CVE-2021-26855Exploited in the wildNVD status: Analyzed
Microsoft Exchange Server Remote Code Execution
CVE-2021-26855 is a critical-severity server-side request forgery vulnerability in Microsoft Exchange Server 2013 Cumulative Update 21 from Microsoft, with a CVSS 3.1 base score of 9.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists version 15.00.1395.012 as the first without the flaw, so updating Microsoft Exchange Server 2013 Cumulative Update 21 to that release or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on November 3, 2021, which means attacks have been observed in the wild, including in ransomware campaigns.
Patch now
Attackers are already exploiting this vulnerability, according to CISA.
What to do
- Update Microsoft Exchange Server 2013 Cumulative Update 21 to 15.00.1395.012 or a later version.
- Update Microsoft Exchange Server 2013 Cumulative Update 22 to 15.00.1473.006 or a later version.
- Update Microsoft Exchange Server 2013 Cumulative Update 23 to 15.00.1497.012 or a later version.
- CISA's required action: Apply updates per vendor instructions.
- Read the advisory on msrc.microsoft.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 21 | 15.00.0 up to, but not including, 15.00.1395.012 | 15.00.1395.012 |
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 22 | 15.00.0 up to, but not including, 15.00.1473.006 | 15.00.1473.006 |
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 23 | 15.00.0 up to, but not including, 15.00.1497.012 | 15.00.1497.012 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 10 | 15.01.0 up to, but not including, 15.01.1531.012 | 15.01.1531.012 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 11 | 15.01.0 up to, but not including, 15.01.1591.018 | 15.01.1591.018 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 12 | 15.01.0 up to, but not including, 15.01.1713.010 | 15.01.1713.010 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 13 | 15.01.0 up to, but not including, 15.01.1779.008 | 15.01.1779.008 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 14 | 15.01.0 up to, but not including, 15.01.1847.012 | 15.01.1847.012 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 15 | 15.01.0 up to, but not including, 15.01.1913.012 | 15.01.1913.012 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 16 | 15.01.0 up to, but not including, 15.01.1979.008 | 15.01.1979.008 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 17 | 15.01.0 up to, but not including, 15.01.2044.013 | 15.01.2044.013 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 18 | 15.01.0 up to, but not including, 15.01.2106.013 | 15.01.2106.013 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 19 | 15.01.0 up to, but not including, 15.01.2176.009 | 15.01.2176.009 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 8 | 15.01.0 up to, but not including, 15.01.1415.010 | 15.01.1415.010 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 9 | 15.01.0 up to, but not including, 15.01.1466.016 | 15.01.1466.016 |
| Microsoft | Microsoft Exchange Server 2019 | 15.02.0 up to, but not including, 15.02.0221.018 | 15.02.0221.018 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 1 | 15.02.0 up to, but not including, 15.01.1979.006 | 15.01.1979.006 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 2 | 15.02.0 up to, but not including, 15.02.0397.011 | 15.02.0397.011 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 3 | 15.02.0 up to, but not including, 15.02.0464.015 | 15.02.0464.015 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 4 | 15.02.0 up to, but not including, 15.02.0659.012 | 15.02.0659.012 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 5 | 15.02.0 up to, but not including, 15.02.0595.008 | 15.02.0595.008 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 6 | 15.02.0 up to, but not including, 15.02.0529.013 | 15.02.0529.013 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 7 | 15.02.0 up to, but not including, 15.02.0721.013 | 15.02.0721.013 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 8 | 15.02.0 up to, but not including, 15.02.0792.010 | 15.02.0792.010 |
| Microsoft | Exchange Server | 2013; 2016; 2019 | Not stated |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 9.1 | Critical | CNA (microsoft.com) |
| CVSS 3.1 | 9.8 | Critical | NVD |
| CVSS 2.0 | 7.5 | High | NVD |
Official description
Microsoft Exchange Server Remote Code Execution Vulnerability
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Server-Side Request Forgery
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.
How developers prevent this class of flaw
- If there is a known list of applications, IP, hostname, and/or domains to which the product is allowed to communicate, then use allowlists to ensure that outgoing requests are restricted to these known entities. The allowlists could be in the application code or possibly enforced by firewalls or network segmentation.
- If the product is expected to be able to connect to arbitrary IP addresses or domains, consider using a denylist that includes private network addresses, internal domain names, etc. The denylist could be in the application code or possibly enforced by firewalls or network segmentation.
Timeline
- CISA published its SSVC assessment (exploitation: active, automatable: yes, technical impact: total).
- CVE record published.
- Added to CISA's Known Exploited Vulnerabilities catalogue.
- Deadline for US federal agencies to remediate, set by CISA.
- Record last changed materially (score, affected products, status or references).
References
Advisories
Third-party and government advisories
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855US Government Resource
Technical write-ups (may include proof of concept)
- packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.htmlExploit, Third Party Advisory, VDB Entry
- packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.htmlExploit, Third Party Advisory, VDB Entry
- packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.htmlExploit, Third Party Advisory, VDB Entry
- packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.htmlExploit, Third Party Advisory, VDB Entry
Other vulnerabilities in Microsoft Exchange Server 2013 Cumulative Update 21
- 7.8 High severity
- 7.8 High severity
- 7.8 High severity
Recent Server-Side Request Forgery vulnerabilities
- 5.3 Medium severity
- 9.8 Critical severity
- 7.2 High severity
Adminer Server-Side Request Forgery Exploited
- 9.8 Critical severity
- 7.5 High severity
- 5.5 Medium severity
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