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CVE-2021-31207· NVD / CVE Program· CNA microsoft

Microsoft Exchange Server Security Feature Bypass Vulnerability

MediumCVSS 6.6 · v3.1KEVWeaponized No fix recorded
Published
May 11, 2021
Updated
Jun 16, 2026
EPSS
99.8% · 100th percentile
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Action score

86

Now

Patch today or shut off access.

CVSS
26 / 40 · 6.6 / 10
CISA KEV
30 / 30 · Listed
EPSS
30 / 30 · 99.8%

CISA SSVC decision

Exploitation
active
Automatable
no
Technical impact
total

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · microsoft
6.6
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Actively exploited in the wild

CISA added this to its Known Exploited Vulnerabilities catalog on Nov 3, 2021. That signals real attacks, not theory.

Remediation deadline for government agencies: Nov 17, 2021.

Known to be used in ransomware campaigns.

Affected systems

VendorProduct
microsoftexchange server

Using exchange server? Add it to your stack and be the first to know about CVEs like this one. (Email server)

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Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • microsoft exchange server2013
  • microsoft exchange server2016
  • microsoft exchange server2019

Versions reported by the vendor

Affected version ranges reported by the assigning authority (microsoft). Independent of NVD's CPE analysis and usually ahead of it.

  • Microsoft Microsoft Exchange Server 2013 Cumulative Update 23

    • 15.00.0 and later · before 15.00.1497.018affected

  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 19

    • 15.01.0 and later · before 15.01.2176.014affected

  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 20

    • 15.01.0 and later · before 15.01.2242.010affected

  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 8

    • 15.02.0 and later · before 15.02.0792.015affected

  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 9

    • 15.02.0 and later · before 15.02.0858.012affected

Same product

microsoft: all records

Other highest-scoring records for the same primary product.

  • CVE-2021-34523Microsoft Exchange Server Elevation of Privilege VulnerabilityKEV
    96Now
  • CVE-2021-34473Microsoft Exchange Server Remote Code Execution VulnerabilityKEV
    96Now
  • CVE-2021-26855Microsoft Exchange Server Remote Code Execution VulnerabilityKEV
    96Now
  • CVE-2022-41040Microsoft Exchange Server Elevation of Privilege VulnerabilityKEV
    95Now
  • CVE-2020-0688A remote code execution vulnerability exists in Microsoft Exchange software when the software fails to properly handle objects in memory, akKEV
    95Now
  • CVE-2021-42321Microsoft Exchange Server Remote Code Execution VulnerabilityKEV
    93Now

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

No fix version is recorded for this entry. Check the references for vendor advisories.

Exploit status

Weaponized

A ready exploit tool or active in-the-wild use is known for this vulnerability. This should be your top priority.

Evidence

  • Metasploit module ×1

Noroxi does not host or link to exploit code. Metasploit module names and Nuclei template ids are shown (names, not code); Exploit-DB entries and PoC repositories are given as counts only.

Public exploit evidence
SourceKindDate
Metasploit moduleexploit/windows/http/exchange_proxyshell_rceMicrosoft Exchange ProxyShell RCEexploit2021-04-06

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

  1. Metasploit-36 d
  2. Published
  3. CISA KEV+175 d
  4. TodaySep 29, 2026+1967 d
EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

—

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

  • CVE-2021-311950 days apartMicrosoft Exchange Server Remote Code Execution Vulnerability
    48Plan
  • CVE-2021-312090 days apartMicrosoft Exchange Server Spoofing Vulnerability
    27Monitor

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • Administrator privileges are required.
  • No user action is required.
  • Special conditions such as timing or configuration are required.

If successful

Confidentiality
high · data can be read
Integrity
high · data or configuration can be modified
Availability
high · the service can be disrupted
Attack vector
Network
Attack complexity
High
Privileges required
High
User interaction
None
Scope
Unchanged
Confidentiality impact
High
Integrity impact
High
Availability impact
High

Weakness class (CWE)

CWE-434 · Unrestricted Upload of File with Dangerous Type

CVSS vector

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

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Noroxi analysis

No Noroxi analysis for this record yet

We don't hand-write analysis for all 385,000+ vulnerabilities; that wouldn't be honest. For notable, high-impact vulnerabilities our team writes the mechanism, detection and remediation steps.

We use this product, ask for help

Change log

No changes recorded on tracked fields yet. Score, KEV, exploit maturity and fix status changes appear here.

References

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records