website’s origin and the last time media was engaged.

…and many more. For brevity, not all tables are detailed, but Chrome has tables capturing data from USB devices, search engines, form data, local storage, etc.


Firefox, an open-source browser by Mozilla, similarly has numerous tables critical to forensic investigations:

  • moz_logins: Contains saved website logins. Columns like formSubmitURL, hostname, encryptedUsername, and encryptedPassword provide details about the website and encrypted login credentials.
  • moz_autofill: Houses autofill data. The name and value columns depict the autofill form data.
  • moz_cookies: Contains stored browser cookies. host, name, and value columns describe the cookie’s host website, name, and value.
  • moz_bookmarks: Holds bookmark data. url and title columns detail the bookmarked URL and title.
  • moz_historyvisits: Focuses on user browsing history. from_visit, place_id, and visit_date provide data on website visits, the place ID, and the visit date.

…among others. Firefox tables also contain data on user extensions, search history, downloaded files, etc.

Microsoft Edge

Microsoft’s Edge browser, though it has a foundation in Chrome’s Chromium project, has its unique tables:

  • Logins: Similar to Chrome, it contains saved login credentials. The action_url, username_value, and password_value columns provide data on the website’s URL and saved login details.
  • Autofill: Stores the browser’s autofill data. Columns name and value depict the autofill data for forms.
  • Cookies: Like other browsers, it captures stored browser cookies. Columns host_key, name, and value offer insights into the cookie’s host, name, and value.

…and more. Edge, similar to Chrome, captures data on user bookmarks, browsing history, extensions, and other user activities.

The Challenge of Patch Management

This report provides a technical analysis of a vulnerability identified in Firefox version 102.8 on Linux where the browser goes into an infinite loop of opening tabs, leading to a potential Denial-of-Service (DoS) scenario.

Vulnerability Overview

Name: Infinite Tab Loop Vulnerability

Affected Version: Firefox 102.8 on Linux

Impact: Browser crash, potential data loss

Vulnerability Type: Denial-of-Service (DoS)

Technical Details

The vulnerability manifests itself when the firefox-trunk launcher file, provided by Ubuntu, is set as the default opener application. If a user is tricked into opening a file with a specific pattern, such as a .patch file (though other file types might also be vulnerable), the browser goes into an infinite loop, continuously opening tabs.

Vulnerability Root Cause Analysis

The primary cause of this vulnerability seems to reside in how the Ubuntu-specific firefox-trunk launcher script interacts with Firefox’s file handling. When a file is attempted to be opened, the script may unintentionally invoke a new instance of Firefox, rather than passing the file to an already opened instance.

The problem is exacerbated by potential misconfigurations in the xdg-mime system, a MIME type database for desktop environments on Linux. If the MIME type for .patch files is set to open with Firefox by default, it triggers the infinite loop.

The xdg-mime utility is a part of the xdg-utils suite on Linux systems, which assists in managing MIME types and their associated default applications. When a file type, like an RSS feed, is to be opened, xdg-mime determines the default application set to handle it.

Under specific circumstances, when Firefox is set as the default handler for certain RSS or Atom files and such a file is malformed or not correctly validated, an infinite loop scenario is triggered. When attempting to process the file, Firefox refers to xdg-mime, which in turn redirects back to Firefox, leading to endless tab openings until Firefox becomes unresponsive.

Firefox is set (either by user action or misconfiguration) as the default handler for .rss or .atom files.

A user tries to open a malformed or unvalidated .rss or .atom file.

Firefox defers to xdg-mime to determine the file’s handler.

xdg-mime identifies Firefox as the handler.

Firefox attempts to open the file in a new tab.

Due to the file’s malformed nature, Firefox again queries xdg-mime.

Steps 3-6 repeat indefinitely.

Here’s a simplified, conceptual assembly snippet illustrating the loop:


  CALL load_file ; Load the RSS or Atom file

  CALL check_file_validity ; Validate the file format

  CMP AL, invalid ; Check if file is invalid

  JZ query_xdg_mime ; If file is invalid, query xdg-mime


  CALL check_xdg_mime ; Ask xdg-mime for file handler

  CMP AL, firefox ; Check if Firefox is the handler

  JZ open_tab ; If yes, jump to open_tab


  OPEN new_tab ; Open the file in a new tab

  JMP start ; Loop back to start

Here are some commands illustrating the interaction:

Set Firefox as the default handler for .rss files:

xdg-mime default firefox.desktop application/rss+xml

Attempt to open a malformed RSS file:

xdg-open malformed.rss

Kill Chain

Reconnaissance: Attacker identifies the victim is using the vulnerable version of Firefox on Linux.

Weaponization: Prepare a .patch file, potentially named sample.atom either with malicious content or leave it empty.

Delivery: Send the .patch file to the victim via email, chat, or any other medium.

Exploitation: Instruct or trick the victim into opening the sample.atom file using Firefox.

Installation: Not applicable for this attack.

Command & Control: Not applicable for this attack.

Actions on Objectives: The browser crashes due to resource exhaustion.


The attacker needs to:

Prepare a file, like “sample.atom” or a .patch file.

Trick the victim into downloading the file.

Instruct the victim to open this file using their Firefox browser.

Command to check the current default handler for .patch files:

xdg-mime query default text/x-patch

And for reproduce the exploitability:

# Create a sample .patch file

echo "This is a sample patch file." > sample.atom

# Instruct the victim to open this file using Firefox

firefox sample.atom


To temporarily address this issue:

Open Firefox and navigate to about:preferences.

Under Applications, find the .patch file type or “differences between files”.

Change the action from “Open in Firefox” to “Text Editor” or any other preferred application.

Defense in Depth

1. Continuous Discovery of Vulnerabilities

Vulnerabilities in browsers are discovered almost daily. These vulnerabilities can range from minor ones with little impact to severe zero-day vulnerabilities that can be exploited as soon as they’re discovered.

Example: The command below demonstrates how to query the National Vulnerability Database (NVD) for known vulnerabilities related to Firefox:

curl | gunzip | jq '.CVE_Items[] | select(.cve.Affects.sw[].sw_cpe.uri:contains("firefox"))'

2. Complexity of Browsers

Modern browsers are no longer just tools to view web pages; they are complex software that supports web apps, extensions, and plugins. This complexity increases the chances of vulnerabilities.

Example: To check for outdated plugins in Firefox, you can navigate to about:plugins. Any outdated plugin can be a potential security risk.

3. Dependency on Third-party Libraries

Browsers often rely on third-party libraries. If any of these libraries have vulnerabilities, it affects the browser too.


To check shared library dependencies of a program, such as Firefox:

ldd /path/to/firefox-bin

4. Diverse User Base with Different Needs

Not all users can apply patches immediately due to custom configurations, extensions, or integration with enterprise systems. Ensuring patches don’t disrupt user configurations is challenging.

5. Deciding What to Patch

Sometimes, applying a patch to fix one problem might introduce another. Deciding what to patch and testing patches are resource-intensive tasks.


Before applying a patch, you might want to test it in a staging environment first. Using Docker can help:

docker run -d --name firefox-test -v /path/to/patched/firefox:/firefox ubuntu:latest /firefox/firefox

6. Automatic Updates vs. User Consent

While automatic updates ensure users get the latest patches immediately, they may also disrupt work or change browser behavior. Getting the balance right between auto-updates and user consent is tricky.


To disable automatic updates in Firefox via about:config, you can set the preference to false.

7. Managing Legacy Systems

Older systems that don’t support newer browser versions pose a significant challenge, as they might be left unpatched and vulnerable.

8. The Threat of Malicious Patches

There’s always a risk that threat actors could introduce malicious patches. Ensuring the integrity of patches is crucial.


To verify the integrity of a downloaded Firefox patch:

sha256sum firefox-patch.tar.gz | grep <expected_checksum>


The vulnerability stemming from the interaction between Firefox and xdg-mime regarding malformed RSS and Atom files underscores the significance of rigorous MIME type management and inter-application coordination in modern computing systems. An infinite loop, as illustrated in this case, can not only disrupt the user experience but can also lead to potential exploitation avenues for malicious actors. Proper file validation, an understanding of the implications of default application settings, and periodic review of system configurations are paramount in mitigating such issues. This specific vulnerability serves as a poignant reminder of the intricate interdependencies within software ecosystems and the continuous vigilance required to maintain their security and stability.

Security Researchers

Negin Nourbakhsh

Fazel Mohammad Ali Pour

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