CVE-2021-26857

Deeper analysis

Microsoft Exchange Server is affected by a remote code execution vulnerability tracked as CVE-2021-26857 and assigned CWE-502 for deserialization of untrusted data. The flaw carries a CVSS 3.1 score of 7.8 with an attack vector of local access, low complexity, no privileges required, and user interaction needed, resulting in high impact to confidentiality, integrity, and availability.

An attacker who can supply a malicious serialized object to the affected Exchange component can trigger code execution on the server. Because the vector is local and requires user interaction, exploitation typically involves an attacker with the ability to place or induce processing of a crafted file or message on the target system.

Microsoft’s security advisory provides patching guidance, and the vulnerability appears in CISA’s catalog of known exploited vulnerabilities, confirming observed in-the-wild exploitation.