CVE-2021-1730Network attack vector

User impersonation via same-origin inline images

Published Feb 25, 2021 · Updated Aug 3, 2024

Spoofing in OWA in Microsoft Exchange Server 2016 CU18 and 2019 CU7 allows remote attackers to impersonate users via inline images. OWA serves inline-message attachments from its authenticated origin, so the browser can attach OWA cookies to requests for attacker-controlled message content. A victim must view a crafted message in OWA; exposure persists when Download Domains are disabled or reuse an OWA hostname.

CVSS severity5.4
Medium
EPSS probability1.82%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Spoofing in OWA in Microsoft Exchange Server 2016 CU18 and 2019 CU7 allows remote attackers to impersonate users via inline images. OWA serves inline-message attachments from its authenticated origin, so the browser can attach OWA cookies to requests for attacker-controlled message content. A victim must view a crafted message in OWA; exposure persists when Download Domains are disabled or reuse an OWA hostname.

The record

CVE
CVE-2021-1730
Published
Feb 25, 2021
Updated
Aug 3, 2024
Vendor
Microsoft
Product
Microsoft Exchange Server 2016 Cumulative Update 18
Classifications
Unavailable
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 25, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=15.01.0 <publication

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2016 Cumulative Update 18version=15.01.0 <publication
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 7version=15.02.0 <publication

Published CVSS scores

5.4MicrosoftCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
5.4NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
5.8NVDAV:N/AC:M/Au:N/C:P/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

No sourced classifications are available.

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo