CVE-2021-31207Microsoft Exchange Server Security Feature Bypass Vulnerability
CVE-2021-31207 is a unrestricted file upload in Microsoft Exchange Server, rated medium (6.6 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
Files of dangerous types can be uploaded, and the server may run or serve them.
How it is reached
An attacker can exploit it over the network, with an administrator account, and nobody has to click anything. Success also depends on conditions the attacker does not fully control.
What an attacker gains
A successful attack can read the data it protects, change data or behaviour and take it offline. In practice, this kind of flaw lets an attacker place a file the server will run or serve to 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: 15.00.1497.018; Microsoft Exchange Server 2019 Cumulative Update 9: 15.02.0858.012; Microsoft Exchange Server 2016 Cumulative Update 20: 15.01.2242.010; Microsoft Exchange Server 2016 Cumulative Update 19: 15.01.2176.014; Microsoft Exchange Server 2019 Cumulative Update 8: 15.02.0792.015. The patch notes are linked below.
- Until it is patched, review who holds accounts that can reach it.
- Check exposed instances for signs of compromise, not only for the version.
Description from NVD
Microsoft Exchange Server Security Feature Bypass 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
- Upload endpoints and the storage behind them. The flaw sits between uploaded content and the servers that store or serve it.
Trust boundaries in play
- Authenticated network access. An account is needed, so exposure follows who can sign in: customers, partners or staff.
- Between uploaded content and the servers that store or serve it. An attacker who crosses it can place a file the server will run or serve to others.
Questions to answer about your own systems
- Where does Microsoft Exchange Server run in your systems, and at which version?
- Which of those instances can be reached from the internet or partner networks?
- 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.