CVE-2026-21577

Executive Summary

This report details CVE-2026-21577, a High-severity Denial of Service (DoS) vulnerability affecting Atlassian Confluence Data Center. The vulnerability, reported through Atlassian's internal penetration testing program, allows an authenticated attacker to disrupt services of a host connected to a network, rendering the Confluence instance unavailable to legitimate users.

Business Impact: Organizations relying on Confluence Data Center for internal documentation, project management, and knowledge sharing are at risk. A successful attack can halt business operations, prevent access to critical documentation, and disrupt development workflows. The likely attacker is an authenticated user with low privileges, potentially a disgruntled insider or an external attacker who has compromised a low-privilege account.

Vulnerability Class: This is a resource exhaustion vulnerability (CWE-400) that is network-accessible but requires authentication. The attack vector is network-based, with low attack complexity.

Urgency Assessment: Despite a CVSS score of 7.1 (High) and an EPSS probability of 0.0%, the vulnerability should be treated with high priority due to the widespread use of Confluence Data Center and the potential for significant business disruption. The 0.0% EPSS score indicates no observed exploitation in the wild as of the publication date, but this is likely due to the vulnerability being newly disclosed.

Key Facts:

• Vendor: Atlassian
• Affected Software: Confluence Data Center (versions 9.0.1, 9.1.0, 9.2.0, 9.3.1, 9.4.0, 9.5.1, 10.0.2, 10.1.0, 10.2.0)
• Patch Availability: Yes, fixed in versions 9.2.17 and 10.2.7.
• CISA KEV: Not listed.

CVSS Breakdown & Threat Level

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Qualitative Severity Label: High

EPSS Interpretation: The EPSS score of 0.0% indicates that as of the report date, there is no evidence of exploitation in the wild. This is typical for a newly published CVE and does not reflect the potential for future exploitation.

Historical Comparison: This vulnerability is similar to other DoS vulnerabilities in Confluence, such as CVE-2022-26138 (a hardcoded password vulnerability that could lead to DoS) and CVE-2023-22515 (a broken access control vulnerability that could lead to system takeover). All share the common theme of affecting Confluence Data Center and requiring authentication.

Temporal & Environmental Considerations: The availability of a patch (9.2.17 and 10.2.7) reduces the temporal score. Organizations should prioritize patching. The environmental score may be higher for organizations with Confluence instances exposed to the internet or with a large number of low-privilege users.

Technical Deep Dive & Attack Mechanics

Root Cause Analysis: The vulnerability is a resource exhaustion flaw (CWE-400) likely stemming from improper handling of a specific API request or input. An authenticated attacker can send a crafted request that causes the Confluence server to allocate excessive resources (CPU, memory, or file handles), leading to a denial of service.

Step-by-Step Technical Explanation:

1. Authentication: The attacker authenticates to the Confluence instance with a valid low-privilege account.
2. Crafted Request: The attacker sends a specially crafted HTTP request to a vulnerable endpoint. The exact endpoint is not publicly disclosed, but it likely involves a feature that performs resource-intensive operations (e.g., search, export, or rendering).
3. Resource Exhaustion: The Confluence server processes the request without proper limits, consuming all available resources (e.g., memory, CPU threads, database connections).
4. Denial of Service: The server becomes unresponsive to legitimate requests, effectively taking the Confluence instance offline.

Attack Surface:

• Interface: HTTP/HTTPS web interface (typically port 8090 or 443).
• Protocol: HTTP/HTTPS.
• Endpoint: A specific API or web endpoint that is accessible to authenticated users.

Pre-conditions:

• Network access to the Confluence instance.
• Valid user credentials with low-level privileges (e.g., "confluence-users" group).

Attack Variants: The attacker could automate the sending of multiple requests to amplify the DoS effect.

Why Standard Defenses May Fail:

• WAF: A Web Application Firewall may not detect the malicious request as it appears to be a legitimate, authenticated API call.
• Rate Limiting: If the vulnerability is triggered by a single request, standard rate limiting will not prevent it.
• Authentication: The attacker is authenticated, so the request passes standard access controls.

Vulnerable vs. Patched Code Analysis

Note: Specific code details are not publicly available. The following is a hypothetical analysis based on the vulnerability class.

Vulnerable Code (Hypothetical):

// Vulnerable endpoint that processes user input without limits
@POST
@Path("/export")
public Response exportData(@QueryParam("format") String format, @QueryParam("pages") String pages) {
    // Directly uses user input to create a resource-intensive operation
    ExportTask task = new ExportTask(pages);
    task.run(); // No timeout or resource limit
    return Response.ok(task.getResult()).build();
}

Patched Code (Hypothetical):

// Patched endpoint with resource limits
@POST
@Path("/export")
public Response exportData(@QueryParam("format") String format, @QueryParam("pages") String pages) {
    // Validate and limit the number of pages
    if (pages.split(",").length > MAX_PAGES) {
        return Response.status(400).entity("Too many pages requested").build();
    }
    ExportTask task = new ExportTask(pages);
    // Execute with a timeout
    ExecutorService executor = Executors.newSingleThreadExecutor();
    Future<ExportResult> future = executor.submit(task);
    try {
        ExportResult result = future.get(30, TimeUnit.SECONDS);
        return Response.ok(result).build();
    } catch (TimeoutException e) {
        future.cancel(true);
        return Response.status(503).entity("Request timed out").build();
    }
}

What the Patch Changes: The patch likely introduces input validation, resource limits (e.g., maximum number of items to process), and timeouts to prevent a single request from consuming all server resources.

Regression Considerations: Teams applying the patch should test that legitimate large exports or operations still function correctly, albeit with new limits.

Timeline & Disclosure History

• CVE Publication Date: 2026-07-21T18:16:57.327
• Estimated Discovery: The vulnerability was discovered during Atlassian's internal Penetration Testing program, suggesting it was found by a security researcher or internal team.
• Disclosure Process: The vulnerability was reported to Atlassian, who coordinated the fix and public disclosure.
• Known Exploitation in the Wild: No known active exploitation as of 2026-07-21.
• CISA KEV: Not listed.

Threat Intelligence Context

Threat Actor Profile: This vulnerability is most likely to be exploited by:

• Insider Threats: Disgruntled employees or contractors with low-level access.
• Script Kiddies: Individuals using automated tools to disrupt services.
• Ransomware Operators: As a precursor to a ransomware attack, to disrupt backups or monitoring.

Attack Campaign Context: No specific campaigns are associated with this CVE at this time.

Geographic Targeting: No specific geographic targeting is expected.

Industry Verticals Most at Risk: All industries using Confluence Data Center are at risk, with particular impact on technology, finance, and government sectors where Confluence is a critical part of daily operations.

Connection to Similar Vulnerabilities: This DoS vulnerability is similar to CVE-2023-22518 (Improper Authorization leading to DoS) and CVE-2024-21677 (Resource Exhaustion in Confluence).

Exploitation & Lab Setup

Red Team Section

Lab Setup:

1. Environment: Docker or a virtual machine running a vulnerable version of Confluence Data Center (e.g., 10.2.0).
2. Prerequisites:
   • Docker installed.
   • A vulnerable Confluence Data Center image (e.g., atlassian/confluence-server:10.2.0).
   • A valid user account on the Confluence instance.
3. Setup Steps:

   docker run -d -p 8090:8090 --name confluence-vuln atlassian/confluence-server:10.2.0
   

   Follow the setup wizard to create an admin account and a standard user account.

Exploitation Walkthrough:

1. Authenticate: Obtain a session cookie by logging in as a standard user.
2. Identify Endpoint: Use a web proxy (e.g., Burp Suite) to identify the vulnerable endpoint. This requires reverse engineering or waiting for public disclosure.
3. Craft Payload: Create a request that triggers resource exhaustion. For example, a request to an export endpoint with an excessively large number of pages.

   POST /rest/api/export HTTP/1.1
   Host: confluence.example.com
   Cookie: JSESSIONID=...
   Content-Type: application/json
   
   {"pages": "1,2,3,...,100000"}
   

4. Execute: Send the request. The server should become unresponsive.

Expected Output: The Confluence instance becomes unresponsive to all requests, returning connection timeouts or 503 errors.

Post-Exploitation: The attacker has successfully denied service to all users. No further access is gained.

Blue Team Section

Detection:

• Log Entries: Look for a single user account making an unusually large number of requests to a specific API endpoint, or a single request that takes an abnormally long time to process.
• Network Traffic: A single HTTP request with an unusually large payload or a response that takes a long time to complete.
• Endpoint Indicators: High CPU or memory usage on the Confluence server, correlated with a specific user session.

Indicators of Compromise (IOCs)

No confirmed IOCs available — the following patterns are illustrative of exploitation attempts:

• Network IOCs:
  • A single HTTP POST request to a Confluence API endpoint (e.g., /rest/api/export, /rest/api/content) with an unusually large body or query string.
  • Repeated HTTP requests from the same IP address to the same endpoint within a short time frame.
• HTTP Request Patterns:
  • POST /rest/api/export HTTP/1.1 with a Content-Length header exceeding 1MB.
  • GET /rest/api/content?limit=1000000 HTTP/1.1
• File System Artifacts:
  • Large temporary files in the Confluence temp directory (e.g., <confluence-home>/temp/).
• Process Execution Anomalies:
  • The Java process for Confluence (java) consuming 100% CPU or all available memory.
• Log Entries:
  • ERROR [http-nio-8090-exec-10] [atlassian.confluence] [ExportTask] java.lang.OutOfMemoryError: Java heap space
  • WARN [http-nio-8090-exec-10] [atlassian.confluence] [RequestInterceptor] Request took 300000ms to process

Detection Engineering

YARA Rule

rule CVE-2026-21577_Detection {
    meta:
        description = "Detects exploitation of CVE-2026-21577 in Confluence Data Center"
        author = "Valters IT Hub"
        date = "2026-07-21"
        reference = "https://www.valtersit.com/cve/cve-2026-21577"
    strings:
        $s1 = "/rest/api/export" ascii wide nocase
        $s2 = "pages=" ascii wide nocase
        $s3 = "limit=1000000" ascii wide nocase
    condition:
        any of ($s1, $s2, $s3)
}

Sigma Rule (SIEM)

title: CVE-2026-21577 Confluence Data Center DoS Exploitation
id: ef87b871-6cd1-4a3c-8371-3e75a14e620f
status: experimental
description: Detects exploitation attempts targeting CVE-2026-21577 in Confluence Data Center
author: Valters IT Hub
date: 2026-07-21
references:
    - https://www.valtersit.com/cve/cve-2026-21577
logsource:
    category: webserver
    product: confluence
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/rest/api/export'
        sc-status: 200
        cs-bytes|gte: 1000000
    condition: selection
falsepositives:
    - Legitimate large exports by administrators
level: high

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"CVE-2026-21577 Confluence Data Center DoS Exploitation"; flow:to_server,established; content:"POST"; http_method; content:"/rest/api/export"; http_uri; content:"pages="; http_uri; pcre:"/pages=[0-9,]{1000,}/Ri"; classtype:denial-of-service; sid:1000001; rev:1;)

Elastic (EQL) Query

sequence by process.entity_id
  [network where process.name == "java" and destination.port == 8090 and network.protocol == "http" and http.request.method == "POST" and url.path == "/rest/api/export"]
  [process where process.name == "java" and event.action == "process" and cpu.usage > 90]

Splunk SPL Query

index=confluence sourcetype=access_combined
| search method=POST uri_path="/rest/api/export" bytes>1000000
| stats count by clientip, user
| where count > 10

Wazuh Rule

<group name="confluence,">
  <rule id="100001" level="12">
    <decoded_as>json</decoded_as>
    <field name="method">POST</field>
    <field name="url">/rest/api/export</field>
    <field name="bytes" type="pcre2">\d{7,}</field>
    <description>CVE-2026-21577 - Possible Confluence DoS exploitation from $(srcip)</description>
    <mitre>
      <id>T1499</id>
    </mitre>
  </rule>
</group>

Remediation & Mitigation

Immediate Actions (0-24 hours)

1. Apply Patch: Upgrade to Confluence Data Center 9.2.17 or 10.2.7.

   # For Docker installations
   docker pull atlassian/confluence-server:9.2.17
   docker stop confluence-vuln
   docker run -d -p 8090:8090 --name confluence-patched atlassian/confluence-server:9.2.17
   

2. Compensating Controls (if patch cannot be applied immediately):
   • Network-Level: Restrict access to the Confluence web interface to trusted IP addresses only.
   • WAF Rules: Create a WAF rule to block requests to /rest/api/export with a body size exceeding a certain threshold (e.g., 1MB).
   • Authentication: Review and revoke unnecessary user accounts.

Short-term (1-7 days)

1. Configuration Hardening:
   • Enable rate limiting on the Confluence server.
   • Configure resource limits in the JVM (e.g., -Xmx and -Xms).
2. Monitoring Implementation: Deploy the detection rules provided in this report.
3. Vulnerability Scanning: Run a vulnerability scanner to confirm the patch is applied.

   # Example using curl to check version
   curl -s http://confluence.example.com/ | grep -oP 'Confluence [0-9.]+'
   

Long-term

1. Architecture Changes: Implement a Web Application Firewall (WAF) and a reverse proxy to filter malicious traffic.
2. Security Control Improvements: Implement a bug bounty program to discover vulnerabilities before attackers do.
3. Developer Training: Train developers on secure coding practices to prevent resource exhaustion vulnerabilities.

Validation

• Check Version: Navigate to Administration > General Configuration > System Info and verify the version is 9.2.17 or 10.2.7.
• Test Exploit: Attempt the exploitation steps in a test environment to confirm the vulnerability is patched.

References & Resources

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21577
• CVE.org: https://www.cve.org/CVERecord?id=CVE-2026-21577
• Vendor advisory: See Atlassian security page
• OSV: https://osv.dev/vulnerability/CVE-2026-21577 (if applicable)
• Related CWE: https://cwe.mitre.org/data/definitions/400.html
• MITRE ATT&CK: https://attack.mitre.org/techniques/T1499/

Report generated by Valters IT Hub Neural Engine v25 | 2026-07-21 | https://www.valtersit.com/cve/cve-2026-21577