Google Reader “preview” and “lens” script improper feed validation

I. DESCRIPTION

Google Reader (http://www.google.com/reader/) helps organise the contents of
those rss or atom feeds for which the user is interested in or subscribed
to. The user instead of continuously checking his/her favorite sites or
discussion groups for updates, (s)he can let Google Reader do it for them.
From news sites to your friends’ blogs, Google Reader helps stay up-to-date
with all the online information that matters most to the user.

II. VULNERABILITY DETAILS

Google reader is supposed to display only those contents which the user has
subscribed to however two vulnerabilities has been identified which may
allow an attacker to entice it’s victim (using google reader service) to
view unwanted web contents carrying malicious payloads.

a. Google reader “preview” script improper feed validation (without user
authentication)
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Google feed reader “preview” script: The script
(http://www.google.com/reader/preview/*/feed/) is normally used for
displaying the feed contents within the reader.

For example, the following request will display the rss content of the link
http://www.microsoft.com/athome/security/rss/rssfeed.aspx:

http://www.google.com/reader/preview/*/feed/http://www.microsoft.com/athome/
security/rss/rssfeed.aspx

Note: ‘*’ in the above link can be replace with any word of your choice
otherwise it can be left as it is.

This ‘preview’ script is only available to authenticated user but if a
direct link is provided it doens’t ask for user authentication. It can be
very usefull for an attacker to mount an attack on its victim by directing
them to view the content of malicious sites (carrying evil payloads).

b. Google reader “lens” script improper feed validation (with user
authentication)
—————————————————————————-
——
Google feed reader “lens” script: The script
(http://www.google.com/reader/lens/feed/) is normally used for displaying
contents of only those feeds to which an authenticated user has subscribed
to.

However, it is possible to pass any rss / atom feed to the script as
parameter to which the user has not subscribed but the un-subscribed feed
contents can still be loaded within the user reader page.

For example, the following request will display the rss content of the link
http://www.securityfocus.com/rss/news.xml:
http://www.google.com/reader/lens/feed/http://www.securityfocus.com/rss/news
.xml

This ‘lens’ script is only available to authenticated user and can be
usefull for an attacker to mount an attack on its victim by directing them
to view the content of malicious sites (carrying evil payloads) even though
the user is not subscribed to.

III. VENDOR
Google.com

V. CREDITS
Debasis