Known exploited vulnerability
CVE-2021-26855
The short answer
CVE-2021-26855 affects Microsoft Microsoft Exchange Server 2013 Cumulative Update 21 and is rated critical (CVSS 9.1). CISA added it to the Known Exploited Vulnerabilities catalog on 2021-11-03: attackers are using it, so patch it first. The vendor has linked a patch.
What it is
Microsoft Exchange Server Remote Code Execution Vulnerability
Weakness: CWE-918.
Severity
9.1 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Exploited (CISA KEV)
Yes, since Nov 3, 2021
Federal due date May 3, 2022
EPSS, next 30 days
100%
Higher than 100% of scored CVEs
How it can be exploited
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality impact
- High
- Integrity impact
- High
- Availability impact
- None
What CISA says to do
Apply updates per vendor instructions.
Affected products
Microsoft Exchange Server 2013 Cumulative Update 21, Microsoft Exchange Server 2013 Cumulative Update 22, Microsoft Exchange Server 2013 Cumulative Update 23, Microsoft Exchange Server 2016 Cumulative Update 10, Microsoft Exchange Server 2016 Cumulative Update 11, Microsoft Exchange Server 2016 Cumulative Update 12, Microsoft Exchange Server 2016 Cumulative Update 13, Microsoft Exchange Server 2016 Cumulative Update 14, Microsoft Exchange Server 2016 Cumulative Update 15, Microsoft Exchange Server 2016 Cumulative Update 16, Microsoft Exchange Server 2016 Cumulative Update 17, Microsoft Exchange Server 2016 Cumulative Update 18. NVD's record lists the exact affected versions.
References
- msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855 PatchVendor Advisory
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855 PatchVendor Advisory
- www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855 US Government Resource
Questions
Is CVE-2021-26855 being exploited?
Yes. CISA added CVE-2021-26855 to its Known Exploited Vulnerabilities catalog on 2021-11-03, with a federal remediation due date of 2022-05-03. CISA lists it as used in ransomware campaigns.
How do I fix CVE-2021-26855?
CISA's required action: Apply updates per vendor instructions.
From NVD, CISA KEV, FIRST EPSS and OSV.dev, checked . Status on NVD: Analyzed. The description is NVD's text, quoted as published.
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