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CVE-2021-26858 is a vulnerability in on-premises Microsoft Exchange Server that allows an authenticated attacker to perform an arbitrary file write on the Exchange server. It affects Exchange Server 2013, 2016, and 2019 and was one of the flaws used in the ProxyLogon exploitation chain. The issue permits a threat actor who has already authenticated to the Exchange server—either directly with valid credentials or by first abusing another flaw such as CVE-2021-26855—to write a file to an attacker-chosen path on the server. In practical exploitation, this arbitrary write capability can be used to place server-executable content such as a web shell into locations served by IIS, enabling subsequent remote command execution and persistent access.
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No valid public exploits. Mallory filtered out 2 candidates as fakes, detection scripts, or README-only repos.
All candidate exploits were filtered out by Mallory's validation.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
35 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
A specific Microsoft Exchange Server vulnerability that is part of the ProxyLogon exploit chain used by SHADOW-EARTH-053 for initial compromise.
A Microsoft Exchange Server vulnerability that is part of the ProxyLogon exploit chain used to compromise unpatched internet-facing Exchange servers for espionage operations.
A Microsoft Exchange Server vulnerability that is part of the ProxyLogon exploit chain used by the threat group for initial access into unpatched Exchange environments.
A Microsoft Exchange Server vulnerability referenced as part of the ProxyLogon exploit chain used to achieve remote code execution.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.