# Microsoft Exchange Server Remote Code Execution Vulnerability

CVE record · `CVE-2021-26857`

- **Identifiers:** `CVE-2021-26857`
- **State:** Confirmed
- **Severity:** CVSS 7.8 High
- **Critical:** Yes: KEV, CVSS 7+ and EPSS
- **CNA:** microsoft
- **Weakness (CWE):** [CWE-502](https://cwe.mitre.org/data/definitions/502.html)
- **Tags:** [Insecure deserialization](https://agentprime.io/advisories/tag/deserialization.md)
- **Technologies:** [Microsoft Exchange Server 2010 Service Pack 3](https://agentprime.io/advisories/technology/microsoft-exchange-server-2010-service-pack-3.md) · [Microsoft Exchange Server 2013 Cumulative Update 21](https://agentprime.io/advisories/technology/microsoft-exchange-server-2013-cumulative-update-21.md) · [Microsoft Exchange Server 2013 Cumulative Update 22](https://agentprime.io/advisories/technology/microsoft-exchange-server-2013-cumulative-update-22.md) · [Microsoft Exchange Server 2013 Cumulative Update 23](https://agentprime.io/advisories/technology/microsoft-exchange-server-2013-cumulative-update-23.md) · [Microsoft Exchange Server 2013 Service Pack 1](https://agentprime.io/advisories/technology/microsoft-exchange-server-2013-service-pack-1.md) · [Microsoft Exchange Server 2016 Cumulative Update 10](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-10.md) · [Microsoft Exchange Server 2016 Cumulative Update 11](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-11.md) · [Microsoft Exchange Server 2016 Cumulative Update 12](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-12.md) · [Microsoft Exchange Server 2016 Cumulative Update 13](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-13.md) · [Microsoft Exchange Server 2016 Cumulative Update 14](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-14.md) · [Microsoft Exchange Server 2016 Cumulative Update 15](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-15.md) · [Microsoft Exchange Server 2016 Cumulative Update 16](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-16.md) · [Microsoft Exchange Server 2016 Cumulative Update 17](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-17.md) · [Microsoft Exchange Server 2016 Cumulative Update 18](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-18.md) · [Microsoft Exchange Server 2016 Cumulative Update 19](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-19.md) · [Microsoft Exchange Server 2016 Cumulative Update 8](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-8.md) · [Microsoft Exchange Server 2016 Cumulative Update 9](https://agentprime.io/advisories/technology/microsoft-exchange-server-2016-cumulative-update-9.md) · [Microsoft Exchange Server 2019](https://agentprime.io/advisories/technology/microsoft-exchange-server-2019.md) · [Microsoft Exchange Server 2019 Cumulative Update 1](https://agentprime.io/advisories/technology/microsoft-exchange-server-2019-cumulative-update-1.md) · [Microsoft Exchange Server 2019 Cumulative Update 2](https://agentprime.io/advisories/technology/microsoft-exchange-server-2019-cumulative-update-2.md)
- **Published:** 2 Mar 2021
- **Updated:** 19 Aug 2026
- **Reviewed by a person:** No

Published by the CVE Program (CNA: microsoft) at 2 Mar 2021, 23:55 UTC. agentprime has not analysed this yet, so whether it affects any app is not known. This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it.

## Critical

Critical, because:

- In CISA’s KEV catalogue since 3 Nov 2021: it is known to be exploited.
- CVSS score 7.8 (7.0 or more) and EPSS 0.96 (0.1 or more): a high score and a real chance of exploitation in the next 30 days.

How we decide what’s critical: <https://agentprime.io/advisories/critical.md>

## Affected products, as the CNA wrote them

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

## Not analysed yet

We haven’t published our own analysis of this CVE. If a person reviews it, this page will say what’s affected and what has to be true for it to matter.

## Severity and exploitation

- 7.8 High, CVSS 3.1: `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`, from CNA (microsoft)

- **CISA KEV:** Listed since 3 Nov 2021: known to be exploited. CISA reports use in ransomware campaigns.
- **EPSS:** 0.96, the 99th percentile, on 6 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

This record came from the full history of the CVE Program, published 2 Mar 2021. New records show when each source confirmed them.

## Sources

- [CVE List: CVE-2021-26857](https://www.cve.org/CVERecord?id=CVE-2021-26857)
- [NVD: CVE-2021-26857](https://nvd.nist.gov/vuln/detail/CVE-2021-26857)

### References from the CNA

- <https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26857> (vendor-advisory, patch)

Facts on this page come from the CVE List as the CNA published them, with CVSS from the sources named above, CISA KEV and FIRST EPSS. Vendors, products and versions are shown as the CNA wrote them. We don’t reproduce the CVE’s description.

## Check whether this affects you

This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it. Compare the affected versions above with what you run.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), `get_advisory` returns this record and `search_advisories` finds related ones. Setup: <https://agentprime.io/mcp.md>
- As JSON: <https://api.agentprime.io/v1/advisories/CVE-2021-26857>
- Everything that needs attention now: <https://agentprime.io/advisories/critical.md>

---

The web page: <https://agentprime.io/advisories/CVE-2021-26857/>. Everything for agents: <https://agentprime.io/llms.txt>.