CVE-2021-31207 microsoft · exchange server

Microsoft Exchange ProxyShell RCE

Vulnerability at a glance

Severity, exploitation likelihood & attack requirements

CVSS v3.1 severity
6.6Medium
EPSS · exploitation likelihood
99.782% Probability within 30 daysPercentile 100.0
EPSS trend
UnavailableNo comparable history
Known exploitation
Known exploited CISA KEV catalog
Attack vector
NetworkRemotely reachable
Attack complexity
HighMore obstacles to exploitation
Privileges required
HighElevated privileges required
User interaction
NoneNo user action needed
Exploit availability & maturity
Not scored Exploit reference available No explicit exploit maturity assessment View exploit evidence
How to read the colors & trend
Higher exposureElevatedModerateLower exposureUnknown / unassessed

CVSS colors follow its reported severity. Attack-factor colors describe each requirement, not an overall risk rating. Gray also covers absence from KEV; absence does not establish safety.

EPSS display bands: below 1% blue, 1–10% amber, 10–50% orange, 50% and above red. These are presentation bands, not FIRST severity categories. The trend compares daily EPSS probabilities, not percentile ranks; “pp” means percentage points. The sparkline shows relative movement, with its vertical scale fitted to the observed scores.

Already protected by existing Atomicorp rules No new CVE-specific rule required.

Atomicorp ModSecurity protection

Protected by Atomicorp rules

Atomicorp ModSecurity rules block attacks exploiting this vulnerability when blocking is enabled. Keep your rules current.

By targeting underlying attack techniques and CWE weakness classes, existing rules can protect against newly disclosed vulnerabilities. Rule changes are regression-tested against attack and false-positive corpora.

1 rule identified Verified 2026-09-29 View rules & research details →

WAF protection applies to the tested attack paths; it does not change the CVSS or EPSS score. Keep your software patched and your Atomicorp rules current.

CISA SSVC assessment

CISA ADP · Assessed 2025-02-04 · SSVC 2.0.3

Exploitation
Active exploitation
Automatable
No
Technical Impact
Total

Stakeholder-Specific Vulnerability Categorization records CISA’s assessment of exploitation, automation, and technical impact. “None reported” reflects the assessment date; it does not establish that no exploit exists. View SSVC source details.

On this page

CVE Details

  • Protection status: Protected by Atomicorp rules · About protection statuses
  • CVE: CVE-2021-31207
  • Name: Microsoft Exchange ProxyShell RCE
  • Severity: Medium
  • CVSS: 6.6 (v3.1) (Medium)
  • CVSS version: 3.1
  • CVSS source: microsoft CNA
  • CVSS vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
    • Attack vector: Network
    • Attack complexity: High
    • Privileges required: High
    • User interaction: None
    • Scope: Unchanged
    • Confidentiality impact: High
    • Integrity impact: High
    • Availability impact: High
  • CWE: CWE-434 — Unrestricted Upload of File with Dangerous Type
  • Related CAPEC attack patterns (via MITRE CWE relationships): CAPEC-1
  • Vendor: microsoft
  • Vulnerable product: exchange server
  • CVE published: 2021-05-11
  • CVE last modified: 2026-06-17
  • Affected versions (NVD configurations):
    • exchange server: 2013 cumulative_update_23
    • exchange server: 2016 cumulative_update_19
    • exchange server: 2016 cumulative_update_20
    • exchange server: 2019 cumulative_update_8
    • exchange server: 2019 cumulative_update_9
  • EPSS: 0.9978; 100.0th percentile as of 2026-09-29 (FIRST )
  • CISA KEV: Listed (added 2021-11-03)
    • Known ransomware campaign use: Known
  • Latest Update: 2026-09-29
  • Selected Rule IDs observed: 390716
  • Matching rules: 1; rules with blocking actions: 1
  • Would-block Rule IDs: 390716
Technical vulnerability details · CVSS assessments, exploit evidence & CPE data

CVSS assessments

CVSS v3.1 · secure@microsoft.com

  • Base score: 6.6
  • Base severity: MEDIUM
  • Exploitability subscore: 0.7
  • Impact subscore: 5.9
  • Assessment type: Secondary
  • Source identifier: secure@microsoft.com
  • Full vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
    • Attack vector: Network
    • Attack complexity: High
    • Privileges required: High
    • User interaction: None
    • Scope: Unchanged
    • Confidentiality impact: High
    • Integrity impact: High
    • Availability impact: High
  • Exploit maturity: Not scored

CVSS v3.1 · NVD

  • Base score: 6.6
  • Base severity: MEDIUM
  • Exploitability subscore: 0.7
  • Impact subscore: 5.9
  • Assessment type: Secondary
  • Source identifier: nvd@nist.gov
  • Full vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
    • Attack vector: Network
    • Attack complexity: High
    • Privileges required: High
    • User interaction: None
    • Scope: Unchanged
    • Confidentiality impact: High
    • Integrity impact: High
    • Availability impact: High
  • Exploit maturity: Not scored

CVSS v2.0 · NVD

  • Base score: 6.5
  • Base severity: MEDIUM
  • Exploitability subscore: 8.0
  • Impact subscore: 6.4
  • Assessment type: Primary
  • Source identifier: nvd@nist.gov
  • Full vector: AV:N/AC:L/Au:S/C:P/I:P/A:P
    • Attack vector: Network
    • Attack complexity: Low
    • Authentication: Single
    • Confidentiality impact: Partial
    • Integrity impact: Partial
    • Availability impact: Partial
  • Exploit maturity: Not scored

Exploitability and impact subscores describe the vulnerability’s technical characteristics. Exploit maturity describes the reported state of exploitation; these are separate measures.

Assessment source: NVD record · CVE record

CISA SSVC assessments

These are CISA’s published decision points at the assessment time. They are not a complete organization-specific prioritization decision.

  • Provider: CISA ADP
  • Provider identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0
  • SSVC version: 2.0.3
  • Role: CISA Coordinator
  • Assessment timestamp: 2025-02-04T18:16:59.637651Z
  • Exploitation: active
  • Automatable: no
  • Technical Impact: total
  • Source: CISA ADP in the CVE record

Exploit references

The CVE source data tags these references as exploits. A reference alone does not establish reliability or active exploitation.

Vulnerable CPE configurations

  • cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_23:*:*:*:*:*:*
    • NVD match criteria ID: DA166F2A-D83B-4D50-AD0B-668D813E0585
  • cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_19:*:*:*:*:*:*
    • NVD match criteria ID: 28FCA0E8-7D27-4746-9731-91B834CA3E64
  • cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_20:*:*:*:*:*:*
    • NVD match criteria ID: 19C1EE0C-B8DD-4B91-BE4B-1C42D72FB718
  • cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_8:*:*:*:*:*:*
    • NVD match criteria ID: 44ECF39A-1DE1-4870-A494-06A53494338D
  • cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_9:*:*:*:*:*:*
    • NVD match criteria ID: 71CDF29B-116B-4DE2-AFD0-B62477FF0AEB

Vulnerability Description

Microsoft Exchange Server Security Feature Bypass Vulnerability

  1. Maintain the observed Atomicorp protections. Keep your Atomicorp commercial rules current and confirm that the selected observed rules (390716 ) have not been locally disabled. Atomicorp commercial rules are enabled by default. If you use Atomic WAF, review the applicable policy and rule settings.

  2. Apply the vendor’s remediation. WAF protection is defense in depth and does not replace upgrading or following the affected vendor’s guidance.

    • Affected versions: Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 versions earlier than 15.00.1497.018.
    • Affected versions: Microsoft Microsoft Exchange Server 2019 Cumulative Update 9 versions earlier than 15.02.0858.012.
    • Affected versions: Microsoft Microsoft Exchange Server 2016 Cumulative Update 20 versions earlier than 15.01.2242.010.
    • Affected versions: Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 versions earlier than 15.01.2176.014.
    • Affected versions: Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 versions earlier than 15.02.0792.015.
    • The CVE record does not include written upgrade or workaround instructions, but it does publish the following patch or mitigation reference:
    • CISA KEV required action: Apply updates per vendor instructions.
    • CISA KEV federal remediation due date: 2021-11-17

  3. Validate after changes. Confirm rule updates are current, review WAF events for the affected application, and test application behavior after vendor upgrades, mitigations, or local policy changes.

Research Notes

Atomicorp WAF protections are designed around attack techniques and exploit behaviors, allowing existing protections to apply across multiple vulnerabilities without requiring a unique rule for every CVE.

Public Vulnerability References

MITRE ATT&CK Context

The following mappings are published by the MITRE Center for Threat-Informed Defense and describe exploitation behavior or potential impact; they do not claim that a WAF rule implements the ATT&CK technique.

Test or Research Source

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

  • Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
  • Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
  • Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
  • Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
  • Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ .