CVE-2021-26857
CVE-2021-26857 is a security flaw in Microsoft Exchange Server, Microsoft’s email server, which companies run for their staff’s mail. Attackers can run their own code on Microsoft Exchange Server. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 3 Nov 2021, and says ransomware gangs have used it.[1]
Microsoft Exchange Server is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 14.03.0513.000, 15.00.1395.012, 15.00.1473.006 and 15.00.1497.012 and others.
Description
CISA calls it a remote code execution in Microsoft Exchange Server. In its words: “Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.”[1]
The underlying weakness is deserialization of untrusted data (CWE-502), in MITRE’s list of common software weaknesses.[2]
It scores 7.8 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Microsoft Exchange Server 2010 Service Pack 3 | 14.03.0513.000 |
| Microsoft Exchange Server 2013 Cumulative Update 21 | 15.00.1395.012 |
| Microsoft Exchange Server 2013 Cumulative Update 22 | 15.00.1473.006 |
| Microsoft Exchange Server 2013 Cumulative Update 23 | 15.00.1497.012 |
| Microsoft Exchange Server 2013 Service Pack 1 | 15.00.0847.064 |
| Microsoft Exchange Server 2016 Cumulative Update 10 | 15.01.1531.012 |
| Microsoft Exchange Server 2016 Cumulative Update 11 | 15.01.1591.018 |
| Microsoft Exchange Server 2016 Cumulative Update 12 | 15.01.1713.010 |
Most families won’t have Microsoft Exchange Server; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 245 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 3 May 2022 to fix it.[1] CISA says ransomware gangs have used it.
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 96%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Microsoft has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.Microsoft Exchange Server is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 14.03.0513.000, 15.00.1395.012, 15.00.1473.006 and 15.00.1497.012 and others), or not in use. CISA gave US government agencies until 3 May 2022 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 7.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks, noting that ransomware gangs have used it.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 96%.[4] |
See also
Other Microsoft Exchange Server flaws
- CVE-2023-21529: deserialization of untrusted data ·
- CVE-2021-31196: information disclosure ·
- CVE-2024-21410: privilege escalation ·
- CVE-2022-41080: privilege escalation ·
- CVE-2022-41082: remote code execution ·
- CVE-2022-41040: server-side request forgery ·
Other Microsoft flaws
- CVE-2026-96951, Microsoft Windows: command injection ·
- CVE-2026-65660, Microsoft SharePoint: code injection ·
- CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
- CVE-2026-81963, Microsoft Windows: link following ·
- CVE-2019-1068, Microsoft SQL Server: remote code execution ·
Other deserialization of untrusted data flaws
- CVE-2021-23758, Ajax.NET Professional: deserialization of untrusted data ·
- CVE-2026-63077, JetBrains TeamCity: deserialization of untrusted data ·
- CVE-2026-45247, Mirasvit Full Page Cache Warmer: full page cache warmer deserialization of untrusted data ·
- CVE-2026-20131, Cisco Secure Firewall Management Center (FMC): software and cisco security cloud control (SCC) firewall management deserialization of untrusted data ·
- CVE-2025-26399, SolarWinds Web Help Desk: deserialization of untrusted data ·
Added the same day
- CVE-2021-42258, BQE BillQuick Web Suite: SQL injection ·
- CVE-2021-42013, Apache HTTP Server: path traversal ·
- CVE-2021-41773, Apache HTTP Server: path traversal ·
- CVE-2021-40539, Zoho ManageEngine: aDSelfService plus authentication bypass ·
- CVE-2021-40444, Microsoft MSHTML: remote code execution ·
Every Microsoft flaw on CISA’s list · Everything added in November 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Exchange Server Remote Code Execution Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2021-26857 Detail”. National Vulnerability Database, published 3 Mar 2021.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com, portal.msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.958, percentile 99.9, 31 Oct 2026.
- CVE Program. “CVE-2021-26857”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.