DaystoPatch

MicrosoftExploited incident

Exchange Server OWA cross-site scripting zero-day: mitigation May 14, patch June 9, 2026 (CVE-2026-42897)

CVE-2026-42897High, CVSS 8.1In CISA's Known Exploited Vulnerabilities catalog since 15 May 2026

  • Exploited before a fix
FixedFixed for every supported version 26 days after the first public report.

Response

Silence
0h
First public report to first vendor statement
Exposure
26d
First public report to first fix
Follow-through
26d
First public report to a fix for every supported version
  • Vendor silent
  • Vendor statement, no fix
  • Fixed for some versions
  • Fixed when disclosed
  • Date only, no time of day

Timeline and evidence

Open a moment to see its source and archived quote.

  1. First public report

    CVE-2026-42897 record (CNA: Microsoft)CuratedQuote verified

    "datePublished":"2026-05-14T17:00:36.515Z"

    Disclosed out of band (not on a Patch Tuesday) with 'Exploited: Yes' in Microsoft's CVE entry; no earlier public report was found.

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    6 more sources for this moment
    Microsoft advisory CVE-2026-42897ClaimedQuote not found in archive14 May 2026

    Microsoft Exchange Server Spoofing Vulnerability. Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Exchange Server allows an unauthorized attacker to

    The vendor's own disclosure.

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    Microsoft advisory CVE-2026-42897 vendor advisory
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    CVE.org record CVE-2026-42897ClaimedQuote verified14 May 2026, 17:00 UTC

    Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Exchange Server allows an unauthorized attacker to perform spoofing over a network.

    CVE record published.

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    Bluesky postClaimedQuote not checked yet14 May 2026, 17:01 UTC

    Addressing Exchange Server May 2026 vulnerability CVE-2026-42897 | Microsoft Community Hub! 🦋

    Earliest third-party post we hold.

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    Bluesky post social
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    CISA KEV (CVE-2026-42897)ClaimedQuote verified15 May 2026

    Microsoft Exchange Server Cross-Site Scripting Vulnerability

    Added to CISA's KEV catalog on 2026-05-15.

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    Mastodon postClaimedQuote not checked yet15 May 2026, 07:37 UTC

    Microsoft Exchange Servers Targeted by Active CVE-2026-42897 Exploit Microsoft warns of a high-severity vulnerability, CVE-2026-42897, in its Exchange Servers, allowing attackers to spoof network communications via a cleverly

    Earliest third-party post we hold.

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    Hacker NewsClaimedQuote not checked yet16 May 2026, 20:09 UTC

    https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-42897

    Earliest third-party post we hold. An independent author on another platform posted within 48 hours. Used for unreviewed incidents only, never graded.

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  2. Vendor statement

    at disclosure

    CVE-2026-42897 record (CNA: Microsoft)CuratedQuote verified

    "datePublished":"2026-05-14T17:00:36.515Z"

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    2 more sources for this moment
    Microsoft advisory CVE-2026-42897ClaimedQuote not found in archive14 May 2026

    Microsoft Exchange Server Spoofing Vulnerability. Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Exchange Server allows an unauthorized attacker to

    Source
    Microsoft advisory CVE-2026-42897 vendor advisory
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    Exchange Team blog: Addressing Exchange Server May 2026 vulnerability CVE-2026-42897 (Wayback capture 2026-05-15)CuratedQuote verified14 May 2026

    On May 14, 2026, Microsoft disclosed

    Opening sentence: 'On May 14, 2026, Microsoft disclosed CVE-2026-42897, a reported vulnerability affecting Exchange Outlook Web Access (OWA).' (re-quoted 2026-10-23 so the quote does not cross the CVE link markup).

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  3. First workaround

    date onlysame day

    Exchange Team blog: Addressing Exchange Server May 2026 vulnerability CVE-2026-42897 (Wayback capture 2026-05-15)CuratedQuote verified

    Customers with EM Service enabled can verify that their servers have applied the mitigation for CVE-2026-42897 (the ID of mitigation is M2.1.x) by doing the following:

    Mitigation M2.1.x via the Exchange Emergency Mitigation Service (on by default) or the EOMT script, published with the disclosure on 2026-05-14. The post states 'Microsoft is working on and will release and announce a security update for impacted versions of Exchange Server in the future.'

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  4. First fix

    date only+26d

    Microsoft Support KB5094139 (Exchange SE RTM)CuratedQuote verified

    Description of the security update for Microsoft Exchange Server Subscription Edition RTM: June 09, 2026 (KB5094139)

    The KB lists CVE-2026-42897 among the CVEs it resolves. MSRC CVRF revision 2.0 (2026-06-09): 'Added links to June 2026 Exchange Server security updates.'

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    1 more source for this moment
    Microsoft advisory CVE-2026-42897ClaimedQuote verified14 May 2026

    Information published.

    Vendor Fix listed at the first revision

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    Microsoft advisory CVE-2026-42897 vendor advisory
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  5. Fixed for all versions

    date only+26d

    Microsoft Support KB5094144 (Exchange 2016 CU23)CuratedQuote verified

    Description of the security update for Microsoft Exchange Server 2016 CU23: June 09, 2026 (KB5094144)

    2019 CU15 (KB5094140) and CU14 (KB5094142) are also dated June 09, 2026. The Exchange team said 2016 and 2019 updates go only to Period 2 ESU customers ('Exchange 2016 and 2019 updates will be released only to customers who are enrolled in the Period 2 Exchange Server ESU program'), since those versions are past mainstream support.

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    1 more source for this moment
    Microsoft advisory CVE-2026-42897ClaimedQuote verified14 May 2026

    Information published.

    every Known Affected product has a Vendor Fix and no later revision announces more updates

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    Microsoft advisory CVE-2026-42897 vendor advisory
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Send this to your boss

No brief for this incident yet. We write one for every open incident and every critical bug exploited before a fix.

Affected products

  • Microsoft Exchange Server Subscription Edition RTM
  • Exchange Server 2019 CU14 and CU15 (ESU)
  • Exchange Server 2016 CU23 (ESU)
How we built this timeline

Selected as a patch-day exception: exploited zero-day disclosed out of band with a mitigation only; the security update came 26 days later on June 2026 Patch Tuesday, so 'disclosed and fixed in the same release' would be wrong. R1 decision (correction pass 23 Oct 2026): the June 9, 2026 update is kept as the complete fix.

On 14 Jul 2026 Microsoft added to this CVE's FAQ: 'Mitigations that have been applied either using the Exchange Emergency Mitigation Service or the EOMT script can be removed after July 2026 update is installed.' The reason is a different bug: CVE-2026-55008 (published 14 Jul 2026, CVSS 9.6, 'Exploited:No', not in KEV) is a separate OWA cross-site scripting flaw, and its FAQ says 'Customers who have installed the July 2026 Security Updates on all affected Exchange servers are fully protected and may safely remove the CVE-2026-42897 mitigation (which is similar in nature and mitigates CVE-2026-55008 also).' Microsoft never describes the June update as incomplete for CVE-2026-42897: this CVE's revision 2.0 (9 Jun 2026) 'Added links to June 2026 Exchange Server security updates', its update table still lists the June KBs (5094139/40/42/44), and the June KBs list CVE-2026-42897 as resolved. R7 does not merge CVE-2026-55008: it is not reported as exploited or chained, and was disclosed 61 days later.

Microsoft notes Internet Explorer / IE mode are not protected by the CSP-based mitigation. CVSS 8.1 from Microsoft.

KEV added 15 May 2026.

Related

Microsoft report card