CVE Intelligence
CISA KEV Medium File handling exploit reference

CVE-2021-31207 — Microsoft Exchange Server Security Feature Bypass Vulnerability

Microsoft Exchange Server Security Feature Bypass Vulnerability

Published May 11, 2021 · Updated Oct 13, 2026 · Sources: CISA Known Exploited Vulnerabilities, NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP, GitHub Security Advisories · CNA: microsoft

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

CISA KEV · Nov 3, 2021 R alphvR hive ☠ FIN7☠ Magic Hound Metasploit modules · 1
How the score was calculated
  • Confirmed exploitation (CISA KEV)+35
  • Used in ransomware campaigns (CISA)+10
  • Exploited by 2 threat actor(s) (MITRE ATT&CK)+10
  • Public exploit/PoC (metasploit)+12
  • EPSS 99.8% (30-day exploitation probability)+15
  • Technical severity (medium)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation
Confirmed
CISA KEV, listed Nov 3, 2021
EPSS FIRST
99.78%
ahead of 100% of scored CVEs · Oct 14, 2026
Affects
Microsoft
Exchange Server and 5 more
CVSS base
6.6
Medium · 4 source(s)
Prioritization reference: 7.2 editorial decision

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation
Active
none · proof-of-concept · active
Automatable
No
can an attacker script the kill-chain steps
Technical impact
Total
partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

CISA required action CISA

Apply updates per vendor instructions.

US federal deadline: Nov 17, 2021 — passed 1793 day(s) ago

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

  1. Identify exposed assets running the affected vendor/product/version combinations (Microsoft Exchange Server, Microsoft Microsoft Exchange Server 2013 Cumulative Update 23, Microsoft Microsoft Exchange Server 2016 Cumulative Update 19…).
  2. Treat as emergency remediation: the CVE is in CISA KEV (confirmed exploitation).
  3. Apply the fixed version or the patch documented under Remediation.
  4. EPSS ahead of 100% of scored CVEs: prioritize it in the patch queue.
  5. Search available logs for exploit probes matching the vulnerability class (file handling), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Microsoft Exchange Server version 2013-cumulative_update_23version 2016-cumulative_update_19version 2016-cumulative_update_20version 2019-cumulative_update_8version 2019-cumulative_update_9 Vulnerable NIST National Vulnerability Database
Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 ≥ 15.00.0 and < 15.00.1497.018 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 ≥ 15.01.0 and < 15.01.2176.014 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server 2016 Cumulative Update 20 ≥ 15.01.0 and < 15.01.2242.010 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 ≥ 15.02.0 and < 15.02.0792.015 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server 2019 Cumulative Update 9 ≥ 15.02.0 and < 15.02.0858.012 Vulnerable CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector
Network
Attack complexity
High
Privileges required
High
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
7.2 CVSS 3.1 High — — GitHub, Inc. via GitHub Security Advisories
6.6 CVSS 3.1 Medium 0.7 5.9 secure@microsoft.com via NIST National Vulnerability Database
6.6 CVSS 3.1 Medium 0.7 5.9 nvd@nist.gov via NIST National Vulnerability Database
6.6 CVSS 3.1 Medium — — microsoft via CVE Program (cve.org) e CISA ADP
6.5 CVSS 2.0 Medium 8.0 6.4 nvd@nist.gov via NIST National Vulnerability Database
CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H (score shown at the top: secure@microsoft.com)
Sources disagree; every score is kept with its origin. Reference adopted after editorial review: 7.2 (GitHub, Inc. via GitHub Security Advisories).

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-434 · Unrestricted Upload of File with Dangerous Type

Other CVEs with this weakness →

CISA KEV details CISA

CISA lists a CVE once it has evidence of exploitation against real targets.

Listed
Nov 3, 2021
Federal deadline
Nov 17, 2021
passed 1793 day(s) ago
Ransomware use
Known

CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a reference date.

Public exploits, PoCs and detection templates

Public code that demonstrates or tests the flaw raises the odds of mass exploitation — and Nuclei templates help find vulnerable assets in your environment.

Metasploit modules

Warning: public PoCs are sometimes disguised malware. Use them only for authorized testing in an isolated environment. This portal does not host exploit code — it only points to the sources.

Threat actors exploiting this CVE MITRE ATT&CK

According to MITRE ATT&CK.

FIN7 G0046Magic Hound G0059

Associated ransomware groups ransomware.live

alphvhive

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 99.78%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

  • Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
  • Monitor file creation in web directories and paths with traversal sequences (../).
  • Hunt retroactively in logs from before Nov 3, 2021 (KEV listing): exploitation may have started earlier.
web/proxy/WAF logsnetwork IDS/IPSfile integrity (FIM)endpoint telemetry (EDR)

Timeline

What happened to this CVE, newest first.

  • 2026
  • Affected products added · microsoft/microsoft exchange server 2013 cumulative update 23, microsoft/microsoft exchange server 2019 cumulative updat
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • SSVC assessment updated
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • Record published
    Oct 13, 2026
  • Fix documented
    Oct 13, 2026 · NIST National Vulnerability Database
  • Affected products added · microsoft/exchange server
    Oct 13, 2026 · NIST National Vulnerability Database
  • CVSS changed · 6.6
    Oct 13, 2026 · NIST National Vulnerability Database
  • Source status changed · → Analyzed
    Oct 13, 2026 · NIST National Vulnerability Database
  • Added to CISA KEV (detected)
    Oct 13, 2026 · CISA Known Exploited Vulnerabilities
  • Record created in this database
    Oct 13, 2026 · CISA Known Exploited Vulnerabilities
  • 2021
  • Added to CISA KEV catalog
    Nov 2, 2021 · CISA
  • CVE published
    May 11, 2021 · microsoft

Source activity

Repeated readings are counted rather than listed.

  • 2× References added · Oct 13, 2026 – Oct 13, 2026
  • 1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

7 on the record

Elsewhere on this site

Microsoft
every CVE for this vendor
2021-11
what else CISA listed that month
File handling
same class
CWE-434
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-96940 Weak authorization in Microsoft Exchange Server allows an authenticated attacker to elevate... 13d
  • CVE-2026-69641 Microsoft Exchange Server Elevation of Privilege Vulnerability Sep 8, 2026
  • CVE-2026-69382 Microsoft Exchange Server Information Disclosure Vulnerability Sep 8, 2026
  • CVE-2026-69380 Microsoft Exchange Server Elevation of Privilege Vulnerability Sep 8, 2026
  • CVE-2026-69375 Microsoft Exchange Server Tampering Vulnerability Sep 8, 2026
  • CVE-2026-69378 Microsoft Exchange Server Denial of Service Vulnerability Sep 8, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

  • No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
  • No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
  • No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2021-31207 is "Microsoft Exchange Server Security Feature Bypass Vulnerability", rated Medium. It affects Exchange Server (Microsoft); versions are in the scope table. It is in CISA KEV: there is evidence of real-world exploitation. CVSS 6.6 describes technical severity. EPSS 99.78% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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