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Date: 18 May 2005 17:16:43 -0000
From: Torseq Tech. <>
Subject: Yahoo! Messenger may be storing all session data 'Unencoded' on
    the local machine

Title: Yahoo! Messenger May Be Storing All Session Data 'Unencoded' On The Local Machine
Discovered By: Torseq Tech. <>
Date: Wednesday, May 18, 2005
Application affected: Yahoo! Messenger ver. 5.x - 6.0 (all builds) Windows, *Nix/Mac ? (not tested)
Vendor: Yahoo! Inc.
Proof-of-Concept included: Yes
Fix Available: Yes (temporary)
Description: By activating the "Logfile" feature in Yahoo! Messenger a person (perhaps unauthorized) is able to secretly log and view virtually all communications sent and received by Yahoo! Messenger from all IDs logged into Messenger on the local computer. Awareness of this logging is virtually none unless this feature is exclusively known about beforehand by the users and they know exactly where to look for the feature's presence (not likely). When using this feature you may be susceptible to privacy breaches and increased risk for potential remote DoS attacks to be launched successfully.


A much lesser-known feature in Yahoo! Messenger is one called the "Logfile" feature, which can be enabled/disabled remotely or locally through the YMSGR: URL handler. If given physical access to a publicly "shared" system (or home) it is possible to create a registry key and set the value enabling the feature without having Messenger alert future users following the reg write of the logging that's taking place. When Messenger is started up the entire login, all chat room conversations, all PMs, conferences, chat invitations, add buddy requests/successes/denies, and mobile text messages both in/out will be logged. User's cookies, login challenge strings and challenge responses, user's buddy lists, ignore lists, aliases on the accounts and personal status messages etc. will all be stored in a text file on the local computer, all completely unencrypted and in a readable (though ugly) raw packet dump of clear-text.


Unlike the 'recognized' and preference-accessible Yahoo! Messenger Archive feature that allows for archiving of PMs, conferences, and mobile text messages the Logfile feature isn't accessible from anywhere in Messenger other than through invoking the YMSGR: handler with the correct arguments to activate/deactivate it's functionality. This feature is also not very documented (not documented officially), with the exception of 1 search result on google with information about it from a third party (at the time of this writing).

Unlike the Archiving feature which is enabled/disabled for each invidual user on a per-ID basis, this Logfile feature, when enabled, causes Messenger to log all communications both sent and received and append their contents in clear-text to an existing text file called ypager.log. By default this text file, in the same directory as Messenger, is used to log basic connection successes and failures. When the Logfile feature is activated ypager.log then becomes an entirely "different" file storing detailed log information above and beyond it's normal troubleshooting statistics.

Several problems exist when this feature is activated. The first problem is that you're able to make a manual registry write (which can be done even on limited and guest accounts on XP) that when made, after being queried and found by Messenger, is acted upon without alerting all future user(s) that logging will take place on their IDs for all sessions thereafter. You can enable this feature locally by making a registry write and creating the DWORD "Use Logfile" with a value of '1', or disable by setting the value to '0' or deleting the DWORD entirely from "HKEY_CURRENT_USER\Software\Yahoo\pager". When a manual reg write is made it logs all user's data, it's scope isn't limited to individual IDs but any IDs used in Messenger on the computer at any time.

This feature can be enabled remotely using the YMSGR: handler link YMSGR:enablelog?1 and clicking on "enable" through an IFRAME or in a hyperlink in pm or chat. The feature can be disabled remotely using YMSGR:enablelog? and clicking on "disable" to stop the logging after shutting down Messenger and restarting it. When logging is enabled through the use of the YMSGR: handler the current user is prompted from the enable/disable Warning dialog box but the message does not cearly indicate exactly what will be logged, where it will be stored, who will be logged or whether or not the data will be encoded or not when stored. A user could be tricked into enabling this feature remotely, and quite easily, without knowing what it does or how it works, much less how to turn it off if they needed to for performance or privacy reasons.

Since many businesses communicate using Yahoo! Messenger employees and/or even outsiders could keep track of business conferences that took place, chat room conversations and private messages (PMs) that might have taken place during workdays. Colleges with Yahoo! Messenger installed on their campus computers could be used to harvest all conversational data as well as private data (cookies, buddylists and ignore lists) from students using it between classes.

Comparing and contrasting the known Yahoo! Archive feature against this "Logfile" feature to indicate the differences is as follows:

Yahoo! Messenger Archive feature:

- Is Messenger GUI-accessible through preferences for enabling and disabling.
- Selective logging exists for PMs, conferences and/or mobile text messages and alerts.
- Logged data is stored on the local computer in the form of .dat files of which the contents are XORd (though very minimal) with the user ID to prevent your typical local user from viewing the contents. Ability to view the contents of the logfiles is granted when signed into the account on which the logs were created (where the user ID and password would be needed to read them).
- Is enabled and disabled on a per-ID basis, meaning each ID logged into Messenger can choose whether to enable archiving or not (default is disabled for all).
- PMs received from senders that are not replied back to during the session are not archived.

Yahoo! Messenger Logfile feature:

- Is not Messenger GUI-accessible from anywhere for simple enabling and disabling.
- No selective logging exists per service type, virtually everything that can be sent or received through Messenger is logged (much broader scope of logged activity vs. Archiving).
- Logged data is stored on the local computer in the ypager.log text file of which the contents are not encoded at all to prevent onlookers from viewing the contents.
- Is enabled and disabled on a "global" basis, meaning once enabled the logging takes place for all communications on all IDs logged into and used in Messenger.
- Received PMs and all communications from senders that are not replied back to during the session are still logged in the text file (unlike Archiving's behavior).

Potential remote DoS vulnerability:

If in a highly 'active' chat room environment, PM or conference where there's multiple lines of text being sent back and forth at high rates there's the possibility that this log could get quite huge due to the actual packet-like structure being used for the logging and the additional details about each packet's contents being appended. This text file could grow into megabytes if enough elapsed time and activity has taken place. A Denial-of-Service condition could possibly be made if logging was activated and the attacker was able to force large amounts of PM or chat room data to the target's Messenger client forcing it to spend a lot of resources and time logging all of the data flood's contents to the text file, possibly resulting in a client crash or decreased system performance. This scenario would be all too real as PM bombing and various other sorts of flooding (booting) exist practically everywhere on Yahoo! Chat.

Logfile Analysis:

Recv 5/18 0:47:45| type=6, status=1, data=5torseq_tech4my_name_is_tr0nix244278527252PpuInhgAiRFwuCJpNC4=97114Hello there sir63;0640, len=121

The line above from ypager.log is an ASCII packet dump that indicates that a PM was received on the ID "torseq_tech" from the ID "my_name_is_tr0nix" timestamped with the date and time of receipt. The message here is "Hello there sir". The YMSG header status= field value is also provided as well as the YMSG service type being "6" in decimal (Private Message). The YMSG payload length, being 121 Bytes (derived from the YMSG 20 Byte header subtracted from the payload continuation) is also present telling us exactly how large the messages received were in octets. Due to the rawness of the structure from the dump a parser could be written to parse the type= value, status= value, data= and len= values along with the direction (Sent & Recv) and the timestamp to be displayed with better human-readability when viewing the log's contents as a whole. The miscellaneous letters and numbers within the data= field could also be stripped out if each service type's delimiters (being those numb
 ers and letters) were extracted from the actual viewable message text. Reconstructing whole chat room conversations, PM conversations, conferences and harvesting buddy lists, ignore lists and cookies etc. could all be accomplished for multiple IDs used from the computer's Messenger client.


To manually activate the Logfile feature without prompting the user upon it's activation (physical access normally required) go here:

To trick users into activating this feature remotely you could have them visit a web page (like this one) or have them click on a hyperlink with the handler embedded. When they see the "Do you want to enable logging on Yahoo! Messenger?" Warning dialog box popup you could inform them that this just activates the Yahoo! Messenger Archive feature for them or something similar to get them to do it.


Possible data theft by local users of the computer, compromised IDs and friends list communications (through cookie logins from the IDs and cookies gathered) via impersonation and increased risk for potential remote DoS attacks to be launched successfully due to such heavy loads of logging that can be induced.

Solution (temporary):

Set ypager.log permissions to Read-only or check for the logging when Messenger is started up each time it's used from a shared computer using the URL handler to disable it when signing in. Deleting the file before Messenger is started won't help as the file is recreated (it's needed even if the Logfile feature is disabled) if it isn't found in the Messenger folder.

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