CVE-2021-34523Microsoft Exchange Server Privilege Escalation Vulnerability
CVE-2021-34523 is a vulnerability in Microsoft Exchange Server, rated critical (9.0 on CVSS 3.1).
Why this priority: Exploited in the wild: listed in CISA's KEV catalog.
Published · Updated · NVD status: Analyzed
What it is
The record does not classify the weakness yet; the description below says what the reporter found.
How it is reached
An attacker can exploit it by someone who can already run code or sign in on the machine, without an account, and nobody has to click anything.
What an attacker gains
A successful attack can read the data it protects and change data or behaviour. The damage can spread past the vulnerable component into others.
Is it being exploited?
CISA added it to the Known Exploited Vulnerabilities catalog on 2021-11-03: it is being exploited in the wild. CISA links it to ransomware campaigns. EPSS estimates a 100% chance of exploitation activity in the next 30 days, higher than 100% of scored CVEs.
What to do
- Treat it as urgent: it is on CISA's KEV list. CISA's deadline for US federal agencies: 2021-11-17.
- Apply the update for your edition of Microsoft Exchange Server. Fixed builds: 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. The patch notes are linked below.
- Limit who can run code or sign in on the hosts that run it.
- Check exposed instances for signs of compromise, not only for the version.
Description from NVD
Microsoft Exchange Server Elevation of Privilege Vulnerability
What this means for your architecture.
A CVE names a product. Whether it matters depends on where that product sits in your systems: what reaches it, which trust boundary it guards and what it can reach once compromised. Veriom maps your components, their relationships and your trust boundaries from your code, cloud and delivery evidence, so you can see where a component like this sits. The radar itself does not look at your systems.
Components it touches
- Microsoft Exchange Server. Start from where it runs and what it is connected to.
Trust boundaries in play
- Host boundary. The attacker needs a foothold on the machine first; the flaw turns a small foothold into a bigger one.
- Isolation between components. The impact crosses out of the vulnerable component into others it should be isolated from.
Questions to answer about your own systems
- Where does Microsoft Exchange Server run in your systems, and at which version?
- Which of those hosts also run code or users you do not fully trust?
- What can it reach once compromised: which data stores, secrets and services?
Read more
Patches and commits
Advisories
Exploits and proofs of concept
Sources
- CVE records and CVSS scores from the NVD CVE API. This product uses the NVD API but is not endorsed or certified by the NVD.
- Exploitation status from CISA’s Known Exploited Vulnerabilities catalog.
- Exploit likelihood from FIRST’s Exploit Prediction Scoring System (EPSS), refreshed daily.
- Explanations are written by Veriom from fixed templates over each record’s data, not by a person reviewing each CVE. Descriptions are quoted from the records as published.