CVE-2026-21588

Executive Summary

CVE-2026-21588 is a high-severity Denial of Service (DoS) vulnerability in Atlassian Confluence Data Center, introduced in versions 8.9.0 through 10.2.0 (specific releases listed by the vendor). An authenticated attacker can exploit this flaw to cause a resource to become unavailable for its intended users by temporarily or indefinitely disrupting services on a network-connected host. Atlassian rated the issue CVSS 7.1 and has published fixed versions; there is currently no public proof-of-concept or evidence of exploitation in the wild.

Technical Analysis

CVE-2026-21588 is classified under CWE-400 (Uncontrolled Resource Consumption), which describes a condition where a product does not properly control the allocation and maintenance of a limited resource, enabling an attacker to influence the amount or rate of resource consumption. The NVD description characterizes the impact explicitly as a Denial of Service: an authenticated attacker can cause "a resource to be unavailable for its intended users by temporarily or indefinitely disrupting services of a host connected to a network."

The vulnerability was introduced in the following Confluence Data Center versions: 8.9.0, 9.0.1, 9.1.0, 9.2.0, 9.3.1, 9.4.0, 9.5.1, 10.0.2, 10.1.0, and 10.2.0. The explicit enumeration of introducing versions, combined with the authenticated attack vector described by the vendor, indicates a flaw in a feature or code path that was added or modified at those release points and can be driven by a user with valid credentials. The specific request endpoint, parameter, or code construct is not disclosed in the available NVD description or CNA title; therefore, this report does not speculate about implementation names.

At the vulnerability-class level, a CWE-400 DoS in a Java-based web application such as Confluence Data Center typically manifests through one of several mechanisms: unbounded allocation of memory or threads in response to a crafted request, algorithmic complexity that scales pathologically with attacker-controlled input, or exhaustion of a bounded pool (thread pool, connection pool, database connection pool, or query budget). The attack surface for this class is a network-reachable HTTP(S) endpoint on the Confluence Data Center service. Because the attacker is authenticated, the vulnerable code path is reachable by any provisioned user — broadening the practical threat model to any account on the instance, including low-privilege or external-facing accounts.

Confluence Data Center is the clustered, horizontally scaled deployment model of Confluence, used for large-enterprise knowledge management, documentation, and collaborative workspaces. Availability is a core service expectation for these deployments; disruption can halt documentation workflows, ticketing integrations, and internal knowledge retrieval across an entire organization.

Exploitation Scenarios

No public PoC, exploit code, or Nuclei template is available for CVE-2026-21588. The attack chain below is inferred from the vulnerability class (CWE-400, authenticated DoS) and the vendor's advisory text. No specific endpoints, parameters, or function names are asserted, because none appear in the available data.

1. Reconnaissance / target identification. The attacker identifies an exposed Confluence Data Center instance — for example via Shodan queries using title and body keywords, or by scanning corporate infrastructure. The attacker determines the Confluence version (for example, through version banners or release-note disclosures) to confirm it falls within the affected range (8.9.0 – 10.2.0 branches as enumerated).

2. Credential acquisition. Because the vulnerability requires authentication, the attacker must first obtain valid Confluence account credentials. This could be via credential stuffing, reuse of leaked passwords, purchase of corporate credentials, social engineering, or abuse of a low-privilege account provisioned for a contractor or partner. The privilege required for the DoS itself may be minimal (any authenticated user), based on the NVD description, which does not distinguish privilege level beyond "authenticated."

3. Triggering the flaw (vulnerability-class level). The attacker sends one or more requests to the vulnerable Confluence functionality. Consistent with CWE-400, the requests cause the application to consume a limited resource in an uncontrolled or unbounded manner — for example, progressively exhausting heap, threads, or database connections. Depending on the mechanism, a single request or a series of repeated requests may be sufficient; the NVD text does not specify whether the DoS is single-shot or requires sustained traffic.

4. Post-exploitation impact. The resource becomes unavailable to legitimate users. Per the vendor, the disruption may be temporary or indefinite. In a clustered Data Center deployment, the realistic blast radius is the affected node and, if resource saturation propagates, the cluster as a whole (for example via load balancer failover, session re-establishment, or shared database/backing-store pressure). Impact is availability-only: confidentiality and integrity are not affected per the stated CVSS score.

This section is intentionally non-specific regarding implementation because the published intelligence contains no endpoint, parameter, or code-level detail. Any report claiming a specific URL, function, or payload for this CVE should be treated as unverified.

Impact Assessment

Per the vendor advisory, CVE-2026-21588 has a CVSS score of 7.1 (High). The NVD description provides no CVSS vector string in the supplied data, but the score, the authenticated network vector, and the availability-only impact are consistent with a vector of the form AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H. That interpretation should be treated as an inference; the authoritative vector is whatever Atlassian published.

• Confidentiality: none. The vulnerability class does not involve data disclosure.
• Integrity: none. No modification of data is described.
• Availability: high. This is the defining impact — the resource becomes unavailable to its intended users.
• Privileges required: low. The attacker must be authenticated. On Confluence, an authenticated user may be an internal employee, contractor, or, if external sharing is enabled, a guest or partner user.

Business impact. Confluence Data Center typically serves as the central documentation, runbook, and process repository for a large enterprise. Outages degrade incident response (runbooks unreachable), onboarding, change-management documentation, and cross-team knowledge sharing. Downstream integrations (Jira, service management, CI/CD documentation links) may also fail or time out. A sustained or repeatable DoS against a critical knowledge base can be used as a diversionary tactic during a larger intrusion, or as a coercion/disruption action.

Blast radius. Affected versions span a substantial deployment base: 8.9.0 through 10.2.0 release points. Data Center deployments are large, customer-facing or organization-wide systems, so a single successful exploitation can affect thousands of users. However, the authenticated precondition significantly reduces the population of capable attackers compared with a pre-auth DoS.

EPSS. The supplied EPSS score is 0.00289 (21.5th percentile), indicating a low near-term probability of exploitation activity. There is no CISA KEV listing and no known in-the-wild exploitation.

Detection Guidance

Because the specific trigger endpoint and parameters are not published, the detection rules below are necessarily generic: they target anomalous request patterns consistent with a resource-exhaustion DoS against Confluence (repetition, burstiness, concurrency, and error-rate spikes) rather than a known signature. They should be tuned to the environment and treated as heuristic detections.

Sigma Rule

title: Potential Authenticated Denial of Service Against Confluence Data Center (CVE-2026-21588)
id: 9f4b2c1e-7a53-4d8e-b6f1-21588cve2026
status: experimental
description: Heuristic detection of probable resource-exhaustion DoS activity against Confluence Data Center by an authenticated user. Tune thresholds to baseline.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-21588
author: valtersit.com Threat Intelligence
date: 2026-01-01
tags:
  - attack.t1499
  - cve.2026.21588
logsource:
  category: webserver
  product: confluence
detection:
  selection_method:
    cs-method:
      - GET
      - POST
  selection_path:
    cs-uri-stem|contains: '/confluence'
  selection_status:
    sc-status:
      - 500
      - 502
      - 503
      - 504
  condition: selection_method and selection_path and selection_status | count() by src_ip > 50 within 5m
falsepositives:
  - Legitimate bulk operations, automation, or scheduled jobs
  - Load balancer health-check flaps
level: medium

YARA Rule

rule CVE_2026_21588_Confluence_DoS_Indicators
{
    meta:
        author = "valtersit.com Threat Intelligence"
        description = "Heuristic detection of Confluence Data Center resource-exhaustion DoS artifacts. No public signature is available; rule flags verbose resource-exhaustion error strings."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-21588"
        date = "2026-01-01"
        severity = "medium"

    strings:
        $a = "java.lang.OutOfMemoryError" ascii wide nocase
        $b = "java.lang.StackOverflowError" ascii wide nocase
        $c = "java.util.concurrent.RejectedExecutionException" ascii wide nocase
        $d = "connection pool exhausted" ascii wide nocase
        $e = "Too many open files" ascii wide nocase
        $f = "GC overhead limit exceeded" ascii wide nocase
        $g = "Confluence" ascii wide nocase

    condition:
        $g and 2 of ($a, $b, $c, $d, $e, $f)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-21588 Potential Confluence Data Center DoS - Repeated Request Burst"; flow:established,to_server; http.method; content:"GET"; http.uri; content:"/confluence"; nocase; threshold:type both, track by_src, count 100, seconds 60; classtype:attempted-dos; reference:cve,2026-21588; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-21588; sid:202621588; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=5m
  [http where http.response.status_code >= 500 and url.path contains "/confluence"] with runs=20

Splunk SPL Query

index=web sourcetype=access_common (uri_path="/confluence*" OR uri="*confluence*") (status=500 OR status=502 OR status=503 OR status=504)
| bin _time span=1m
| stats count AS err_count, dc(clientip) AS unique_clients, values(status) AS statuses BY _time, clientip
| where err_count >= 50
| table _time, clientip, err_count, unique_clients, statuses
| sort - err_count

Wazuh OSSEC Rule

<group name="web,appsec,confluence,attack.dos,">
  <rule id="202621588" level="10" frequency="20" timeframe="300">
    <if_matched_sid>31151</if_matched_sid>
    <same_source_ip />
    <description>Possible authenticated Denial of Service against Confluence Data Center (CVE-2026-21588): repeated 5xx responses from one source.</description>
    <mitre>
      <id>T1499</id>
    </mitre>
    <group>cve_2026_21588,</group>
  </rule>
</group>

Note: the detection rules above are heuristic and version-agnostic. Since no fixed request signature is publicly documented for CVE-2026-21588, defenders should validate against internal baselines and rate-limit/concurrency telemetry, and correlate with authentication logs to identify the responsible account.

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The vendor advisory specifies fixed versions, but no independent patch-commit or tracking-source evidence is available.

1. Immediate mitigations. (a) Restrict Confluence Data Center exposure to trusted networks, VPN, or zero-trust access proxies; do not publish it directly to the internet. (b) Enforce strong authentication and least privilege on Confluence accounts — the attack requires authentication, so reducing the number of valid accounts (disable unused/contractor/guest accounts, revoke stale tokens and API credentials) reduces exposure. (c) Apply rate limiting and concurrency caps at the reverse proxy/load balancer per authenticated user or session to blunt resource-exhaustion bursts. (d) Configure resource limits (JVM heap, thread pools, database connection pools) so a single abuse path cannot exhaust the whole node, and monitor for OOM/thread-pool saturation alerts. (e) Monitor authentication logs for anomalous login patterns that could indicate credential stuffing preceding a DoS.

2. Patch/upgrade steps. Atlassian's advisory states the fixed releases are:

   • Confluence Data Center 9.2 branch: upgrade to >= 9.2.24.
   • Confluence Data Center 10.2 branch: upgrade to >= 10.2.17. Atlassian also recommends upgrading to the latest available release. For all other affected branches (8.9.0, 9.0.1, 9.1.0, 9.3.1, 9.4.0, 9.5.1, 10.0.2, 10.1.0), upgrade to the latest fixed release on the corresponding supported branch. See the Confluence release notes and download archives linked in the references. Test in staging and follow the standard Confluence Data Center upgrade runbook (backup, maintenance window, cluster rolling restart).

3. Long-term hardening. Establish a patch-management cadence with version-awareness monitoring for Atlassian advisories; subscribe to Atlassian security bulletins. Segment the Confluence cluster from general corporate networks and enforce egress/ingress rules. Enable centralized authentication (SSO/MFA) to reduce credential-reuse risk. Baseline normal request and resource-utilization patterns to make anomalies detectable. Maintain documented runbooks in an out-of-band location so that a Confluence outage does not impede incident response.

References

1. NVD — CVE-2026-21588: https://nvd.nist.gov/vuln/detail/CVE-2026-21588
2. Atlassian Confluence release notes: https://confluence.atlassian.com/doc/confluence-release-notes-327.html
3. Atlassian Confluence download archives: https://www.atlassian.com/software/confluence/download-archives
4. CWE-400 — Uncontrolled Resource Consumption: https://cwe.mitre.org/data/definitions/400.html
5. MITRE ATT&CK T1499 — Endpoint Denial of Service: https://attack.mitre.org/techniques/T1499/

Threat actor campaigns / exploit-in-the-wild evidence: None. The CVE is not listed in CISA KEV, no public PoC or exploit code is available, and the supplied EPSS score is 0.00289 (21.5th percentile), indicating low near-term exploitation likelihood. CVE-2026-21588 was reported to Atlassian via its Penetration Testing program.