7.8

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

CVSS: 7.8v3.1HighKEVRansomwareAnalyzed
Published 02-03-2021Last modified 19-08-2026
Open JSON

Vulnerability summary

CVE-2021-26857 is a high (CVSS 7.8) Deserialization of Untrusted Data vulnerability in Microsoft Exchange Server. It is known to be exploited: CISA lists it, first on 03-11-2021. It has been used in ransomware campaigns. EPSS estimates a 95.76% chance of exploitation in the next 30 days, higher than 99.8% of scored vulnerabilities.

Overall risk
90/100
0100
Critical · default modelHow is this calculated?
CVSS score
7.8/10
010
High · v3.1 (CVE.org)
EPSS score
95.76%
0%100%
Critical · 0.00 pp in 30 days
KEVListedCISA
RansomwareKnown useFlagged by CISA
Public exploits0No public exploits recorded

Timeline

  1. 02-03-2021
    PublishedVulnerability first disclosed.
  2. 03-11-2021
    Added to CISA KEVMicrosoft Exchange Server Remote Code Execution Vulnerability
  3. 03-05-2022
    CISA remediation dueApply updates per vendor instructions.
  4. 19-08-2026
    Last modifiedVulnerability information updated.

Description

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS metrics

3 scores · 2 sources · 2 versions

EPSS trend

Probability

95.76%

Chance of exploitation in the next 30 days

Percentile

100%

Versus all other CVEs

30-day change

0.00 pp

From 95.76% on 19-09-2026

19-09-202619-10-2026
EPSS score
95.76%

Known exploited

Compare KEV catalogs
CISA KEVListed
Added
03-11-2021
Federal due date
03-05-2022
Ransomware

CISA required action

Apply updates per vendor instructions.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Microsoft27 products

1 vendor · 27 products

References

3 links