Browser-Based Phishing Attacks: A Penetration Tester’s Guide

Introduction

Phishing remains one of the most pervasive threats in the cyber security landscape, with browser-based phishing attacks becoming increasingly sophisticated. These attacks leverage web browsers to deceive users into divulging sensitive information, bypassing traditional security measures. For penetration testers, understanding these threats is crucial to identifying vulnerabilities and recommending robust countermeasures.

This in-depth blog post explores the mechanics of browser-based phishing attacks, their business impact, real-world examples, and proactive mitigation strategies. We will also examine how penetration testers can simulate these attacks to assess organisational defences effectively.

Understanding Browser-Based Phishing Attacks

What Are Browser-Based Phishing Attacks?

Browser-based phishing attacks exploit web browsers to deceive users into interacting with fraudulent content. Unlike traditional phishing methods (e.g., emails or SMS), these attacks manipulate browser functionalities, web-based applications, and social engineering tactics to appear legitimate.

Such attacks are particularly dangerous because they:

• Evade conventional email-based security controls.
• Exploit inherent trust in web browsers.
• Target a wide range of users, from employees to executives.

How Do These Attacks Work?

Browser-based phishing attacks typically follow a structured process:

1. Reconnaissance – Attackers gather intelligence about their targets, including commonly visited websites, login portals, and communication patterns.
2. Website Spoofing – Cybercriminals create fraudulent sites that mimic legitimate ones, using homograph attacks (e.g., substituting “rn” for “m” in domain names).
3. Credential Harvesting – Victims enter login credentials or financial data into the fake website, unknowingly handing over sensitive information.
4. Session Hijacking – In more advanced scenarios, attackers use techniques such as Man-in-the-Browser (MitB) to intercept user sessions in real time.
5. Data Exfiltration – Stolen credentials are either used for further attacks or sold on dark web marketplaces.

Types of Browser-Based Phishing Attacks

1. Miscreants-in-the-Browser (MitB) Attacks

A MitB attack involves injecting malicious code into a victim’s browser via a compromised plugin, extension, or malware. Once installed, it can:

• Modify web pages in real-time.
• Capture keystrokes, including login credentials.
• Perform unauthorised transactions on banking sites.

Real-World Example

The Zeus Trojan, a notorious banking malware, leveraged MitB techniques to steal financial information from millions of users.

2. Homograph Attacks (IDN Spoofing)

Homograph attacks exploit similarities between characters in different alphabets to deceive users. For instance, replacing “a” with “а” (Cyrillic “a”) creates an almost identical domain name.

Real-World Example

Attackers have cloned legitimate banking websites with domains such as “раypal.com” (using a Cyrillic ‘p’), leading unsuspecting victims to fraudulent login pages.

3. Evilginx2 & Reverse Proxy Phishing

Tools like Evilginx2 allow attackers to act as intermediaries between users and legitimate services, capturing credentials and session tokens in real time. Unlike conventional phishing, these attacks bypass two-factor authentication (2FA).

Real-World Example

Google and Microsoft 365 login pages have been successfully spoofed using reverse proxy phishing techniques, leading to corporate account takeovers.

4. Clickjacking Attacks

Clickjacking tricks users into clicking hidden elements on a webpage, such as a “Submit” button overlaid on a transparent iframe. This can lead to:

• Unauthorised permission grants.
• Account hijacking.
• Activation of malware downloads.

5. Malicious Browser Extensions

Attackers distribute browser extensions that appear harmless but contain malicious code. Once installed, they can:

• Steal login credentials.
• Capture clipboard data.
• Inject malicious scripts into web pages.

Real-World Example

The “Copyfish” Chrome extension was hijacked in 2017, allowing attackers to inject malicious ads and steal data from users’ browsers.

Common Techniques of Browser-Based Phishing: A Penetration Tester’s Guide

Browser-based phishing attacks have evolved significantly, leveraging advanced techniques to bypass security measures and deceive even the most vigilant users. These attacks exploit weaknesses in web browsers, social engineering tactics, and user trust to harvest credentials, financial information, and session tokens.

For penetration testers, understanding these attack techniques is critical for assessing security vulnerabilities and recommending effective countermeasures. In this blog post, we will explore the most common browser-based phishing techniques, their implications for businesses, and how penetration testers can simulate these attacks to strengthen organisational defences.

1. Fake Login Pages

How It Works

Attackers create counterfeit versions of legitimate websites—such as banking portals, corporate email logins, or cloud service providers—to trick users into entering their credentials. These fake login pages often:

• Use HTTPS certificates to appear legitimate.
• Clone design elements from the real website.
• Redirect victims to the actual website after stealing credentials, making detection difficult.

Example Attack Scenario

A phishing email claims the user’s Microsoft 365 account requires verification. The email contains a link to office365-support.com, a near-identical replica of Microsoft’s real login page. Once the user enters their credentials, they are sent to the legitimate site—none the wiser that their credentials have been stolen.

Penetration Testing Strategy

• Use tools like Evilginx2 to create reverse proxy phishing pages that can capture credentials and session cookies.
• Analyse how corporate email filters handle phishing links and identify weaknesses.
• Educate employees by simulating phishing campaigns to test awareness levels.

2. Miscreants-in-the-Browser (MitB) Attacks

How It Works

MitB attacks involve malware that injects scripts into a victim’s web browser, allowing attackers to:

• Intercept sensitive data (e.g., login credentials, banking details).
• Modify transactions in real-time (e.g., changing account numbers during financial transfers).
• Bypass two-factor authentication (2FA) by injecting malicious scripts.

Example Attack Scenario

A user visits their online banking website. An undetected Trojan running in their browser modifies the recipient’s bank account details in the background. The user confirms the transaction, unaware that funds are being redirected to an attacker’s account.

Penetration Testing Strategy

• Deploy MitB simulation tools like BeEF (Browser Exploitation Framework) to assess browser vulnerabilities.
• Test whether endpoint detection and response (EDR) solutions can detect browser-based script injections.
• Identify weaknesses in client-side security mechanisms and recommend mitigations.

3. Tabnabbing

How It Works

Tabnabbing exploits user inattentiveness by dynamically changing an inactive browser tab’s content to mimic a legitimate login page. When the user returns to the tab, they believe they were logged out and unknowingly enter their credentials.

Example Attack Scenario

• A user opens multiple tabs, including a work email and a suspicious website.
• After some time, the suspicious tab’s content refreshes to resemble the company’s login page.
• The user, thinking their session expired, re-enters their credentials, which are then sent to an attacker.

Penetration Testing Strategy

• Test the organisation’s susceptibility to tabnabbing by creating a proof-of-concept phishing site.
• Assess how browsers handle tab refreshes and whether security policies (e.g., Content Security Policy) prevent such attacks.
• Educate employees about the dangers of logging into sites via inactive tabs.

4. Browser-in-the-Browser (BitB) Attacks

How It Works

BitB attacks simulate a pop-up authentication window within a webpage, tricking users into believing they are signing into a legitimate service (e.g., Google, Microsoft). Unlike traditional phishing, these attacks:

• Create a fake browser authentication pop-up within the webpage.
• Mimic single sign-on (SSO) login prompts.
• Capture credentials and session tokens in real time.

Example Attack Scenario

An attacker creates a phishing page that includes a fake Google login pop-up. When the user enters their credentials, the attacker captures them and logs into their account before the victim realises they’ve been compromised.

Penetration Testing Strategy

• Use tools like BitB Framework to create proof-of-concept attacks and demonstrate risks.
• Assess whether security measures, such as WebAuthn or security keys, mitigate BitB attacks.
• Educate users on checking for legitimate pop-up behaviour (e.g., dragging the window outside the main browser area).

5. Malicious Browser Extensions

How It Works

Attackers distribute rogue browser extensions that appear legitimate but contain malicious scripts. Once installed, they:

• Capture keystrokes (e.g., usernames, passwords).
• Modify web page content to inject fake forms or phishing elements.
• Steal cookies and session tokens to hijack user accounts.

Example Attack Scenario

A user installs a browser extension promising to improve email productivity. The extension has hidden code that captures all keystrokes entered on the browser, including login credentials for corporate accounts.

Penetration Testing Strategy

• Develop a proof-of-concept malicious extension and demonstrate its impact.
• Identify corporate policies around browser extension security.
• Recommend endpoint security controls that detect unauthorised extension activity.

6. Homograph Attacks (URL Spoofing)

How It Works

Homograph attacks exploit similarities between different character sets (e.g., Latin, Cyrillic) to register deceptive domain names. For example:

• Real domain: google.com
• Spoofed domain: gооgle.com (where ‘о’ is a Cyrillic character)

These attacks are effective because:

• Modern browsers display URLs in a way that hides character differences.
• Victims often don’t scrutinise URLs closely.
• Attackers obtain valid SSL certificates, making the sites appear secure.

Example Attack Scenario

An executive receives an email inviting them to a Zoom meeting. The email contains a link to zооm.us, which loads a near-identical Zoom login page. The executive enters their credentials, unknowingly handing them to an attacker.

Penetration Testing Strategy

• Register a similar-looking domain and assess employee susceptibility.
• Use IDN homograph attack generators to create phishing test cases.
• Implement DNS security measures to block lookalike domains.

Real-World Examples of Browser-Based Phishing Attacks

Browser-based phishing attacks are not just theoretical threats—they have been used in high-profile cyberattacks against organisations, government agencies, and individuals worldwide. Below are real-world examples that illustrate how cybercriminals have successfully exploited browser-based phishing techniques.

1. The Google Docs Phishing Worm (2017)

Attack Type: Fake Login Pages

What Happened?

In May 2017, a sophisticated phishing attack targeted Google users by impersonating Google Docs. Victims received emails that appeared to be from a trusted contact, inviting them to collaborate on a document. Clicking the link led them to a legitimate-looking Google OAuth login page, where they were prompted to grant access to a third-party app named “Google Docs.”

Why Was It Effective?

• The attack leveraged Google’s OAuth authentication, making the request seem legitimate.
• The phishing page was hosted on Google’s infrastructure, bypassing traditional security filters.
• Once users granted permissions, the attacker gained full access to their Google accounts, including emails, contacts, and drive contents.

Impact

• Affected over 1 million Google users within hours.
• The attacker gained access to corporate and personal Google accounts.
• Google had to revoke OAuth permissions and strengthen its API security.

Lessons for Penetration Testers

• Assess whether an organisation’s employees can differentiate real OAuth prompts from phishing attempts.
• Implement security awareness training to teach users to verify app permissions before granting access.
• Recommend multi-factor authentication (MFA) and Google’s Advanced Protection Programme for high-risk accounts.

2. The BitB Attack Against Cloudflare Employees (2022)

Attack Type: Browser-in-the-Browser (BitB) Attack

What Happened?

In August 2022, Cloudflare employees were targeted by a sophisticated Browser-in-the-Browser (BitB) phishing attack. Attackers crafted a fake Cloudflare Okta login page that mimicked a single sign-on (SSO) pop-up. The fake authentication window was displayed within the browser itself, making it nearly indistinguishable from a legitimate SSO prompt.

Why Was It Effective?

• The fake SSO login window appeared identical to real Okta pop-ups.
• Victims had no way to verify if they were interacting with an actual authentication window.
• Attackers bypassed 2FA by capturing session cookies, allowing them to access Cloudflare’s internal systems.

Impact

• Cloudflare identified and mitigated the attack before any major breach occurred.
• The company strengthened security measures, including hardware security keys (YubiKey) for authentication.

Lessons for Penetration Testers

• Simulate BitB phishing attacks using tools like Evilginx2 to test security awareness.
• Encourage organisations to enforce WebAuthn-based authentication to eliminate reliance on password-based logins.
• Train employees to verify authentication pop-ups by dragging them outside the browser window (BitB windows are part of the webpage, unlike real pop-ups).

3. The SolarWinds Supply Chain Attack (2020-2021)

Attack Type: Malicious Browser Extensions & Man-in-the-Browser (MitB) Attacks

What Happened?

The SolarWinds cyberattack, linked to Russian nation-state hackers (APT29), involved a multi-stage attack that compromised thousands of organisations, including U.S. government agencies. A key aspect of the attack involved phishing tactics to steal credentials and gain persistence inside corporate networks.

Attackers used:

• Malicious browser extensions to capture authentication tokens from users accessing Office 365.
• MitB techniques to alter Microsoft authentication requests and bypass MFA.
• OAuth consent phishing to trick victims into granting access to malicious third-party apps.

Why Was It Effective?

• Attackers used legitimate-looking OAuth apps that appeared safe to end-users.
• Malicious browser extensions captured authentication tokens in real-time.
• SolarWinds Orion software was already trusted by organisations, allowing lateral movement inside networks.

Impact

• Affected 18,000 organisations, including Microsoft, Cisco, and U.S. federal agencies.
• Led to massive data breaches and espionage activities.
• Exposed vulnerabilities in OAuth authentication, token security, and browser-based attacks.

Lessons for Penetration Testers

• Test corporate security against malicious browser extensions by simulating data exfiltration from compromised browsers.
• Identify weaknesses in OAuth security policies and recommend strict app approval processes.
• Encourage companies to implement session monitoring and automatic token revocation in case of suspicious activities.

4. The PayPal Homograph Attack (2023)

Attack Type: Homograph Attack (URL Spoofing)

What Happened?

In early 2023, security researchers discovered a homograph phishing attack targeting PayPal users. Attackers used Unicode domain tricks to register a lookalike domain that replaced Latin characters with visually identical Cyrillic letters:

• Real domain: paypal.com
• Spoofed domain: раураl.com (with Cyrillic ‘р’ and ‘а’)

The phishing email urged users to verify their accounts by clicking a link to the fraudulent PayPal login page. Once victims entered their credentials, attackers stole their login details and used them for fraudulent transactions.

Why Was It Effective?

• The spoofed domain appeared visually identical to the real one.
• Attackers obtained SSL certificates, making the site appear secure (https://).
• Many users did not scrutinise the URL closely before entering credentials.

Impact

• Thousands of PayPal users were tricked into revealing credentials.
• Cybercriminals exploited compromised accounts for money laundering.

Lessons for Penetration Testers

• Register IDN homograph domains to test if employees can detect fraudulent URLs.
• Implement enterprise DNS security solutions to block suspicious domains.
• Educate users on hovering over links before clicking to verify legitimacy.

5. The Coinbase Tabnabbing Attack (2021)

Attack Type: Tabnabbing

What Happened?

Cryptocurrency exchange Coinbase was targeted by a phishing campaign using tabnabbing techniques. Attackers tricked users into visiting a phishing page that appeared harmless. When users switched to another tab, the phishing site dynamically changed to mimic the Coinbase login page.

Victims, believing they had been logged out, unknowingly re-entered their credentials, which were then stolen by the attackers.

Why Was It Effective?

• The phishing site looked identical to the real Coinbase login page.
• Victims did not suspect tab refreshes were malicious.
• Attackers bypassed 2FA by stealing session tokens.

Impact

• Many Coinbase users reported unauthorised withdrawals from their accounts.
• Cybercriminals exploited compromised accounts for cryptocurrency theft.

Lessons for Penetration Testers

• Test browser behaviour for tab refresh hijacking using JavaScript-based tabnabbing techniques.
• Recommend using password managers, which do not autofill credentials on phishing pages.
• Educate users to always manually reload the page before entering login credentials.

The Business Impact of Browser-Based Phishing Attacks

Browser-based phishing attacks pose significant risks to enterprises, including:

1. Financial Losses

• Fraudulent transactions can drain corporate accounts.
• Regulatory fines for non-compliance with GDPR, CCPA, and other data protection laws.

2. Reputational Damage

• Data breaches erode customer trust.
• Negative media coverage impacts brand value.

3. Intellectual Property Theft

• Competitors or cybercriminals can exploit stolen trade secrets.
• Confidential client data may be leaked or sold on dark web marketplaces.

4. Operational Disruptions

• Account takeovers can lock out executives from critical services.
• Internal network infections lead to downtime and recovery costs.

Mitigation Strategies for Browser-Based Phishing Attacks

1. Implement Browser Security Controls

• Enable HTTPS-Only mode in enterprise browsers.
• Block third-party cookie tracking.
• Enforce strict Content Security Policy (CSP) rules.

2. Deploy Phishing-Resistant Authentication

• Use hardware security keys (e.g., YubiKey) for 2FA.
• Implement passkeys and FIDO2 authentication.

3. Conduct Regular Security Awareness Training

• Simulate phishing attacks to educate employees.
• Train executives to identify domain spoofing techniques.

4. Monitor Browser Extensions and Plugins

• Restrict installations to verified corporate-approved extensions.
• Use endpoint detection and response (EDR) solutions to detect suspicious activity.

5. Leverage AI-Powered Threat Detection

• Deploy machine learning algorithms to identify phishing attempts.
• Use real-time URL scanning to block malicious websites.

How Penetration Testers Can Simulate Browser-Based Phishing Attacks

Penetration testers play a critical role in assessing an organisation’s resilience against browser-based phishing. Key strategies include:

1. Conducting Evilginx2 Attacks in a Controlled Environment

• Set up Evilginx2 to perform reverse proxy phishing simulations.
• Capture session cookies to demonstrate risks of 2FA bypass.

2. Developing and Deploying Malicious Extensions

• Create a proof-of-concept browser extension that logs keystrokes.
• Demonstrate how employees could unknowingly install malicious add-ons.

3. Performing IDN Homograph Tests

• Register similar-looking domains to assess employee susceptibility.
• Evaluate the effectiveness of browser-based phishing protection tools.

4. Executing Clickjacking Tests

• Overlay transparent iframes to test clickjacking vulnerabilities.
• Assess whether Content Security Policies prevent such attacks.

Final Thoughts

Browser-based phishing attacks are evolving, leveraging sophisticated techniques to bypass traditional security measures. For penetration testers, understanding these threats is crucial in helping organisations strengthen their cyber security posture.

By simulating real-world phishing scenarios, businesses can identify vulnerabilities before cybercriminals exploit them. Investing in robust security practices, from phishing-resistant authentication to AI-driven threat detection, will significantly mitigate the risks associated with these attacks.

Organisations must remain vigilant, continuously educating employees and deploying cutting-edge security solutions to stay ahead of cyber adversaries.

Would your organisation survive a browser-based phishing attack? Conduct a penetration test today to find out.

Secure your Cyber Risk. Business Risk

If you need expert penetration testing services to assess your organisation’s security posture, contact us today. Our team specialises in simulating sophisticated browser-based phishing attacks to help you stay protected.

Browser-based phishing attacks continue to evolve, leveraging fake login pages, BitB techniques, malicious extensions, and homograph attacks to deceive users. These real-world examples highlight the devastating impact of browser-based phishing on organisations and individuals.

For penetration testers, simulating these attacks helps identify vulnerabilities before cybercriminals exploit them. Implementing strong authentication mechanisms, browser security policies, and user education can significantly reduce the risk of browser-based phishing attacks.

Would your organisation survive a browser-based phishing attack? Conduct a penetration test today to find out.

Which real-world phishing attack surprised you the most? Let us know in the comments! 🚀