CVE-2021-42278
CVE-2021-42278 is a security flaw in Microsoft Active Directory, the system companies use to manage staff accounts and computers. Someone who already has a foothold can use Microsoft Active Directory to take full control of the machine. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 11 Apr 2022, and says ransomware gangs have used it.[1]
Microsoft Active Directory 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 6.1.7601.25769, 6.0.6003.21282, 6.2.9200.23517 and 6.3.9600.20174.
Description
CISA calls it a domain services privilege escalation in Microsoft Active Directory. In its words: “Microsoft Active Directory Domain Services contains an unspecified vulnerability that allows for privilege escalation.”[1]
The underlying weakness is improper input validation (CWE-20), in MITRE’s list of common software weaknesses.[2]
It scores 7.5 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 |
|---|---|
| Windows Server 2008 R2 Service Pack 1 | 6.1.7601.25769 |
| Windows Server 2008 R2 Service Pack 1 (Server Core installation) | 6.1.7601.25769 |
| Windows Server 2008 Service Pack 2 | 6.0.6003.21282 |
| Windows Server 2008 Service Pack 2 (Server Core installation) | 6.0.6003.21282 |
| Windows Server 2012 | 6.2.9200.23517 |
| Windows Server 2012 (Server Core installation) | 6.2.9200.23517 |
| Windows Server 2012 R2 | 6.3.9600.20174 |
| Windows Server 2012 R2 (Server Core installation) | 6.3.9600.20174 |
Most families won’t have Microsoft Active Directory; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 11 Apr 2022, 152 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 2 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 73%. 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 Active Directory 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 6.1.7601.25769, 6.0.6003.21282, 6.2.9200.23517 and 6.3.9600.20174), or not in use. CISA gave US government agencies until 2 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 7.5.[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 73%.[4] |
See also
Other Microsoft Active Directory flaws
- CVE-2022-26923: domain services privilege escalation ·
- CVE-2021-42287: domain services privilege escalation ·
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 improper input validation flaws
- CVE-2026-93952, Arista VeloCloud Orchestrator: improper input validation ·
- CVE-2026-12569, PTC Windchill and FlexPLM: improper input validation ·
- CVE-2026-34910, Ubiquiti UniFi OS: improper input validation ·
- CVE-2026-6973, Ivanti Endpoint Manager Mobile (EPMM): improper input validation ·
- CVE-2026-34197, Apache ActiveMQ: improper input validation ·
Added the same day
- CVE-2022-23176, WatchGuard Firebox and XTM: privilege escalation ·
- CVE-2021-42287, Microsoft Active Directory: domain services privilege escalation ·
- CVE-2021-39793, Google Pixel: out-of-bounds write ·
- CVE-2021-27852, Checkbox Survey: survey deserialization of untrusted data ·
- CVE-2021-22600, Linux Kernel: privilege escalation ·
Every Microsoft flaw on CISA’s list · Everything added in April 2022
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Active Directory Domain Services Privilege Escalation Vulnerability”. Known Exploited Vulnerabilities Catalog, added 11 Apr 2022.
- ^ National Institute of Standards and Technology. “CVE-2021-42278 Detail”. National Vulnerability Database, published 10 Nov 2021.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com, portal.msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.733, percentile 99.5, 31 Oct 2026.
- CVE Program. “CVE-2021-42278”. 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.