Phishing Attack Exposes Yahoo Accounts

Fugitif · 27 Oct 2008

Ongoing Phishing Attack Exposes Yahoo Accounts

The Netcraft toolbar community has detected a vulnerability on a Yahoo website, which is currently being used to steal authentication cookies from Yahoo users — transmitting them to a website under the control of a remote attacker. With these stolen details, the attacker can gain access to his victims' Yahoo accounts, such as Yahoo Mail.

The attack exploits a cross-site scripting vulnerability on Yahoo's HotJobs site at hotjobs.yahoo.com, which currently allows the attacker to inject obfuscated JavaScript into the affected page. The script steals the authentication cookies that are sent for the yahoo.com domain and passes them to a different website in the United States, where the attacker is harvesting stolen authentication details.

When websites use cookies to handle authenticated sessions, it is extremely important to protect the cookie values and ensure they are not seen by other parties. Cross-site scripting vulnerabilities often allow these values to be accessed by an attacker and transmitted to a website under their control, which then allows the attacker to use the same cookie values to hijack their victim's session without needing to log in. This type of attack can be mitigated to some extent by using HttpOnly cookies to prevent scripts gaining access to the cookies — a feature that is now supported by most modern browsers.

Earlier this year, Netcraft blocked a similar flaw on another Yahoo website. The previous attack targeted a cross-site scripting vulnerability on Yahoo's ychat.help.yahoo.com site, which was served securely using a valid SSL certificate, adding further credibility to the attack. The attacker used the vulnerability to inject malign JavaScript into one of the site's webpages. Unlike the current attack, the injected code was sourced from a server in Spain, but also resulted in the victim's cookies being stolen and transmitted to a PHP script on the same server. ,
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In both cases, Netcraft found that the Yahoo cookies stolen by the attacker would have allowed him to hijack his victims' browser sessions, letting him gain access to all of their Yahoo Mail emails and any other account which uses cookies for the yahoo.com domain.

Simply visiting the malign URLs on yahoo.com can be enough for a victim to fall prey to the attacker, letting him steal the necessary session cookies to gain access to the victim's email — the victim does not even have to type in their username and password for the attacker to do this. Both attacks send the victim to a blank webpage, leaving them unlikely to realise that their own account has just been compromised.

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Fugitif · 28 Oct 2008

Attack of the killer XSS

Attack of the killer XSS

Yahoo has closed a gaping hole that attackers were exploiting to gain access to victims' Yahoo Mail accounts and other restricted areas of site.

The cross site scripting error in the hotjobs.yahoo.com domain allowed the attackers to inject cleverly obfuscated javascript into the page that silently siphoned the cookies used to authenticate Yahoo users when they log in to sections of the portal that require a password. Armed with the cookies, attackers were then given broad control over the victim's Yahoo account, including Yahoo Email and any other service that uses authentication cookies belonging to the yahoo.com domain.

"I guess the beautiful bit about it from an attacker's viewpoint is quite a lot of people would be unaware of what's happened" after accessing a booby-trapped hotjobs URL, said Paul Mutton, an internet services developer for Netcraft who helped discover the exploit. "Not many people will think of changing their password after that happens."

To Yahoo's credit, Mutton said the XSS error was closed within hours of him reporting it to Yahoo's security team. But the episode is a reminder that even the biggest sites can be needlessly sloppy when it comes to handling authentication cookies. The attack would have been impossible to carry out had Yahoo bothered to use http-only cookies.

Yahoo is hardly alone here. Last month, bankofamerica.com, register.com, netflix.com and dozens of other big name sites were caught transmitting credentials that are vulnerable to a new tool called CookieMonster. That attack is neutered when sites use https-only cookies.

A Yahoo spokeswoman said she was looking in to the Netcraft report.

According to Mutton, XSS vulnerability was exploited by hotjobs.com URLs that contained a long series of digits. When transformed into javascript it redirected users to a blank webpage on a different domain. A victim need not enter a user name or password. Visiting the blank page was all that was required to steal an authentication cookie that acts as a universal key across the the entire yahoo.com domain.

At time of writing, the blank page remained up and running. It's unknown if it's being used to attack other websites,
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Phishing Attack Exposes Yahoo Accounts

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Phishing Attack Exposes Yahoo Accounts

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