CVE-2021-26857Exploited in the wildNVD status: Analyzed
Microsoft Exchange Server Remote Code Execution
CVE-2021-26857 is a high-severity deserialization of untrusted data vulnerability in Microsoft Exchange Server 2010 Service Pack 3 from Microsoft, with a CVSS 3.1 base score of 7.8 out of 10. It can be exploited with local access to the system by an unauthenticated attacker, but only if a user takes part, for example by opening a link or file. The record lists version 14.03.0513.000 as the first without the flaw, so updating Microsoft Exchange Server 2010 Service Pack 3 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 2010 Service Pack 3 to 14.03.0513.000 or a later version.
- 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.
- CISA's required action: Apply updates per vendor instructions.
- Read the advisory on msrc.microsoft.com for workarounds and exact fixed builds.
- Because the attack needs a user to take part, warn users about unexpected links and files related to this product until it is patched.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Microsoft | Microsoft Exchange Server 2010 Service Pack 3 | 14.0.0.0 up to, but not including, 14.03.0513.000 | 14.03.0513.000 |
| 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 2013 Service Pack 1 | 15.00.0 up to, but not including, 15.00.0847.064 | 15.00.0847.064 |
| 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 |
Version data comes from the organisation that assigned the CVE; 2 further products are omitted here. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- LocalThe attacker needs local access or must trick a user into running something.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- RequiredA user must take an action such as opening a file or link.
- 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:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 7.8 | High | CNA (microsoft.com) |
| CVSS 3.1 | 7.8 | High | NVD |
| CVSS 2.0 | 6.8 | Medium | 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: Deserialization of Untrusted Data
The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.
How developers prevent this class of flaw
- If available, use the signing/sealing features of the programming language to assure that deserialized data has not been tainted. For example, a hash-based message authentication code (HMAC) could be used to ensure that data has not been modified.
- When deserializing data, populate a new object rather than just deserializing. The result is that the data flows through safe input validation and that the functions are safe.
- Explicitly define a final object() to prevent deserialization.
More Deserialization of Untrusted Data vulnerabilities (CWE-502)
Timeline
- CISA published its SSVC assessment (exploitation: active, automatable: no, 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-2021-26857US Government Resource
Recent Deserialization of Untrusted Data vulnerabilities
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