A now-patched vulnerability in Microsoft Exchange Server, dubbed ProxyToken, could be abused by an unauthenticated attacker to perform configuration actions on targeted mailboxes.

This latest flaw in the beleaguered platform is tracked as CVE-2021-33766 and is rated 7.3 out of ten on the threat severity scale, and might give rise to the disclosure of personal information if abused.

A hypothetical example of exploitation, according to researchers with the Zero Day Initiative, could lead to an attacker copying all email addresses on a targeted account and forwarding them to an account controlled by the attacker.  

The flaw lies in the Delegated Authentication feature, a mechanism in which the front-end site passes authentication requests to the back-end system when it detects the presence of a SecurityToken cookie.

Because Microsoft Exchange needs to be specifically configured to use the feature and have the backend carry out checks, the module that handles this delegation isn’t loaded under a default configuration. 

This leads to a bypass as the back-end fails to authenticate incoming requests based on the SecurityToken cookie. The back-end will be completely unaware that it needs to authenticate incoming requests, which means requests can sail through without being subject to authentication on either the front or back-end systems.

Microsoft patched this vulnerability as part of its Patch Tuesday round of fixes for July, with no evidence so far that hackers have exploited it.

Businesses will be put on high alert in light of the existence of another Microsoft Exchange Server flaw, however, following the supply-chain attack earlier in the year. 

Hackers linked with the Chinese state exploited four flaws in the platform to launch a series of attacks against potentially hundreds of thousands of victims in March, according to security researchers.

The incident was one of many similar supply-chain attacks during 2021, including the infamous SolarWinds hack towards the end of last year.