Microsoft Exchange Server up to 2019 CU8 deserialization

CVSS Meta Temp Score
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Current Exploit Price (≈)
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CTI Interest Score
Our Cyber Threat Intelligence team is monitoring different web sites, mailing lists, exploit markets and social media networks. The CTI Interest Score identifies the interest of attackers and the security community for this specific vulnerability in real-time. A high score indicates an elevated risk to be targeted for this vulnerability.
7.2$0-$5k0.00

Summaryinfo

A vulnerability has been found in Microsoft Exchange Server 2013 CU23/2016 CU18/2016 CU19/2019 CU7/2019 CU8 and classified as critical. Impacted is an unknown function. This manipulation causes an unknown weakness. This vulnerability appears as CVE-2021-26857. The attack may be initiated remotely. In addition, an exploit is available. It is suggested to install a patch to address this issue.

Detailsinfo

A vulnerability was found in Microsoft Exchange Server 2013 CU23/2016 CU18/2016 CU19/2019 CU7/2019 CU8 (Groupware Software). It has been declared as critical. The CWE definition for the vulnerability is CWE-502. The product deserializes untrusted data without sufficiently verifying that the resulting data will be valid. As an impact it is known to affect confidentiality, integrity, and availability.

The weakness was presented 03/02/2021 as confirmed security guidance (Website). The advisory is shared at portal.msrc.microsoft.com. This vulnerability is known as CVE-2021-26857. It demands that the victim is doing some kind of user interaction. Technical details are unknown but an exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 12/17/2025).

It is declared as attacked. The CISA Known Exploited Vulnerabilities Catalog lists this issue since 11/03/2021 with a due date of 05/03/2022:

Apply updates per vendor instructions.

Applying a patch is able to eliminate this problem. A possible mitigation has been published immediately after the disclosure of the vulnerability.

The vulnerability is also documented in the databases at Zero-Day.cz (611) and EUVD (EUVD-2021-13641). Additional details are provided at techcommunity.microsoft.com. Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Type

Vendor

Name

Version

License

Website

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔍
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 7.4
VulDB Meta Temp Score: 7.2

VulDB Base Score: 6.3
VulDB Temp Score: 6.0
VulDB Vector: 🔍
VulDB Reliability: 🔍

Vendor Base Score (Microsoft): 7.8
Vendor Vector (Microsoft): 🔍

NVD Base Score: 7.8
NVD Vector: 🔍

CNA Base Score: 7.8
CNA Vector: 🔍

CVSSv2info

AVACAuCIA
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VectorComplexityAuthenticationConfidentialityIntegrityAvailability
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VulDB Base Score: 🔍
VulDB Temp Score: 🔍
VulDB Reliability: 🔍

NVD Base Score: 🔍

Exploitinginfo

Class: Deserialization
CWE: CWE-502 / CWE-20
CAPEC: 🔍
ATT&CK: 🔍

Physical: Partially
Local: Yes
Remote: Yes

Availability: 🔍
Status: Attacked

EPSS Score: 🔍
EPSS Percentile: 🔍

KEV Added: 🔍
KEV Due: 🔍
KEV Remediation: 🔍
KEV Ransomware: 🔍
KEV Notice: 🔍

Price Prediction: 🔍
Current Price Estimation: 🔍

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Zero-Day.cz: 🔍

Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Patch
Status: 🔍

Reaction Time: 🔍
0-Day Time: 🔍
Exposure Time: 🔍

Timelineinfo

02/08/2021 🔍
03/02/2021 +22 days 🔍
03/02/2021 +0 days 🔍
03/03/2021 +0 days 🔍
12/17/2025 +1750 days 🔍

Sourcesinfo

Vendor: microsoft.com

Advisory: portal.msrc.microsoft.com
Status: Confirmed

CVE: CVE-2021-26857 (🔍)
GCVE (CVE): GCVE-0-2021-26857
GCVE (VulDB): GCVE-100-170593
EUVD: 🔍
scip Labs: https://www.scip.ch/en/?labs.20161013
Misc.: 🔍

Entryinfo

Created: 03/03/2021 08:23
Updated: 12/17/2025 19:09
Changes: 03/03/2021 08:23 (29), 03/03/2021 08:27 (23), 03/03/2021 12:57 (1), 03/06/2021 12:21 (2), 03/14/2021 12:07 (18), 04/21/2024 11:54 (30), 06/29/2024 23:18 (2), 07/08/2024 19:21 (1), 07/25/2024 20:16 (14), 09/12/2024 23:36 (1), 05/04/2025 05:38 (3), 07/29/2025 01:29 (1), 10/30/2025 21:18 (1), 12/17/2025 19:09 (1)
Complete: 🔍
Cache ID: 216::103

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Discussion

 Hardcoders (+0)
5 years ago
https://techcommunity.microsoft.com/t5/exchange-team-blog/released-march-2021-exchange-server-security-updates/ba-p/2175901
Windows server 2010 is also affected

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