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CVE-2021-26858 is a Microsoft Exchange Server vulnerability in on-premises Exchange Server 2013, 2016, and 2019 that allows post-authentication arbitrary file write and is commonly associated with the ProxyLogon exploitation chain. The flaw has been described as a remote code execution vulnerability because attackers can use it to write attacker-controlled files to locations that are subsequently executed by the server, including deployment of ASPX web shells. Reporting on exploitation indicates it was frequently chained after CVE-2021-26855, which provided the authentication bypass and server-side request forgery needed to reach privileged Exchange functionality. Multiple analyses also describe the vulnerability as being used to manipulate Exchange virtual directories, particularly OAB-related configuration, in order to persist attacker-controlled content to disk and expose it through IIS. In practical attacks, exploitation of CVE-2021-26858 enabled adversaries to place web shells, execute commands, dump credentials, access mail data, and establish durable access on compromised Exchange servers.
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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.
58 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.