U.S. Cybersecurity and Infrastructure Security Agency (CISA) adds Atlassian flaw to its Known Exploited Vulnerabilities catalog.The U.S. Cybersecurity and Infrastructure Security Agency (CISA) added an Atlassian flaw, tracked as CVE-2026-21589 (CVSS score of 9.3), to its Known Exploited Vulnerabilities (KEV) catalog.In early October, threat actors started exploiting CVE-2026-21589, a critical arbitrary file access flaw in Atlassian Data Center products. The vulnerability could allow attackers to access sensitive files under certain conditions. Affected products include Bitbucket, Confluence, Jira Service Management, Jira Software, Bamboo, Crowd, Crucible, and Fisheye.“This Arbitrary File Access vulnerability allows an unauthenticated attacker to access specific files within the web application root directory in affected versions. Exploitation requires prior knowledge of the target file’s exact name and path; this vulnerability does not allow attackers to enumerate or list directory contents.” reads the advisory. “In some configurations, there may be some sensitive files that make this highly severe.”Previdian telemetry has detected 15 exploitation attempts from three IP addresses in Japan and the U.S..“Exploitation has now started to hit our honeypot network” Previdian Founder Ryan Dewhurst wrote on LinkedIn.On October 6, watchTowr Labs published a technical analysis of CVE-2026-21589 after comparing vulnerable and patched Atlassian packages.Researchers found a path traversal flaw where double-colon (::) sequences can be treated as path separators, allowing attackers to read files from the application web root.“While semicolons (;) are very common in path traversal within Java applications, along with the everyday dots (.) and slashes (/), one item stood out: the matching of double colons (::).” reads the analysis. “This is uncommon syntax for this type of vulnerability, but grepping through the codebase we came across an all too familiar sight.”In one Crowd deployment with Jira, researchers were able to read crowd.properties, which exposed application credentials. They then used those credentials to create a user and add it to the jira-administrators group. Crowd IP allowlisting can block this attack path. Exploitation attempts against its honeypot network reportedly began two hours after watchTowr published additional technical details about the vulnerability.According to watchTowr, the problem is in Atlassian’s web-resource handling. It can turn a string such as ..::..::..::..::WEB-INF::web.xml into ../../../../WEB-INF/web.xml, allowing attackers to move through directories and access files they should not be able to read.Atlassian says all versions before the listed fixed versions are affected:ProductFixed versionsBitbucket Data Center9.4.26, 10.2.8, 10.5.1Confluence Data Center9.2.26, 10.2.19Jira Service Management Data Center5.12.40, 10.3.26, 11.3.12Jira Software Data Center9.12.40, 10.3.26, 11.3.12Bamboo Data Center10.2.24, 12.1.12Crowd Data Center6.3.7, 7.0.3, 7.1.7, 7.2.4Crucible4.9.15Fisheye4.9.15watchTowr has released a free tool on GitHub to help check if a server is vulnerable. According to Binding Operational Directive (BOD) 22-01: Reducing the Significant Risk of Known Exploited Vulnerabilities, FCEB agencies have to address the identified vulnerabilities by the due date to protect their networks against attacks exploiting the flaws in the catalog.Experts also recommend that private organizations review the Catalog and address the vulnerabilities in their infrastructure.CISA orders federal agencies to fix the flaw by October 18, 2026.Follow me on Twitter: @securityaffairs and Facebook and MastodonPierluigi Paganini(SecurityAffairs – hacking, CISA)