Security – Pass hash attack for Yahoo Messenger

Authentication is an important process of a internet service for authenticating users. For example, when you register an account for Yahoo mail, you will be asked for an ID and password so that only you can access to your email account. That means Yahoo will save your ID and your password in their servers so that they can compare with your input later. Your ID can be saved in clear text but the password will not be never saved under clear text. Yahoo uses its own algorithmus to make a hash of your password. For example your password is “123456789”, then Yahoo may save it as format “25f9e794323b453885f5181f1b624d0b”. In this way only you know the password and if the server is attacked, your password will be still safe. Furthermore password hash protects you from losing password through snipping on network connection.

A normal authentication looks like below

Normal authentication

Now, let’s consider a scenario. What if I manually establish a connection to the email I would like to access, but rather than feeding it privileged username and password I feed it the username and that password hash that I have stolen? Yahoo is concerned about is receiving a hash that matches what it is expecting (password hash). This means that I don’t have to perform the one-way hashing function on the password, I just have to supply the hash, which is the basis for this attack. We call this attack “password hash attack” and the authentication will look like image below

Password hash attack

So end with theorie and start with a small pratice of “password hash attack” with Yahoo Messenger. If you use Yahoo Messenger, there is an option to save your ID and password.

Yahoo Messenger - Log In

This password again is of course not saved under clear text but its hash will be saved in registry and sent to server for authentication when user logs in Yahoo Messenger. Therefore we can make a password hack attack for Yahoo Messenger as following steps.
0. Somehow take an access to the computer of victim. Open Yahoo Messenger and see if the option of saving ID and Password is ON. If it’s on, we can start our attack.
1. Press Windows + R. Type in “regedit”. Enter.
2. Browse to “HKEY_CURRENT_USER\Software\Yahoo\Pager”.
3. Right click on it and click “Export”. Enter a file name to save registry information into .reg file.

Registry export

4. Copy the .reg file from victim’s computer to our computer. Open it with Notepad, delete all of values but
+ “Save Password”
+ “Yahoo! User ID”
+ “ETS”

4 neccessary entries

5. Double click to import this .reg file into our registry.
6. Open your Yahoo Messenger and click Log In. Suprise you are logging to account of victim.


  1. Ông ơi, sao tui tìm mãi ko thấy cái MD_ID_VALUE ở đâu nhỉ.
    Thiếu cái đó ko login vào được phải ko?
    Tui thử làm mà ko có cái đó thì thấy vẫn đủ ID, Pass ở máy khác, nhưng bấm login vào ko đc.
    Mà cái này xài trên win7 hay tất cả các loại win khác hả ông 😀

  2. Nếu mà user không dùng chức năng lưu ID và Password thì cái MD_ID_VALUE không có đâu ông ah. :)). Win nào cũng được ông. Phụ thuộc vào Yahoo Messenger chứ không phải vào Win. :).

  3. Dạo này lo cày bừa Thiên Long Bát Bộ kiếm trùng lâu giới quá nên chưa coi lại…ha…ha… Để bữa nào rãnh mới xem được ông ah. :)).

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