DUO Two Factor Authentication (2FA)


According to the following IS policies, a 2FA solution, DUO 2FA, is implemented for protecting our accounts:


Please enroll 2FA NOW!  It is mandatory for all University accounts.

Available to

Staff, students and department (please refer to tab 2. Implementation Scope for details)

Service Charge and Application

Free; Self-Service Enrollment is required (please refer to tab 1: Introduction and Prerequisites for using DUO 2FA below.)

Service Availability

24 x 7; except maintenance period


Please submit to ITSC Service DeskInformation Security > General Enquiry > 2FA


Traditionally, we need to provide our username and password for identity verification.  With DUO 2FA, an additional piece of information, i.e. a Duo Push Response / One-time Passcode, is needed before you can get access to the applications.

What you need to do (Prerequisites for using Duo 2FA):

  1. Download the ‘Duo Mobile’ app from App Store and install it in your designated mobile device to be used for 2FA.
  2. Enroll your account and designated mobile device (e.g. your mobile phone, tablet) via the Two-Factor Authentication (2FA) Self-Service User Portal.

Details steps can be referred to tab 6. User Manual > DUO 2FA User Guide.

2nd Factor for Authentication:

After enrolling DUO 2FA, whenever you are logging in to those DUO 2FA integrated applications, you are required to provide either (i) Duo Push Response, or (ii) One-time Passcode from your ‘Duo Mobile’ app for 2FA authentication.

(i) Duo Push Response:
(ii) One-time Passcode:

Once the correct 2nd factor is provided to the application, the access to the application would be granted.

User Group

All staff and student users are mandatory to use the DUO 2FA for access protection:



Please find the list of User Applications which integrated Duo 2FA (via Campus Network or CUHK VPN plus CUHK Login).



 Schedule  Phase
3 Jul 2024 Enforce 2FA on all CADS Student applications
4 Oct 2023 Enforce 2FA on all CADS Staff and Projects applications
2 Feb 2021 Enforcement for all Staff and Project accounts
3 Nov 2020 Enforcement for all Student accounts
Mar 2020 – Jun 2020 2FA applies on 4 Major Applications
Feb 2018 ITSC Pilot Run
Mar 2018 Pilot Users
Q2 2018 Implementation for CUHK Staff


You can perform the following tasks via the Duo 2FA Self-Service User Portal:

  1. Self-Service User Enrollment & Device Registration:  Enroll your O365 account and register designated mobile device in DUO.
  2. Device Management:  Register additional mobile device(s) for getting DUO 2nd factor or manage the registered mobile devices.
  3. Bypass Code Generation:  For temporary login use in case you encounter any problems with your mobile, e.g. forgot to bring, malfunction, lost or stolen, etc. and you cannot generate the 2nd factor from the Duo mobile app on your mobile device.
  4. Rescue Code Generation:  For generating Bypass Code when emergency, i.e. when both your registered mobile device and CU Link Card are not available.


ITSC also keeps some stock of hardware tokens for Duo 2FA for departments’ or individual users (students or staff) purchase if they find it necessary. Departments or individual users who are interested to purchase the hardware token may find the information below for placing order.

Price of each hardware token for Duo 2FA: HK$100

Steps to place order:

For Department order:

  1. Department fills in the Hardware Token for Duo 2FA – Order Form (for department).
  2. Return the signed order form to ITSC (address can be found in the form).
  3. We will inform you by email (according to the email address provided in the order form) for collecting the hardware tokens.  The maximum lead time is 5 working days.
  4. Payment method: by inter-departmental transfer.

For Individual order:

  1. User fills in the “Two-Factor Authentication (2FA) – Hardware Token Order Form (Individual Order)” which can be found under ITSC Service Desk > Information Security.
  2. We will inform you by email (according to the email address provided in the order form) for collecting the hardware tokens.  The maximum lead time is 5 working days.
  3. Payment method: Cash / EPS / AliPay HK / WeChat Pay (HK) / VISA / Master / UnionPay / Octopus

1) Enrollment, Registration and Devices Management via Duo 2FA Self-Service User Portal:

Usage Prerequisites / Supported Clients
A) Enroll 2FA and register your mobile device
(PDF versionQuick Guide / Video demo)
B) Generate Bypass Code when your mobile is not available
(PDF version / Video demo)
Have your CU Link Card or Rescue Code ready
C) Prepare Rescue Code for emergency use
(PDF version)
Successfully login Duo 2FA Self-Service User Portal
D) Device Management
Activate Duo 2FA on a new mobile device

2. Access applications with 2FA:

Applications Prerequisites / Supported Clients
A) Applications using CUHK Central Authentication

(PDF version)

B) CUHK Email Service

(Microsoft 365 Mail for staff / @Link Email for student)


Clients which support modern authentication including:

  • Windows OS: Office 2016 or above, Office 2013 with modern authentication registry key updated
  • Mac OS: native mail app in MacOS 10.14 or above, Outlook 2016 (or above) for Mac
  • iOS: iOS 15 or above + native mail client (bundled in iOS),
    iOS 15 or above + Outlook app (to be downloaded from App Store)
  • Android OS: Android 11 or above + Outlook app for Android
(Setup Guide)
C) MyCUHK (applicable for staff)
(Video demo)

3. Integration with different platforms (for IT support staff):

Platform Prerequisites / Supported Clients
A) Windows clients login

(Deployment Guide / Desktop Logon Guide)

  • .NET Framework 4.5 or later
  • Win 8, 10, 11
B) Windows servers
(Deployment Guide)
  • .NET Framework 4.5 or later
  • Win Server 2016, 2019, 2022
  • Require up-to-date security patch of Duo Mobile App
  • Require mobile device with screen lock
C) Unix/Linux servers
(Deployment Guide)
  • CentOS Stream 8 or above
  • Ubuntu 20.04 LTS or above
  • Linux 8 or above
  • Debian 11 or above

Top 3 FAQ

  1. I have enrolled 2FA already. How to activate Duo 2FA (2-factor authentication) on my new mobile?  Click here for the answer
  2. It says, ‘Enter a passcode or contact your administrator for a bypass code.’ even I typed my email address and OnePass Password correctly.  Click here for the answer
  3. How to Setup multiple accounts in Duo Mobile for 2FA authentication?  Click here for the answer


(A) General Issues:

Q1: I have a mobile phone and a tablet, can I enroll both of them to use Duo 2FA?

A1: Yes, you can.


Q2: I received an unexpected Duo Push notification from my Duo Mobile App, what should I do?

A2: The Duo Push notification is a login request from your account, if it is not initiated by you or is unexpected, please tap ‘Deny’ to reject the request.


Q3: Can I use Duo Mobile on Windows Phone?

A3: As the application support for Duo Mobile on Windows Phone ended on 1 Jan 2019, and the app was removed from the Windows Store on 10 Mar 2020, you will not be able to download the app from this date.  For users who installed and activated the Duo mobile app before 10 Mar 2020, they will not be affected. Details can be found in Duo knowledge base website.


Q4: I try to login my PC with Duo Authentication, but it prompts “Invalid Certificate” on my mobile and cannot send a push to my mobile.

A4: Please turn off the wifi on your mobile and automatically switch to use your data service provider for network connection.  Then try to login again on your PC, a DUO Push should be sent again to your mobile for approval.  The error “Invalid Certificate” is most likely triggered by your Wi-Fi which certification is not trusted by DUO.


Q5: I found the following warning message when I login to Duo Portal, may I know what software it will check? If I skip to update the software, will it block us to login?

A5: Only the browser version will be checked and notify user if their browser is out-of-date. You may skip it first to continue and arrange for update according to your usual practice.


Q6: What should I do if I changed my mobile device?

A6: You need to login the Duo 2FA Self-Service User Portal to register your new mobile and manage (delete) your old mobile.


Q7: What can I do if my enrolled mobile device is malfunction?

A7: You need to generate a bypass code with your CU Link Card via Duo 2FA Self-Service User Portal, then use the bypass code to login the Duo 2FA Self-Service User Portal again, register your new mobile and manage (delete) your old mobile.


Q8: If I forgot to bring my enrolled mobile device, can I skip the 2FA?

A8: You need to generate a bypass code with your CU Link Card via Duo 2FA Self-Service User Portal, then use the bypass code to login the systems.


Q9: Will the Duo Push / SMS use my mobile data?

A9: We only enabled Duo Push and it requires Internet connection, so it will use your mobile data if you are receiving Duo Push while you are connecting with your Internet Service Provider.


Q10: When I tried to login O365, I did not receive any message from the Duo mobile app asking me to approve or deny the login.

A10: Sometimes the Duo push did not prompt in time, you may try to open the Duo Mobile app, ‘pull down’ the screen to force a refresh with server. This should trigger to prompt immediately.  Details can be found in Duo knowledge base website.  Or, you can use the passcode generate from the Duo Mobile app to login 2FA applications.

If you are using Android device, please also check if your OS and Google Play Services installed are up-to-date.  Sometimes the issues on the device, like out-of-date play services, would prevent the device to receive the Duo Push notifications properly.


Q11: After I login the system/application with Duo 2FA, do I need to login with 2FA again if I access the system/application later?

A11: It depends on the system/application settings. Usually, the applications have time-out settings which will request users to re-login again.


Q12: Any information will be collected by Duo mobile app if it is installed on the mobile device?

A12: According to Duo’s Service Privacy Notice (https://duo.com/legal/privacy-notice-services), the information they will collect includes:

  • Device information, such as: device attributes (for example: hardware model; operating system; web browser version; as well as unique device identifiers and characteristics, including if your device is “jailbroken,” if you have a screen lock in place and if your device has full disk encryption enabled), connection information (for example, name of your mobile operator or Internet Service Provider, browser type, language and time zone, and mobile phone number); device locations (for example, internet protocol  (IP) addresses and Wi-Fi); and for some Services, whether a Public Key Infrastructure Certificate is installed on your device.
  • Log data, this includes information that your browser sends whenever you visit a website, included one of ours, or that your Duo mobile app sends whenever you are using it. This log data may include how you access the Services (including the device-specific information discussed above and type of integration – in other words, the application – being protected), the dates and times you access the Services, where you access the Services from (by IP address) and device event information such as crashes, system activity, and hardware settings.


Q13: How can I generate bypass code for my account?

A13: You can generate bypass code for your account via the Duo 2FA Self-Service User Portal.  Please follow the steps below:

  1. Login to Duo 2FA Self-Service User Portal using your account credentials.
  2. provide your CU Link Card information as the 2nd factor.
  3. If the account owner could be associated, a bypass code would be generated.
  4. After the bypass code is generated, a notification email will be sent out to notify the account owner.


Q14: I would like to know how can I generate bypass code for my Tier account?

A14: You can generate bypass code for your Tier account via the Duo 2FA Self-Service User Portal by login with your tier account’s credentials.


Q15: How can I get the bypass code if I don’t have any computer nearby?

A15: You can visit Duo 2FA Self-Service User Portal with your mobile device to generate the bypass code.


Q16: Do I need to generate a bypass code for each login?

A16: The validity of a bypass code is 20 times within 12 hours.  If the bypass code is expired, or you forgot the bypass code, you can generate another new bypass code via the Duo 2FA Self-Service User Portal.


Q17: How can I remove Duo 2FA from my account?

A17: Users cannot remove the Duo 2FA from their account by themselves.  If you attempt to delete the account in the Duo Mobile app on your mobile device, this will only remove the account from your Duo Mobile app but you will still be promoted for 2FA when you login those 2FA-integrated applications.


Q18: My android smartphone was bought in mainland China and there was no Google Playstore in it, how can I install the Duo mobile app?

A18: For those Android smartphones which cannot connect to the Google Play Store, user can download the Duo Mobile APK directly from official Duo website (update information can be reference from Duo knowledge base website).  *Please do not download and install untrusted APK files into your mobile since they may be altered by hackers for malicious purposes.

Then follow the steps in User Manual > under Section 2.1 (vii) for installation.

Please also note that, since Google Play Services is required in order to receive the Duo Push notification on Android devices, for Android devices without Google Play Services installed, you have to open the Duo Mobile app and “fetch” by swiping down in the mobile app (More details can be reference from above Duo knowledge base website and here).


Q19: What is the difference between Passcode, Bypass Code and Rescue Code?

A19: Below table describes the difference:

Passcode Bypass Code Rescue Code
Purpose Provide 2FA Provide 2FA Generate Bypass Code when Duo Mobile or CU Link is not on hand
Length 6-digit 9-digit 16-characters
How to Generate
  • Duo Mobile or
  • Hardware Token
At Duo 2FA Self-Service User Portal using

  • CU Link or
  • Rescue Code
At Duo 2FA Self-Service User Portal

  • Must be generated in advance using 2FA login
  • Safely kept where you can find it in an emergency
Expiry One-off 20 times or 12 hours whatever occurs first One-off


Q20: After authenticating 2FA, it shows “Trust this browser?”, what should I click next?

A20: It is a remember device feature which allows you to skip Duo 2FA when you login any Duo-2FA integrated web application again within 12 hours with the same browser and device.  You can click “Yes, trust browser” only if you trust the browser from your own computer.  Otherwise, click “No, do not trust browser” if you don’t trust the browser, especially when using a public/shared/untrusted computer.


(B) 2FA on VPN:

Q1: I would on leave soon and I would use another SIM card, may I know how i can access VPN with Duo 2FA?

A1: Provided that you are using the same mobile for Duo 2FA currently, no matter you changed a new SIM card or even unable connected to the Internet, you can still establish connection to CUHK VPN as usual with Duo 2FA given.

 Password Format Behavior after inputted password and clicked OK Condition applied
<Onepass> “DUO Push” will be triggered Same mobile as current and able to connect to the Internet
<Onepass>,<passcode> Authentication will be proceeded Unable to be connected to the Internet


Q2: What is the ‘comma’ in VPN password field used for?

A2: The ‘comma’ shows in answer of last question, i.e. Part (B) A1, is used to separate your VPN password and 2FA passcode.


Q3: How can I connect ClassNet VPN with Duo 2FA?

A3: Please refer to the steps below:

(C) 2FA on O365 & other applications:

Q1: Can I use my Android or iOS native mail client to access my O365 mailbox with Duo 2FA?

A1: Duo only work with mail clients which supports modern authentication.  For iOS, both native mail client on iOS 13 or above and Outlook app on iOS 13 or above are supported.  For Android, native mail client is NOT supported, you can only use the Outlook app for Android to access your mailbox with Duo 2FA, the User Manual for Setting up Outlook App for Android is available.  For more details about the supported clients, it can be found in Duo knowledge base website.


Q2: My native mail client which bundled in my iOS does not prompt for Duo 2FA, what can I do?

A2: For iOS, Duo only supports native mail client in iOS 13 or above, so please check and upgrade your iOS if you are using the lower version.  Afterwards, you also need to recreate your email account in order to update the authentication settings in your iOS from basic to modern authentication since Duo can only work with application which support modern authenticatoin.  Details can be found in Duo knowledge base website.


Q3: My Office 2016 Outlook application does not prompt for Duo 2FA login, when will it ask me to login with Duo 2FA?

A3: You need to clear the Windows Credential Manager in order to trigger Duo 2FA login, please follow the steps below:

  1. Quit all Office applications.
  2. Click Start > Control Panel > User Accounts > Credential Manager > Windows Credential.
    • Select each item whose type is MicrosoftOffice16_Data:ADAL:<GUID>, and then press Delete. Repeat this step to delete all items for your account.
  3. Close the Credential Manager window and restart the MS Outlook, it will redirect to CUHK Login page and request for DUO 2FA login.


Q4: My Office 2013 Outlook application does not prompt for Duo 2FA login, when will it ask me to login with Duo 2FA?

A4: Please make sure that you have updated the following Registry Key to enable the modern authentication for your Office 2013:

HKCU\SOFTWARE\Microsoft\Office\15.0\Common\Identity\EnableADAL REG_DWORD 1
HKCU\SOFTWARE\Microsoft\Office\15.0\Common\Identity\Version REG_DWORD 1

Then, clear the Windows Credential Manager in order to trigger Duo 2FA login, steps can be referred to the answer of about question, Part (C) A3.


Q5: My Outlook 2016 for Mac app on my Mac OS does not prompt for Duo 2FA login, when will it ask for my login again?

A5: You need to clear the Keychain Access in order to trigger Duo 2FA login, please follow the steps below:

  1. Quit Outlook and all other Office applications.
  2. Start Keychain Access by using one of the following methods:
    • Select the Finder application, click Utilities on the Go menu, and then double-click Keychain Access.
    • In Spotlight Search, type Keychain Access, and then double-click Keychain Access in the search results.
  3. In the search field in Keychain Access, enter Exchange.
    • In the search results, select each item to view the Account that’s listed at the top, and then press Delete. Repeat this step to delete all items for your Exchange account.
  4. In the search field, enter adal.
    • Select all items whose type is MicrosoftOffice15_2_Data:ADAL:<GUID>, and then press Delete.
  5. In the search field, enter office.
    • Select the items that are named Microsoft Office Identities Cache 2 and Microsoft Office Identities Settings 2, and then press Delete.
  6. Quit Keychain Access and restart the Outlook 2016 for Mac, it will redirect to CUHK Login page and request for DUO 2FA login.


Q6: Can Gmail support Modern Authentication?

A6: No, Duo 2FA only works with email clients which support modern authentication. As Gmail doesn’t support it, please use the supported clients that listed at https://www.itsc.cuhk.edu.hk/all-it/information-security/two-factor-authentication-2fa/#tab_6 (O365 Services)


Q7: I cannot login my CUHK account in the Microsoft OneDrive app and Microsoft Teams app on my iPhone. After input the login ID and OnePass password, I can receive the Duo Push notification on my mobile device, but after I approve it, the login page prompts “Unknown error” and cannot continue.

A7: According to Duo, this occur recently for users running iOS 13.6 or older version, users are recommended to upgrade their iOS to the latest version 13.6.1 as this is known to resolve the issue.


Q8: Is it possible to reduce the number of times for 2FA login?

A8: Yes, but this feature is only available for Duo 2FA-integrated web applications.  After authenticated the 2FA for the web application, it will show a “Trust this browser?” message, it is a remember device feature which allows you to skip Duo 2FA when you login any Duo-2FA integrated web application again within 12 hours with the same browser and device. You can click “Yes, trust browser” only if you trust the browser from your own computer. Otherwise, click “No, do not trust browser” if you don’t trust the browser, especially when using a public/shared/untrusted computer.



(D) 2FA on Desktops / Servers:

Q1: During the Duo installation, is it a mandatory / optional to enable the “Bypass Duo authentication when offline (FailOpen)” if it is installed on servers?

duo win login 4.2.15

A1: By default, Duo Authentication for Windows Logon will “fail open” and permit the Windows logon to continue if it is unable to contact the Duo service. You can set the fail mode during installation to “fail closed” by deselecting the “Bypass Duo authentication when offline” box during installation. This will deny all login attempts if there is a problem contacting the Duo service.

On server systems, you should consider whether security concerns overrides the need for access. For example, in case under man-in-the-middle / DNS spoofing intercepting traffic to *.duosecurity.com, should the server fall back to single factor authentication?

On the other hand, in case of troubleshooting and you have physical access to the system, the “fail closed” setting can be changed in Safe Mode via registry key HKLM\SOFTWARE\Duo Security\DuoCredProv\FailOpen (Set to 1 to allow “fail open” or 0 to restrict to “fail closed”. Default: Fail open.).

Therefore it depends on factors such as security concerns, service design, requirements on availability, accessibility, etc. it is recommended but not mandatory to use set the fail mode to “fail close” on server systems.


Q2: I installed other 2FA solution on my computer before, can I install Duo 2FA additionally?

A2: Please uninstall other 2FA solution on your system before you install the Duo 2FA solution.


Q3: If my notebook installed Duo 2FA, how can I login it when it is not yet connected to Wifi network?

A3: For desktop PC / notebook, if the setting “Bypass Duo authentication when offline (FailOpen)” is selected during the Duo 2FA installation, you can still login the system without 2FA when the system is offline.


Q4: I installed DUO application on my notebook, but after reboot, I cannot login my original Windows account.

A4: Please make sure to match the Windows login username as below:

  • For domain-joint systems: use UPN as username
  • For non domain-joint systems: use either University ID, sAMAccountName, or Computing ID as username


(E) Hardware Token for Duo 2FA:

Q1: How can I purchase the hardware token?  How long do I need to wait for getting the token?

A1: Check “Hardware Token for Duo 2FA” for details.


Q2: My office ordered and assigned a hardware token to me, what should I do next?  How should I register this token?

A2: You are not required to register the hardware token since ITSC would associate the hardware token to your account at the backend of Duo system in advance.  So once you have enrolled to Duo 2FA, you can start to use the token immediately to generate the passcode for 2FA login.


Q3: Can I share the hardware token with my colleagues in my department?

A3: No, each hardware token can only be associated to 1 person.


Q4: How long can the hardware token be used?

A4: According to the hardware specification provided by vendor, the battery life of OTP c100 hardware token is around 3-5 years.


Q5: What should I do if I lost the hardware token?

A5: Please report to ITSC immediately for disabling the hardware token.  Meanwhile, you can generate a bypass code via Duo 2FA Self-Service User Portal for temporary use, or you may consider to register your mobile device and install the Duo mobile app on your device if you don’t yet have it.  And you can order a new hardware token again if necessary.


Q6: How can I set my hardware token as the default device for 2FA?

A6: Sorry, the hardware token cannot be set as a default device if you have more than 1 device registered under your account, only mobile device can be selected as default device to receive the Duo Push notification.


Q7: I have a hardware token and a mobile device registered, which device should I use to generate the passcode for 2FA?

A7: For inputting the passcode as second factor, it is fine to input any passcode generated from your registered mobile device(s) or hardware token(s) assigned to you.


Q8: My office just bought a hardware token for me, why I still received the email reminder from ITSC about enrolling to Duo 2FA?

A8: .If you are the application administrator of the 4 major Admin Systems (CUPIS, CUSIS, UG and PG Admission Systems), you are mandatory to use 2FA to login these system according to the Security Policy for Handling Personal Data.  So, if you had been assigned with a hardware token, but you have not yet enrolled to Duo 2FA, ITSC will remind you to enroll Duo 2FA before the deadline.


Q9: Knowing that ITSC would register the hardware token that assigned to me in advance, and I didn’t registered any other mobile device via the Duo 2FA Self-Service User Portal, will I see my token registered in the portal?

A9: Yes, you can find all the devices registered, including mobile device(s) and hardware token(s), under your account by logging in Duo 2FA Self-Service User Portal.  Please also note that, by logging in Duo 2FA Self-Service User Portal, you are enrolled to Duo 2FA immediately if you have not yet enrolled before.


Q10: Can I register my mobile device via Duo 2FA Self-Service Portal even though my department has bought me a hardware token from ITSC?

A10: Yes, you can register your mobile device(s) if necessary.


Q11: Understand that a hardware token cannot be shared with multiple person, but can I use the same hardware token for multiple accounts of mine?

A11: Yes, the same hardware token can be associated to multiple accounts of same person.


Q12: I bought a hardware token for Duo 2FA from ITSC, can I transfer it to my colleague if I don’t need it anymore?

A12: Yes, you can transfer it to your colleague, but ITSC need to remove the token association from your account and re-associate it to your colleague’s account.  So, please inform your colleague (as the account owner) to submit a request to ITSC Service Desk > Information Security > General Enquiry > 2-Factor Authentication (2FA), together with his/her Computing ID and @cuhk.edu.hk email address for token re-association.  After the token is re-associated, we will inform the account owner about the task is done.


Q13: My hardware token for Duo 2FA is malfunction, what can I do?

A13: If the hardware token is malfunction, you can either dispose it securely by yourself, or return it to ITSC (2/F., Pi Chiu Building) for disposal, and re-order a new one if necessary.



3 keys to go for enrolling 2FA on accounts for shared use. For step-by-step guide, please start reading P. 19 of 2-Factor Authentication for Project / Other Non-individual Accounts (for staff).

DUO releases Universal Prompt in web applications to increase accessibility and streamline verification process. The same features are provided in Universal prompt but appear differently. On the same browser and same device, Universal Prompt speeds up the process by defaulting to your last authentication method.

Changes for Duo Universal Prompt

Current DUO Traditional Prompt New Duo Universal Prompt

  • After you login your username and password, you are prompted to select an option for 2nd authentication.

Login flow change:

  •  After you login your username and password, the URL will be redirected from CUHK web application to a page hosted by Duo at *.duosecurity.com .
  • On the same browser and same device, it speeds up the authentication process by defaulting to your last authentication method or select “Other options” to choose another method.


Duo 2FA-integrated web applications migrated to the Universal Prompt:

DUO announces end-of-support for the Traditional Prompt beginning from 30 Mar 2024.  You can check the migration status of the Universal Prompt for the Duo 2FA-integrated web applications.



Published on 22 Dec 2022