# CVE-2021-26084 — Atlassian Confluence Server

**CISA KEV** · **Ransomware**

> As of 2026-11-05, CVE-2021-26084 is a CRITICAL-severity vulnerability in Atlassian Confluence Server, CVSS v3.1 9.8, EPSS 99.9% (99.9th percentile). It is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), with a US federal remediation deadline of 2021-11-17, and CISA links it to known ransomware campaigns. Threadlinqs Intelligence links 1 tracked threat campaign to CVE-2021-26084, most recently “AtomSilo Ransomware – Reverse Engineering Analysis (RSA-512 + AES hybrid encryption) and Confluence CVE-2021-26084 / DLL Side-Loading Intrusion Chain”.

**Last updated:** 2025-12-16

## What is CVE-2021-26084?

In affected versions of Confluence Server and Data Center, an OGNL injection vulnerability exists that would allow an unauthenticated attacker to execute arbitrary code on a Confluence Server or Data Center instance. The affected versions are before version 6.13.23, from version 6.14.0 before 7.4.11, from version 7.5.0 before 7.11.6, and from version 7.12.0 before 7.12.5.

The record classifies CVE-2021-26084 under weakness class [CWE-917](https://intel.threadlinqs.com/cwe/CWE-917). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality, integrity, availability. 2 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 1897 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 9.8 — CRITICAL (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **EPSS (FIRST):** 99.9% probability of exploitation in the next 30 days, higher than 99.9% of all scored CVEs
- **CISA KEV:** Listed since 2021-11-03, federal remediation deadline 2021-11-17 — used in known ransomware campaigns
- **Threadlinqs priority:** 10/10 — CISA lists it as used in ransomware, which Threadlinqs scores at the maximum
- **Published:** 2021-08-30, last modified 2025-12-16

## Is CVE-2021-26084 being exploited?

CISA added CVE-2021-26084 to the Known Exploited Vulnerabilities catalog on 2021-11-03, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2021-11-17 under BOD 22-01. CISA flags the vulnerability as one used in known ransomware campaigns. Weaponised exploit code for CVE-2021-26084 is publicly available. 11 public proof-of-concept repositories are tracked for this identifier. A ProjectDiscovery Nuclei detection template exists (`http/cves/2021/CVE-2021-26084.yaml`). It currently carries a trending score of 50 in the Threadlinqs vulnerability feed.

- [hev0x/CVE-2021-26084_Confluence](https://github.com/hev0x/CVE-2021-26084_Confluence) (github)
- [0xf4n9x/CVE-2021-26084](https://github.com/0xf4n9x/CVE-2021-26084) (github)
- [alt3kx/CVE-2021-26084_PoC](https://github.com/alt3kx/CVE-2021-26084_PoC) (github)
- [dinhbaouit/CVE-2021-26084](https://github.com/dinhbaouit/CVE-2021-26084) (github)
- [1ZRR4H/CVE-2021-26084](https://github.com/1ZRR4H/CVE-2021-26084) (github)
- [crowsec-edtech/CVE-2021-26084](https://github.com/crowsec-edtech/CVE-2021-26084) (github)
- [ZZ-SOCMAP/CVE-2021-26084](https://github.com/ZZ-SOCMAP/CVE-2021-26084) (github)
- [Vulnmachines/Confluence_CVE-2021-26084](https://github.com/Vulnmachines/Confluence_CVE-2021-26084) (github)

## Affected products and versions

- [Atlassian](https://intel.threadlinqs.com/vendors/atlassian): Confluence Server, Confluence Data Center

## How to fix CVE-2021-26084

The record marks a vendor fix as available for CVE-2021-26084. Patch reference: [https://jira.atlassian.com/browse/CONFSERVER-67940](https://jira.atlassian.com/browse/CONFSERVER-67940). Because CVE-2021-26084 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2021-11-17. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.

## Threat activity tracking CVE-2021-26084

1 tracked threat in the Threadlinqs corpus references CVE-2021-26084, either in the campaign’s CVE list or as an indicator on the campaign record.

- [AtomSilo Ransomware – Reverse Engineering Analysis (RSA-512 + AES hybrid encryption) and Confluence CVE-2021-26084 / DLL Side-Loading Intrusion Chain](https://intel.threadlinqs.com/threat/TL-2026-5288) — HIGH · 2026-11-05

## Sources

Enriched from CVE.org, NVD, FIRST EPSS, CISA KEV, GitHub Security Advisories, public proof-of-concept repositories, ProjectDiscovery Nuclei. Last verified by Threadlinqs on 2026-11-05. This product uses the NVD API but is not endorsed or certified by the NVD.

**Other references**

- [jira.atlassian.com](https://jira.atlassian.com/browse/CONFSERVER-67940)
- [packetstormsecurity.com](http://packetstormsecurity.com/files/167449/Atlassian-Confluence-Namespace-OGNL-Injection.html)

Canonical: https://intel.threadlinqs.com/cve/CVE-2021-26084
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