CVE-2021-33766
CVE-2021-33766 is a security flaw in Microsoft Exchange Server, Microsoft’s email server, which companies run for their staff’s mail. Attackers can read information from Microsoft Exchange Server that should be private from anywhere, without logging in. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 18 Jan 2022.[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 15.00.1497.015, 15.01.2176.012, 15.01.2242.008 and 15.02.0792.013 and others.
Description
CISA calls it an information disclosure in Microsoft Exchange Server. In its words: “Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.”[1]
The underlying weakness is improper authentication (CWE-287), in MITRE’s list of common software weaknesses.[2]
It scores 7.3 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 2013 Cumulative Update 23 | 15.00.1497.015 |
| Microsoft Exchange Server 2016 Cumulative Update 19 | 15.01.2176.012 |
| Microsoft Exchange Server 2016 Cumulative Update 20 | 15.01.2242.008 |
| Microsoft Exchange Server 2019 Cumulative Update 8 | 15.02.0792.013 |
| Microsoft Exchange Server 2019 Cumulative Update 9 | 15.02.0858.010 |
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 18 Jan 2022, 188 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 1 Feb 2022 to fix it.[1]
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 98%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
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 15.00.1497.015, 15.01.2176.012, 15.01.2242.008 and 15.02.0792.013 and others), or not in use. CISA gave US government agencies until 1 Feb 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 7.3.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[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 98%.[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 improper authentication flaws
- CVE-2026-105133, AhsayCBS: improper authentication ·
- CVE-2026-42018, JFrog Artifactory: improper authentication ·
- CVE-2026-82329, JFrog Artifactory: improper authentication ·
- CVE-2026-59822, BerriAI LiteLLM: improper authentication ·
- CVE-2023-49105, ownCloud: improper authentication ·
Added the same day
- CVE-2021-40870, Aviatrix Controller: controller unrestricted upload of file ·
- CVE-2021-32648, October CMS: improper authentication ·
- CVE-2021-25298, Nagios XI: XI OS command injection ·
- CVE-2021-25297, Nagios XI: XI OS command injection ·
- CVE-2021-25296, Nagios XI: XI OS command injection ·
Every Microsoft flaw on CISA’s list · Everything added in January 2022
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Exchange Server Information Disclosure”. Known Exploited Vulnerabilities Catalog, added 18 Jan 2022.
- ^ National Institute of Standards and Technology. “CVE-2021-33766 Detail”. National Vulnerability Database, published 14 Jul 2021.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com, portal.msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.982, percentile 99.9, 31 Oct 2026.
- CVE Program. “CVE-2021-33766”. 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.