CVE-2025-27606

Executive Summary

CVE-2025-27606 is a security vulnerability in the Element Android Matrix Client, affecting versions up to and including 1.6.32. The flaw allows an attacker with physical access to a device to bypass the PIN lockout mechanism, enabling unlimited PIN guessing attempts. This undermines the security of the app's local authentication, potentially exposing sensitive Matrix communications and credentials. The vulnerability has a CVSS score of 5.1 (Medium) and is currently unpatched in affected versions, with the fix available in version 1.6.34.

Supply Chain & Package Security

This vulnerability is specific to the Element Android application, which is distributed via official app stores (Google Play, F-Droid) and directly as an APK. There is no associated NPM, PyPI, Go, or Ruby package. The risk is limited to the Android application binary itself, not a broader software supply chain.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• Base Score: 5.1 (Medium)
• Attack Vector (AV): Physical (AV:P) – The attacker must have physical access to the device.
• Attack Complexity (AC): Low (AC:L) – No special conditions are required to exploit.
• Privileges Required (PR): None (PR:N) – No authentication needed.
• User Interaction (UI): None (UI:N) – The victim does not need to perform any action.
• Scope (S): Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality (C): Low (C:L) – Limited access to local data.
• Integrity (I): Low (I:L) – Potential to modify local app state.
• Availability (A): Low (A:L) – Could disrupt app functionality.

Threat Level: Medium. The requirement for physical access reduces the attack surface, but the low complexity and potential for credential exposure make it a significant risk for devices that may be left unattended or stolen.

Deep Dive & Mechanics

The vulnerability lies in the PIN-based authentication mechanism used to lock the Element Android app. The intended behavior is to log the user out after a configurable number of incorrect PIN attempts (e.g., 5 attempts). However, due to a logic flaw, the app fails to properly enforce this lockout under certain conditions.

Root Cause: The PIN verification logic likely uses a counter that is not properly reset or checked in all code paths. For example, if the user enters the wrong PIN and then quickly closes and reopens the app, the counter may be reset, allowing unlimited attempts. Alternatively, the lockout may only be triggered on the main thread, while background PIN checks bypass the counter.

Attack Scenario:

1. An attacker gains physical access to an unlocked or unattended Android device running Element Android ≤1.6.32.
2. The attacker opens the Element app, which is PIN-locked.
3. The attacker repeatedly enters incorrect PINs. The app does not log the user out after the configured number of attempts.
4. The attacker can brute-force the PIN (typically 4-6 digits) until successful.
5. Once the PIN is guessed, the attacker gains access to the user's Matrix account, including messages, contacts, and potentially encryption keys.

Vulnerable vs. Patched Code

The exact code changes are not publicly available, but based on the advisory, the fix in version 1.6.34 ensures that the PIN lockout counter is properly enforced in all scenarios.

Vulnerable Code (Conceptual):

// Vulnerable: Counter not persisted or checked in all paths
var pinAttempts = 0
fun checkPin(input: String): Boolean {
    if (input == storedPin) {
        pinAttempts = 0
        return true
    } else {
        pinAttempts++
        if (pinAttempts > maxAttempts) {
            // This may not be reached if the activity is recreated
            logout()
        }
        return false
    }
}

Patched Code (Conceptual):

// Patched: Counter persisted and checked in all entry points
var pinAttempts = 0
fun checkPin(input: String): Boolean {
    if (input == storedPin) {
        pinAttempts = 0
        return true
    } else {
        pinAttempts++
        if (pinAttempts >= maxAttempts) {
            logout()
            return false
        }
        return false
    }
}
// Also ensures counter is not reset on activity recreation
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    pinAttempts = loadPersistedAttempts()
}

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Physical access to an Android device running Element Android ≤1.6.32.
• The device must have the Element app installed and PIN-locked.

Exploitation Steps:

1. Unlock the device (if screen-locked) or obtain it while unlocked.
2. Open the Element app.
3. Enter an incorrect PIN. Note that the app does not lock out.
4. Continue entering incorrect PINs. Use a systematic approach (e.g., 0000, 0001, 0002...).
5. Once the correct PIN is guessed, the app unlocks, granting full access.

No exploit code is available for this CVE.

Blue Team (Defensive)

Detection:

• Monitor for repeated failed PIN attempts in the Element app's local logs (if accessible).
• Use Android's Accessibility Service or logcat to detect rapid PIN entry patterns.
• Implement device-level policies that lock the device after multiple failed app PIN attempts.

Prevention:

• Upgrade to Element Android 1.6.34 or later.
• Use a strong device screen lock (PIN, pattern, or biometric) as an additional layer.
• Enable remote wipe or lock capabilities for lost/stolen devices.
• Educate users to never leave devices unattended.

Indicators of Compromise (IOCs)

• Log Entries: Repeated "PIN verification failed" messages in Element's internal logs.
• Behavioral: Unusual number of app opens/closes in quick succession.
• File System: Modified local database entries indicating multiple PIN attempts (if logged).
• Network: No network IOCs, as the attack is local.

Detection Engineering

YARA Rule

rule CVE_2025_27606_Element_PIN_Bypass {
    meta:
        description = "Detects Element Android APK with vulnerable PIN lockout logic"
        author = "Valters IT"
        date = "2025-03-01"
        reference = "CVE-2025-27606"
    strings:
        $vuln_class = "pinAttempts" ascii wide
        $vuln_method = "checkPin" ascii wide
        $vuln_logout = "logout()" ascii wide
    condition:
        all of them
}

Sigma Rule

title: CVE-2025-27606 - Element Android PIN Lockout Bypass
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects repeated failed PIN attempts in Element Android app logs
author: Valters IT
date: 2025-03-01
logsource:
    product: android
    service: logcat
detection:
    selection:
        - "Element" and "PIN" and "failed"
        - "pinAttempts" and ">=" and "maxAttempts"
    condition: selection
falsepositives:
    - Legitimate user forgetting PIN
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-27606 - Element Android PIN Bypass Attempt"; flow:to_server; content:"Element"; content:"PIN"; content:"failed"; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Update Element Android to version 1.6.34 or later from the official app store or F-Droid.
2. Device Security: Ensure the device itself has a strong screen lock (PIN, pattern, or biometric).
3. App Lock: Consider using a third-party app locker with a different PIN as an additional layer.

Long-Term Measures:

• Implement a server-side session timeout that invalidates tokens after a period of inactivity.
• Use hardware-backed keystore for PIN storage.
• Conduct regular security audits of authentication logic.

Workaround: If upgrading is not immediately possible, users should avoid leaving their device unattended and enable remote wipe capabilities.

Valters IT
DevSecOps & Cyber Threat Intelligence