KEV Intelligence

Vulnerability report

Active exploitation observed Confirmed confidence Not in CISA KEV

CVE-2026-55040

Microsoft SharePoint Server Security Feature Bypass Vulnerability

Microsoft / Microsoft SharePoint Enterprise Server 2016 · affected before 16.0.5561.1001

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Severity
CVSS 9.1 · Critical
Confidence
Confirmed
Exploit status
Observed in sensors
EPSS
1.6%
First observed
19 Jul 2026
Last observed
13 Aug 2026

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2026-55040 is an unauthenticated Microsoft SharePoint Server Security Feature Bypass Vulnerability affecting Microsoft Microsoft SharePoint Enterprise Server 2016 and 2 other products. Weak authentication in...

Is it exploited?

Yes. KEV Intelligence sensors observed exploitation attempts with confirmed confidence.

Who is affected?

Microsoft / Microsoft SharePoint Enterprise Server 2016 affected before 16.0.5561.1001.

What should we do?

Patch immediately, validate internet-facing exposure, and monitor for matching requests.

Overview

Microsoft SharePoint Server Security Feature Bypass Vulnerability

Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.

Vendor Product Affected Status
Microsoft Microsoft SharePoint Enterprise Server 2016 Before 16.0.5561.1001 Affected
Microsoft Microsoft SharePoint Server 2019 Before 16.0.10417.20175 Affected
Microsoft Microsoft SharePoint Server Subscription Edition Before 16.0.19725.20434 Affected
View vendor advisory (opens in new tab)
Published
14 Jul 2026
Exploitation Reported
19 Jul 2026
Attack vector
Remote
Complexity
Low
Privileges
None
User interaction
None

Tags

microsoft

CVE References

Exploitation evidence

Why KEV Intelligence marks this CVE as exploited

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploited in the wild

KEV Intelligence

Recorded 19 Jul 2026

Independent exploitation attestation added to the KEV Intelligence record.

Active exploitation observed

KEV Intelligence sensor

First observed 19 Jul 2026

First-party sensor telemetry confirms matching exploitation attempts.

Proof of concept available

GitHub

Recorded 06 Aug 2026

Public scanner or PoC coverage increases practical exploitability.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the KEV Intelligence Pro API.

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Source Added
KEV Intelligence First 2026-08-12 19:37 UTC

Operational indicators for this CVE are listed under Detection.

Sensor telemetry

First-party evidence of exploitation activity

Aggregate observations show the scale, recency, and distribution of activity without overstating sparse data.

13

Attempts observed

9

Unique attacker IPs

5

Attacker countries

HK · JP · NL · TW · US

4

Sensors observed

Exploitation attempts over the last 27 days

Daily events observed by KEV Intelligence sensors

Updated 13 Aug 2026

0
1
0
0
0
1
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
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1
5
4
18 Jul 24 Jul 30 Jul 5 Aug 13 Aug

First observed 19 Jul 2026 · Last observed 13 Aug 2026

Pro adds sensor region and window summaries. Enterprise adds raw IPs, paths, User-Agents, and payloads.

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Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Observed signals

Request targets
4
User-Agents
7

Raw values available in Pro and Enterprise.

Scanner coverage

No scanner integrations recorded yet.

Virtual patch status

No KEV Intelligence virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

Attacker IP Indicators

Attacker IP indicators observed · available in Pro and Enterprise.

Sensor-derived attacker IP indicators are available to Pro and Enterprise accounts under Detection and through the Pro API.

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Risk and context

Severity, weaknesses, and research context

CVSS v3.1

9.1 Critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:U/RL:O/RC:C

EPSS

1.6%

Weaknesses

Weak Authentication CWE-1390

Recent mention · TheHackerNews

Attackers Exploit SharePoint Authentication Bypass After Public PoC Release

Threat actors have begun to exploit a newly disclosed Microsoft SharePoint vulnerability following the release of a proof-of-concept (PoC) code. The vulnerability in question is CVE-2026-55040 (CVSS score: 9.1), which refers to a critical security feature bypass that stems...

Read full advisory

All Mentions

Recent mention · TheHackerNews

Attackers Exploit SharePoint Authentication Bypass After Public PoC Release

TheHackerNews · 13 Aug 2026

Threat actors have begun to exploit a newly disclosed Microsoft SharePoint vulnerability following the release of a proof-of-concept (PoC) code. The vulnerability in question is CVE-2026-55040 (CVSS score: 9.1), which refers to a critical security feature bypass that stems from weak authentication. It was patched by Microsoft as part of its July 2026 Patch Tuesday updates. "The authentication

Recent mention · Rapid7

Patch Tuesday - August 2026

Rapid7 · 11 Aug 2026

Microsoft is publishing 421 vulnerabilities on August 2026 Patch Tuesday, including 236 vulnerabilities in Windows. This is lower volume than last month’s record-breaking behemoth, but still one of the largest Patch Tuesday totals ever. There is no reason to suppose that Patch Tuesday will ever return to the lower volumes we saw prior to 2026. Microsoft is aware of exploitation in the wild for one of the vulnerabilities published today, as well as public disclosure for two others, although the Notable CVEs section of the Security Update Guide omits one of these. As usual, browser vulns are...

Recent mention · TheHackerNews

Researchers Disclose AI-Assisted SharePoint Exploit Chain Reaching Unauthenticated RCE

TheHackerNews · 11 Aug 2026

Security researchers found a way to enter Microsoft SharePoint servers as any user, including an administrator, with no valid account. A significant part of the work that found it was done through an AI agent. The flaw, tracked as CVE-2026-55040 (CVSS 9.1), affects SharePoint Server Subscription Edition, SharePoint Server 2019, and SharePoint Server 2016. Microsoft's

Recent mention · Rapid7

CVE-2026-63520: Microsoft SharePoint Remote Code Execution (FIXED)

Rapid7 · 11 Aug 2026

OverviewRapid7 Labs conducted a zero-day research project against Microsoft SharePoint, resulting in the discovery of two new vulnerabilities that, when chained together, achieve unauthenticated remote code execution (RCE) against a vulnerable SharePoint server. Today, both Rapid7 and Microsoft are disclosing the second vulnerability in this chain, the RCE vulnerability CVE-2026-63520. The first vulnerability in the chain, CVE-2026-55040, was disclosed by Rapid7 and Microsoft last month.Our full disclosure timeline for the exploit chain can be seen below in Figure 1.Figure 1: The road to...

Recent mention · Rapid7

Rapid7 Analysis: Microsoft SharePoint JWT Token Authentication Bypass (CVE-2026-55040)

Rapid7 · 11 Aug 2026

OverviewOn July 14, 2026, Rapid7 and Microsoft disclosed CVE-2026-55040, an authentication bypass vulnerability affecting Microsoft SharePoint. Today we are publishing a technical analysis of the vulnerability along with an accompanying proof-of-concept (PoC) script.Figure 1: The Rapid7 Labs PoC for CVE-2026-55040.⠀A remote unauthenticated attacker can leverage CVE-2026-55040 to bypass authentication on a vulnerable SharePoint server, and perform operations as a SharePoint site user or administrator. The vulnerability is due to several issues in the JWT token validation pipeline.AnalysisThe...

Recent mention · Tenable Blog

CVE-2026-32201, CVE-2026-45659, CVE-2026-56164: Frequently Asked Questions About Active Exploitation of Microsoft SharePoint Server Vulnerabilities

Tenable Blog · 16 Jul 2026

Four Microsoft SharePoint Server vulnerabilities are under active exploitation, prompting CISA to issue a hardening alert. An additional high-severity flaw recently patched adds pressure for organizations running on-premises deployments.Key TakeawaysCISA confirmed active exploitation of three on-premises SharePoint Server vulnerabilities (CVE-2026-32201, CVE-2026-45659, CVE-2026-56164), used to gain unauthorized access, establish remote code execution, steal IIS machine keys and deploy malware for persistence.Two additional SharePoint Server vulnerabilities disclosed on July 14, 2026,...

Recent mention · Rapid7

Patch Tuesday - July 2026

Rapid7 · 14 Jul 2026

Microsoft is publishing 622 vulnerabilities on July 2026 Patch Tuesday, including a record-breaking 416 Windows vulnerabilities. Microsoft is aware of exploitation in the wild for two of the vulnerabilities published today, both of which are listed on CISA KEV, as well as public disclosure for one other. As usual, browser vulns are not included in the Patch Tuesday count above. Rapid7 noted last month that Microsoft no longer enumerates Chromium CVEs in the Security Update Guide. However, Microsoft has now taken the pursuit of minimalism much further, since today’s Security Update Guide no...

Recent mention · Rapid7

CVE-2026-55040: Microsoft SharePoint JWT Token Authentication Bypass (FIXED)

Rapid7 · 14 Jul 2026

OverviewRapid7 Labs conducted a zero-day research project against Microsoft SharePoint, resulting in the discovery of two new vulnerabilities that, when chained together, achieve unauthenticated remote code execution (RCE) against a vulnerable SharePoint server. Today, both Rapid7 and Microsoft are disclosing the first vulnerability in this chain, the authentication bypass vulnerability CVE-2026-55040. The RCE component of the exploit chain is expected to be patched by Microsoft in the next update cycle for August 2026. The exploit chain was developed as an entry for the recent Pwn2Own...

Recent mention · All CISA Advisories

CISA Urges SharePoint Hardening After New Exploitations

All CISA Advisories · 14 Jul 2026

CISA is aware of active exploitation of vulnerabilities CVE-2026-32201, CVE-2026-45659, and CVE-2026-56164, enabling cyber threat actors to gain unauthorized access to on-premises SharePoint Server instances. These vulnerabilities affect all supported on-premises SharePoint Server versions (Subscription Edition, 2019, and 2016) and involve establishing remote code execution (RCE) and post-exploitation activities, such as stealing Internet Information Services (IIS) machine keys and performing deserialization techniques, to gain persistence and deploy malware. Organizations should monitor...

Potential Proof of Concepts

These PoCs are unverified and could contain malware. Use at your own risk.

sfewer-r7/CVE-2026-55040

github · Created 2026-08-06 11:46:34 UTC · 14 stars

Microsoft SharePoint JWT Authentication Bypass (CVE-2026-55040)

l0ggg/CVE-2026-55040

github · Created 2026-07-28 08:46:15 UTC · 0 stars

Exploit code for CVE-2026-55040, it can create auth header for any validate account.

Timeline

From disclosure to observed exploitation

  1. Public PoC available

    Public proof-of-concept code published

  2. Observed by KEV Intelligence sensors

    Evidence-backed exploitation signal

  3. Indicators of compromise added (9)

    Indicators of compromise recorded

  4. CVE published

    Vulnerability disclosed publicly

  5. CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2026-55040

Free JSON includes basic KEV fields 
{
  "cve_id": "CVE-2026-55040",
  "confidence": "Confirmed",
  "cvss_score": 9.1,
  "epss_score": 0.01626,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": true
  },
  "sensor_telemetry": { "attempts": 13, "sensors": 4 }
}

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