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Worm Wriggles Through Yahoo! Mail Flaw
Jasen Bell writes to mention a ZDNet article about a clever new worm affecting users of Yahoo!'s email service. The virus uses a flaw in JavaScript to infect a computer when an email is opened from the user's web-based mail. From the article: "The worm, which was spotted in the wild early this morning, has hit the remote server more than 100,000 times, forwarding Yahoo e-mail addresses harvested from unsuspecting users, Turner said. Although the worm is spreading quickly, and no patch has been issued, Symantec is rating the threat a '2.' The security vendor uses a 1-to-5 rating system, with '5' as its most severe category."
you could also not open werid emails from people you don't know
... and thus I received this worm email "from" one of my friends ... so it's not just coming from random accounts, it's coming from people who have you in their contact list.
Yeah, but this spreads via your Yahoo! contact list
Just copy-pasted this off symantec:
Category 5 - Very Severe
Highly dangerous threat type, very difficult to contain. All machines should download the latest virus definitions immediately and execute a scan. Email servers may need to come down. All three threat metrics must be High.
* Wild: High
* Damage: High
* Distribution: High
Category 4 - Severe
Dangerous threat type, difficult to contain. The latest virus definitions should be downloaded immediately and deployed.
* Wild: High
* Damage or Distribution: High
Category 3 - Moderate
Threat type characterized either as highly wild (but reasonably harmless and containable) or potentially dangerous (and uncontainable) if released into the wild.
* Wild: High
or
* Damage: High and Distribution: High
Category 2 - Low
Threat type characterized either as low or moderate wild threat (but reasonably harmless and containable) or non-wild threat characterized by an unusual damage or spread routine, or perhaps by some feature of the virus that makes headlines in the news.
* Damage: High
or
* Distribution: High
or
* Wild: Low or Moderate
Category 1 - Very Low
Poses little threat to users. Rarely even makes headlines. No reports in the wild.
* Wild: Low
* Damage or Distribution: Low
Lameness filter got me. Here is a link.
--fatboy
Some people tend to think that this worm is harmless (just "spreading itself"). But the worm actually sends the harvested email adresses to an external site - www.av3.net [which I wouldn't dare to browse to].
Here are the technical details of the worm:
1) Arrives on the compromised computer as an HTML email containing Javascript. The email may have the following characteristics:
From: Varies
Subject: New Graphic Site
Message body: Note: forwarded message attached.
2) Once the email is opened the worm exploits a vulnerability in the Yahoo email service to run a script.
3) Sends a copy of itself to certain email addresses gathered from the Yahoo email folders.
4) Targets email addresses from the @yahoo.com and @yahoogroups.com domains.
5) Contacts the following URL:
[http://]www.av3.net/index.htm
6) Sends a list of email addresses gathered to the above URL.
The poster's question is valid. He/she is asking if the JavaScript worm can actually do anything other that work within the browser, as in how can the worm "infect" the computer. The answer is that it can't. It only harvests the email addresses that are on your Yahoo addressbook, and emails itself to them, once again though Yahoo. So everything is done within the browser, and there is no compromise outside the browser's sandbox.
With a little creativity, this could be extended to grab a file off the HD, and send the data to any site it chose, but it does not sound like that is the case here.
The article is wrong when it claims that it's "a flaw in JavaScript", it's a flaw in Yahoo's webmail. So the answer to your question is almost certainly: nobody thought it was a good idea to enable JavaScript in emails, the developers working on Yahoo's webmail didn't escape things properly and nobody was doing decent QA to catch the mistake the developers made. So basically, it's a management error.
There doesn't seem to be detailed technical information available anywhere, but it sounds very much like it's just a specialised form of an XSS attack, where you sneak code into the application in such a way that the application doesn't encode it properly for output to another user.
Bogtha Bogtha Bogtha
It's fixed on yahoo's servers now, but according to the source link posted earlier, the flaw that's being exploited seems to be a bug in how yahoo parses html attributes. The bug sends itself as:
/ ma_mail_1.gif'
<img src='http://us.i1.yimg.com/us.yimg.com/i/us/nt/ma
target=""onload="whole bunch of crappy javascript here that uses only
single quotes and just goes on and on">
Note the lack of a space between the 'target' bit and the 'onload' bit. Now, apparently "target" is one of the HTML attributes that yahoo allows through on an IMG tag (why?). Anyway, it appears that yahoo's servers see both the target and the onload bit as one big long target attribute and let it through, whereas most browsers see that as a separate "target" and "onload" attribute and execute the javascript as soon as the image (one of the standard yahoo mail images, so it'll likely already be in the browser cache) is loaded.
The lesson here? I'm not really sure, beyond "double- and triple-check your parsing routines, since they will be used in security-sensitive code".