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Researchers at Information Security Research Team (INSERT) have dissevered a serious flaw in Google's Gmail service. The group demonstrates how anyone with no special Internet access privileges other than being able to connect to SMTP (TCP port 25) and HTTP (TCP port 80) servers is able to exploit a single Gmail account in order to be granted nearly unrestricted access to Google's massive whitelisted SMTP relay infrastructure.

From the Report:

As part of our recent work on the trust hierarchy that exists among email providers throughout the Internet, we have uncovered a serious security flaw in Google's free email service, Gmail. This vulnerability exposes Google's email servers in a way that allows an attacker to use them as open spam and phishing relays. This issue is related to the risk of a malicious user abusing Gmail's email forwarding functionality. This is possible because Gmail's email forwarding functionality does not impose proper security restrictions during its setup process and can be easily subverted. By exploiting this problem an attacker can send unlimited spam and phishing (i.e. forged) email messages that are delivered by Google's very own SMTP servers. Since the messages are delivered by Google's own servers, an attack based on this flaw is able to bypass all spam filters that are based on the blacklist / whitelist concept. We were able to confirm that this vulnerability is indeed exploitable by crafting a proof of concept attack that allowed us to send forged email messages unrestrictedly through Google's server infrastructure. We have also verified that this flaw allows attackers to bypass spam filters by using our method to send messages that are usually flagged as spam. While sending these messages directly from our network in the traditional way had the messages classified as spam, by sending the very same messages using our exploit, the messages were delivered directly to the victim's inbox, thus bypassing filters. All email providers that offer Google's SMTP servers any special level of trust (e.g. whitelist status) are vulnerable. We have contacted Google about this issue and are waiting for their position before releasing further details.

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Related topics: Security, Spam