CVE-2021-23957
📋 TL;DR
This vulnerability in Firefox for Android allowed malicious websites to bypass iframe sandbox restrictions using Android-specific intent URLs. Only Firefox for Android versions before 85 are affected; desktop browsers and other mobile browsers are not vulnerable.
💻 Affected Systems
- Mozilla Firefox for Android
📦 What is this software?
Firefox by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
An attacker could execute arbitrary code within the browser context, potentially leading to data theft, session hijacking, or further exploitation of the Android device.
Likely Case
Malicious websites could escape sandboxed iframes to access parent page content, perform cross-site scripting, or launch unauthorized intent actions on Android devices.
If Mitigated
With proper browser updates, the vulnerability is completely eliminated. On patched versions, intent URLs are properly handled within sandbox constraints.
🎯 Exploit Status
Exploitation requires user to visit a malicious website. No public exploit code has been released.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firefox for Android 85 and later
Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2021-03/
Restart Required: Yes
Instructions:
1. Open Google Play Store on Android device. 2. Search for 'Firefox'. 3. If update is available, tap 'Update'. 4. Restart Firefox after update completes.
🔧 Temporary Workarounds
Disable JavaScript
androidTemporarily disable JavaScript in Firefox settings to prevent exploitation
Use alternative browser
androidSwitch to a different browser until Firefox is updated
🧯 If You Can't Patch
- Restrict browsing to trusted websites only
- Enable enhanced tracking protection and disable JavaScript for untrusted sites
🔍 How to Verify
Check if Vulnerable:
Check Firefox version in Settings > About Firefox. If version is less than 85, device is vulnerable.Check Version:
Open Firefox > Settings > About FirefoxVerify Fix Applied:
Confirm Firefox version is 85 or higher in Settings > About Firefox.📡 Detection & Monitoring
Log Indicators:
- Unusual intent:// URL patterns in browser logs
- Multiple iframe navigation attempts
Network Indicators:
- HTTP requests containing intent:// URLs from sandboxed contexts
SIEM Query:
Not applicable for typical mobile browser scenarios