High severityKnown ExploitedCVE-2021-26855

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

Published Nov 3, 2021Updated May 3, 2022Source checked Jul 18, 2026

Upstream Advisory

Key facts

Risk: High
Exploitation: Known exploited
Affected software: Not mapped
Fix availability: Not reported

Recommended action

Review the upstream advisory, identify the affected product in your inventory, and apply the vendor update when one is available.

What this means

Microsoft Exchange Server Remote Code Execution Vulnerability is listed in the HOL Guard supply-chain feed, but affected software is not mapped to a package. Review the upstream advisory for the affected product and vendor guidance. This vulnerability is known to be exploited in the wild. Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

Affected software

Affected software not mapped. Review the upstream record for vendor-specific product and version guidance.

Evidence and references

No references were reported by the source.

Source metadata

Published: Nov 3, 2021
Modified: May 3, 2022
Source checked: Jul 18, 2026

Reported by CISA KEV (kev).

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