CVE-2025-4215

Executive Summary

CVE-2025-4215 is a denial-of-service (DoS) vulnerability in the popular browser extension uBlock Origin, affecting versions up to 1.63.3b16. The flaw resides in the currentStateChanged function within src/js/1p-filters.js, where a crafted UI interaction can trigger inefficient regular expression complexity (ReDoS). An attacker can remotely exploit this by luring a user to a malicious page or by injecting a specially crafted payload into the extension's filter list UI, causing the browser tab or the extension itself to become unresponsive. The attack complexity is high, requiring specific conditions, but a public proof-of-concept exists. The vendor has released a patch in version 1.63.3b17.

Supply Chain & Package Security

This vulnerability affects a browser extension distributed through official browser stores (Chrome Web Store, Firefox Add-ons, etc.) and directly from GitHub releases. Users should update the extension through their browser's extension manager or by downloading the latest release from the official GitHub repository. There is no NPM, PyPI, Go, or Ruby package involved.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0 in the NVD data, likely due to incomplete scoring. Based on the vulnerability characteristics, a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Network (AV:N) – The attack can be triggered remotely.
• Attack Complexity (AC): High (AC:H) – Requires specific conditions and user interaction.
• Privileges Required (PR): None (PR:N) – No authentication needed.
• User Interaction (UI): Required (UI:R) – The victim must visit a malicious page or trigger the vulnerable UI.
• Scope (S): Unchanged (S:U) – The vulnerable component is the extension itself.
• Confidentiality (C): None (C:N) – No data breach.
• Integrity (I): None (I:N) – No data modification.
• Availability (A): Low (A:L) – Causes temporary unresponsiveness of the extension or browser tab.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:L Estimated Base Score: 3.1 (Low)

Threat Level: Low. The attack is difficult to execute reliably, and the impact is limited to a denial of service of the extension, not the entire system.

Deep Dive & Mechanics

The vulnerability is a classic ReDoS (Regular Expression Denial of Service) in the currentStateChanged function of src/js/1p-filters.js. This function is responsible for updating the UI when the user modifies their custom "My filters" list. The function uses a regular expression to parse or validate the filter text. By providing a specially crafted input string that causes catastrophic backtracking, the regular expression engine consumes excessive CPU time, freezing the browser tab.

The vulnerable code likely uses a regex pattern with nested quantifiers or alternations that can be exploited. For example, a pattern like (a+)+b can cause exponential backtracking when given a string like "aaaa...ac". The attacker can inject this malicious string into the filter list via a crafted webpage or by tricking the user into pasting it into the UI.

Vulnerable vs. Patched Code

The patch commit eaedaf5b10d2f7857c6b77fbf7d4a80681d4d46c modifies the regex pattern in src/js/1p-filters.js to eliminate the inefficient complexity.

Vulnerable Code (simplified):

// Hypothetical vulnerable regex
const filterRegex = /^([a-z]+)+$/;
function currentStateChanged() {
    const input = document.getElementById('myFilters').value;
    if (filterRegex.test(input)) {
        // Process filter
    }
}

Patched Code (simplified):

// Patched regex with atomic grouping or possessive quantifiers
const filterRegex = /^([a-z]++)+$/;
function currentStateChanged() {
    const input = document.getElementById('myFilters').value;
    if (filterRegex.test(input)) {
        // Process filter
    }
}

The patch likely uses possessive quantifiers (++) or atomic groups to prevent backtracking, or it may have rewritten the regex to be linear in complexity.

Exploitation & Lab Setup

Red Team (Attacker)

Prerequisites:

• A victim running uBlock Origin <= 1.63.3b16.
• Ability to deliver a malicious payload to the victim's "My filters" UI (e.g., via a crafted webpage that uses the extension's API or by social engineering).

Exploit Steps:

1. Craft a payload string that triggers catastrophic backtracking. For example, if the vulnerable regex is (a+)+b, the payload could be "aaaa...ac" (many 'a's followed by a 'c').
2. Deliver the payload to the victim. This could be done by:
   • Hosting a malicious webpage that uses chrome.runtime.sendMessage or similar to inject the payload into the extension's storage.
   • Tricking the user into pasting the payload into the "My filters" text area.
3. Once the payload is processed by currentStateChanged, the browser tab will freeze or become unresponsive.

Proof-of-Concept (PoC) Code:

// PoC: Inject malicious filter via extension API (requires extension permissions)
chrome.runtime.sendMessage({
    type: 'userFilters',
    payload: 'aaaa...ac' // Replace with actual ReDoS payload
});

Blue Team (Defender)

Detection:

• Monitor for high CPU usage in browser processes, especially when uBlock Origin is active.
• Look for unusual patterns in the "My filters" list, such as long strings of repeated characters.
• Use browser developer tools to profile JavaScript execution and identify long-running regex operations.

Mitigation:

• Immediately update uBlock Origin to version 1.63.3b17 or later.
• If update is not possible, disable the "My filters" feature temporarily.
• Educate users not to paste unknown content into the extension's settings.

Indicators of Compromise (IOCs)

• File: src/js/1p-filters.js (vulnerable version)
• Hash (SHA256): eaedaf5b10d2f7857c6b77fbf7d4a80681d4d46c (patch commit hash)
• Network: No specific network IOCs; the attack is local to the browser.
• Behavior: High CPU usage in browser process, unresponsive browser tab, JavaScript stack traces showing currentStateChanged function.

Detection Engineering

YARA Rule

rule CVE_2025_4215_ReDoS_uBlock {
    meta:
        description = "Detects vulnerable uBlock Origin 1p-filters.js with ReDoS pattern"
        author = "Valters IT"
        date = "2025-04-10"
        hash = "eaedaf5b10d2f7857c6b77fbf7d4a80681d4d46c"
    strings:
        $vuln_regex = /\([a-z]+\+\)\+/  // Simplified pattern for demonstration
        $func_name = "currentStateChanged"
    condition:
        $func_name and $vuln_regex
}

Sigma Rule

title: CVE-2025-4215 - uBlock Origin ReDoS Exploitation Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential exploitation of CVE-2025-4215 via high CPU usage or long-running JavaScript in browser
author: Valters IT
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\chrome.exe' or '\firefox.exe'
        CommandLine|contains: 'uBlock'
    condition: selection
falsepositives:
    - Legitimate high CPU usage from other extensions
level: low

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-4215 - uBlock Origin ReDoS Payload Delivery"; flow:to_server,established; content:"|22|aaaa"; depth:100; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Update uBlock Origin to version 1.63.3b17 or later from the official browser store or GitHub releases.
2. Verify the update by checking the extension version in the browser's extension manager.

Long-term Measures:

• Implement input validation and regex timeout mechanisms in the extension.
• Conduct regular security audits of regex patterns in the codebase.
• Monitor for similar ReDoS vulnerabilities in other components.

Valters IT Signature:

Valters IT
Senior DevSecOps Engineer
Cyber Threat Intelligence Expert