Skip to content

CVE-2021-26855: ProxyLogon

Server-Side Request Forgery (SSRF): Is CVE-2021-26855 real, exploitable, or a false positive? Here's the community verdict.

☆ Watch

signals

public sources

Exploited in wild
Yes
CISA KEV
Public exploit
Metasploit +1
Metasploit/EDB/PoC
Base severity
9.1 Critical
CVSS
Exploitation prob.
100%
FIRST EPSS
Weakness
CWE-918 · Server-Side Request Forgery (SSRF)
CWE

Confirmed exploited in the wild. Patch this first, regardless of the base score.

public exploits

links to sources — we don’t host code
MetasploitExploit-DB(4)

A working exploit is publicly available from a maintained source. Treat this as higher urgency and verify your exposure.

baseline read

auto · not a community verdict

Real — exploited in the wild

CISA confirms active exploitation. Treat scanner hits as true positives unless your specific version or config is unaffected.

Based on CISA KEV

Confirm or dispute →

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Microsoft Exchange Server Remote Code Execution Vulnerability

NVD only has a brief summary for this one. The community fills in the real-world detail below.

References

NVD MITRE CVE.org CISA KEV

Published

Embed this verdict
TruePositive verdict for CVE-2021-26855
Markdown
[![TruePositive verdict](https://www.truepositive.app/cve/CVE-2021-26855/badge.svg)](https://www.truepositive.app/cve/CVE-2021-26855)
HTML
<a href="https://www.truepositive.app/cve/CVE-2021-26855"><img src="https://www.truepositive.app/cve/CVE-2021-26855/badge.svg" alt="TruePositive verdict for CVE-2021-26855"></a>

Live badge that updates automatically as the community verdict changes.

Community ground truth

Be the first practitioner to weigh in

So far this is only TruePositive's editorial baseline from public sources. Add your real-world verdict below — it becomes the signal the next person triaging this relies on.

🥇 The first 50 practitioners to contribute earn a Founding Contributor badge.

In your experience, is this finding real and exploitable?

awaiting field verdicts
Real, but not a risk here
Not a real issue

Curated baseline: TruePositive's read from public sources is Real & exploitable — a starting point, not a community verdict.

No account needed. Anonymous verdicts post as an unverified signal. Log in to make yours verified and earn reputation.

Field notes & remediation

Verdicts are the quick signal. Notes are the evidence and fixes behind them.

  • 0
    Field note · TruePositive EditorialCurated

    This is the SSRF bug that starts the ProxyLogon chain, which ends in unauthenticated remote code execution on on-premises Microsoft Exchange. An attacker who can reach the server over HTTPS can read mail and, chained with the related bugs, plant a web shell with no login. The HAFNIUM group used it as a zero-day in early 2021, and then many groups mass-exploited it.

    Who is affected: on-premises Exchange Server 2013, 2016, and 2019. Exchange Online and Microsoft 365 are not affected.

    Very important: this was a zero-day, so patching alone is not the end of the job. If your Exchange was internet-facing and unpatched in early 2021, assume it may have been hit and investigate.

    Commonly flagged by: Nessus, Qualys, Rapid7, Microsoft Defender.

  • 0
    Remediation · TruePositive EditorialCurated

    Apply the Exchange security updates (the March 2021 out-of-band updates and later). If you were exposed and unpatched during the mass-exploitation window, run incident response. Look for aspx web shells in the OAB and ECP folders, check for unusual mailbox exports, and review for new or unknown accounts. Keep Exchange behind a VPN where you can.

Add a field note or remediationoptional
Note type

What are you adding?

Markdown supported · minimum 20 characters.

Related CVEs

Same weakness: CWE-918 · Server-Side Request Forgery (SSRF).