On March 2, 2021 several companies released reports about in-the-wild exploitation of zero-day vulnerabilities inside Microsoft Exchange Server. The following vulnerabilities allow an attacker to compromise a vulnerable Microsoft Exchange Server. As a result, an attacker will gain access to all registered email accounts, or be able to execute arbitrary code (remote code execution or RCE) within the Exchange Server context. In the latter case, the attacker will also be able to achieve persistence on the infected server.
A total of four vulnerabilities were uncovered:
CVE-2021-26855. Server-side request forgery (SSRF) allows an attacker without authorization to query the server with a specially constructed request that will cause remote code execution. The exploited server will then forward the query to another destination.
CVE-2021-26857 caused by unsafe data deserialization inside the Unified Messaging service. Potentially allows an attacker to execute arbitrary code (RCE). As a result of insufficient control over user files, an attacker is able to forge a body of data query, and trick the high-privilege service into executing the code.
CVE-2021-26858. This vulnerability allows an authorized Exchange user to overwrite any existing file inside the system with their own data. To do so, the attacker has to compromise administrative credentials or exploit another vulnerability such as SSRF CVE-2021-26855.
vulnerabilities microsoft exchange server