Client-Side Vulnerabilities

Client-Side Vulnerabilities are security flaws that exist and execute within the user's web browser or local device, rather than on the web server. Unlike server-side vulnerabilities (like SQL Injection) that attack the backend database or application logic directly, client-side attacks target the user, exploiting the trust relationship between the user's browser and the website they are visiting.

In the modern threat landscape, these vulnerabilities are a primary vector for identity theft, session hijacking, and digital skimming attacks (Magecart).

What Are Client-Side Vulnerabilities?

These vulnerabilities occur when an application relies on the client (the browser) to perform security-sensitive operations or when it fails to validate input before rendering it to the user. Because the code (HTML, CSS, JavaScript) is executed on the user's machine, attackers can manipulate it to steal sensitive data like session tokens, cookies, and login credentials.

Common Types of Client-Side Vulnerabilities

• Cross-Site Scripting (XSS): The most prevalent client-side flaw. It allows attackers to inject malicious scripts into trusted websites. When a victim visits the infected page, the script executes and sends their cookies or other data to the attacker.

• Cross-Site Request Forgery (CSRF): This attack tricks a logged-in user into performing unwanted actions on a web application (like changing a password or transferring funds) without their knowledge.

• Clickjacking (UI Redressing): Attackers use transparent or opaque layers (iframes) to trick a user into clicking on a button or link on another page when they intended to click on the top-level page.

• DOM-Based Vulnerabilities: These occur when an application contains client-side JavaScript that processes data from an untrusted source in an unsafe manner, typically by writing it to the Document Object Model (DOM).

• Digital Skimming (Magecart/Formjacking): Attackers compromise third-party JavaScript libraries (like chat widgets or analytics tags) used by a website. When the legitimate site loads the compromised script, it silently captures user input (such as credit card numbers) from forms.

• Open Redirects: An application accepts user-controlled input that specifies a destination URL, allowing attackers to leverage the trusted site's reputation to redirect victims to a phishing page.

Why Are Client-Side Vulnerabilities Dangerous?

Client-side attacks are particularly dangerous because they bypass traditional server-side defenses, such as firewalls. Because the malicious activity occurs within the user's encrypted browser session, it is often invisible to the organization's security monitoring tools until the data has already been stolen.

Common Questions About Client-Side Vulnerabilities

How do I detect client-side vulnerabilities? Detection requires a combination of tools. Dynamic Application Security Testing (DAST) scanners simulate user behavior to detect XSS and CSRF vulnerabilities. Software Composition Analysis (SCA) identifies vulnerabilities in third-party JavaScript libraries. Content Security Policy (CSP) reporting can alert you when unauthorized scripts attempt to execute.

Can a firewall block client-side attacks? Standard Network Firewalls cannot block them because the traffic appears to be legitimate web browsing. Web Application Firewalls (WAFs) can offer some protection by filtering malicious input in requests, but they cannot prevent all client-side execution, especially DOM-based attacks.

What is the best defense against client-side attacks? A strong Content Security Policy (CSP) is the most effective defense. It tells the browser which domains are allowed to load scripts, preventing unauthorized code execution even if a vulnerability exists.

Managing Client-Side Vulnerabilities with ThreatNG

ThreatNG addresses the challenge of client-side security by providing visibility into the external digital footprint where these vulnerabilities manifest. By mapping the organization's web assets and the third-party ecosystem it relies on, ThreatNG helps identify conditions—such as missing security headers or vulnerable JavaScript libraries—that enable client-side attacks.

External Discovery

ThreatNG automates inventorying web assets and the scripts they use, creating a comprehensive map of the client-side attack surface.

• Discovering Web Assets: ThreatNG identifies all subdomains and landing pages owned by the organization. It routinely uncovers forgotten marketing sites, legacy portals, or "shadow" web applications that lack modern security controls. These unmanaged assets are often the path of least resistance for attackers attempting Cross-Site Scripting (XSS) or session hijacking.

• Mapping Third-Party Dependencies: The solution catalogs the external JavaScript files loaded by these assets. It reveals the "supply chain" of scripts—analytics, chatbots, and trackers—that execute in the user's browser. This mapping is critical for identifying dependencies that could introduce digital skimming (Magecart) risks if the third-party provider is compromised.

External Assessment

ThreatNG assesses the security posture of discovered assets to determine their susceptibility to client-side exploitation.

• Header Analysis (CSP & Security Headers): ThreatNG evaluates HTTP response headers of web assets to verify the presence and configuration of critical security controls. Example: It checks for Content Security Policy (CSP), X-Frame-Options, and Strict-Transport-Security (HSTS). If ThreatNG detects that a high-traffic login page is missing the X-Frame-Options header or has a CSP that allows unsafe-inline scripts, it flags the asset as highly susceptible to Clickjacking and XSS, quantifying the risk for immediate remediation.

• JavaScript Library Vulnerabilities: ThreatNG identifies outdated or vulnerable JavaScript libraries running on public-facing pages. Example: If the assessment detects that a customer support portal is loading a version of jQuery or Angular known to contain "High"- severity CVEs related to DOM-based XSS, ThreatNG highlights that specific library. It provides the security team with the exact location and version number, enabling them to upgrade the library before it can be exploited.

Reporting

ThreatNG generates reports that translate technical client-side risks into actionable intelligence for developers and security managers.

• Client-Side Risk Reports: Reports detail which specific subdomains are missing critical headers, such as CSP, and prioritize them by asset business value. This helps development teams understand exactly where to focus their hardening efforts to prevent attacks like UI Redressing.

• Supply Chain Exposure Reports: ThreatNG provides a comprehensive view of all third-party domains trusted by the organization's websites. This report is crucial for understanding the potential "blast radius" if a specific vendor (like an advertising network or analytics provider) were compromised.

Continuous Monitoring

The client-side environment changes with every code deployment. ThreatNG monitors these changes to detect new risks in real-time.

• Script Drift Detection: ThreatNG monitors script behavior on key pages. If a legitimate script suddenly changes its source or starts communicating with a new, unknown domain (a common indicator of a Magecart infection), ThreatNG detects this "drift" and triggers an alert.

• Header Regression Monitoring: If a new deployment accidentally removes the CSP header or weakens the SameSite cookie attribute, ThreatNG detects the regression immediately. This ensures that security controls remain active and effective despite frequent updates by development teams.

Investigation Modules

ThreatNG’s investigation modules enable analysts to examine specific client-side threats and the infrastructure that hosts them.

• Domain Intelligence Investigation: When ThreatNG discovers a web application on an unfamiliar or suspicious subdomain within the organization's perimeter, this module investigates the hosting domain's background. Example: If ThreatNG identifies a legacy marketing page running on a forgotten subdomain like campaign-2021.company-news.com, the Domain Intelligence module analyzes the domain's registration history and DNS records. If it discovers that the domain points to a deprecated cloud resource or has been flagged for reputation issues, it confirms that the asset is vulnerable to Subdomain Takeover or misuse, prompting immediate decommissioning.

• Sensitive Code Exposure Investigation: This module scans client-side code (JavaScript) found on public repositories or within the page source. Example: It looks for hardcoded secrets, such as API keys, OAuth tokens, or internal IP addresses, that developers might have left in the frontend code. Identifying these exposures prevents attackers from scraping the keys to launch further server-side attacks.

Intelligence Repositories

ThreatNG enriches client-side findings with global threat data to identify active exploitation campaigns.

• Vulnerability Intelligence: ThreatNG correlates discovered JavaScript versions with its vulnerability database. It identifies if a library used on the site has a "Must-Patch" vulnerability that is currently being exploited in the wild, allowing teams to prioritize patches based on active threat levels rather than just CVSS scores.

Complementary Solutions

ThreatNG provides the intelligence that powers other defense mechanisms, ensuring a layered defense against client-side attacks.

• Complementary Solution (Content Security Policy Managers): ThreatNG feeds discovered script sources and domains into CSP management tools. This cooperation helps these tools generate accurate allowlists, ensuring that legitimate scripts run while blocking unauthorized ones without breaking site functionality.

• Complementary Solution (DAST Scanners): ThreatNG provides a comprehensive list of URLs and subdomains to Dynamic Application Security Testing (DAST) tools. By feeding the DAST scanner the full "Target List" of assets (including shadow sites), ThreatNG ensures the scanner tests the entire attack surface for XSS and CSRF, rather than just the known main website.

• Complementary Solution (Web Application Firewalls - WAF): ThreatNG shares intelligence on high-risk assets with Web Application Firewalls. If a specific subdomain is identified as hosting a vulnerable library that cannot be patched immediately, the WAF can be configured with virtual patching rules to block exploit attempts targeting that vulnerability.

Examples of ThreatNG Helping

• Helping Prevent Clickjacking: ThreatNG helps an organization secure its customer portal by identifying that the X-Frame-Options header was missing on the login page. The assessment highlighted that the site could be framed by attackers to trick users, leading the team to implement DENY or SAMEORIGIN directives to neutralize the risk.

• Helping Manage Third-Party Scripts: ThreatNG helps a retailer by discovering a "zombie" script from a former marketing vendor still loading on their checkout page. The continuous monitoring alert regarding the script's presence allowed the team to remove the unnecessary code, eliminating a potential vector for data skimming.

• Helping Secure Legacy Applications: ThreatNG helps by finding an old employee login page running on a forgotten subdomain that was using a 5-year-old version of Angular with critical XSS vulnerabilities. The report allowed the security team to decommission the risk before it was discovered and exploited by attackers.