CVE-2026-40479

The client-side escapeForHtml() function in KimaiEscape.js, introduced in commit 89bfa82c (#2959) to fix a JavaScript XSS vulnerability, only escapes <, >, and & but does not escape " (double quote) or ' (single quote). When user-controlled data (profile alias) is placed in an HTML attribute context (title="__DISPLAY__") via the team member form prototype and rendered through innerHTML, the missing quote escaping allows HTML attribute injection, resulting in Stored XSS.

Details

Incomplete security patch. The escapeForHtml() function was meant to prevent XSS but missed quote characters, which are critical for HTML attribute context escaping.

Vulnerable code, assets/js/plugins/KimaiEscape.js:29-33:

const tagsToReplace = {
    '&': '&',
    '<': '&lt;',
    '>': '&gt;',
    // MISSING: '"': '&quot;'
    // MISSING: "'": '&#039;'
};

Affected code files:

• assets/js/plugins/KimaiEscape.js:24-38, incomplete escape function
• assets/js/forms/KimaiTeamForm.js:77,86, replacement + innerHTML
• templates/macros/widgets.html.twig:126, title="{{ tooltip }}" in avatar macro
• templates/form/blocks.html.twig:104, {{ widgets.avatar('__INITIALS__', '__COLOR__', '__DISPLAY__') }}

PoC

poc.zip

Please extract the uploaded compressed file before proceeding

1. ./setup.sh
2. ./poc_xss.sh

• Stored XSS: payload persists in the database (user alias field)
• Privilege escalation: ROLE_USER injects XSS that executes in ROLE_ADMIN/ROLE_SUPER_ADMIN browser session

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-40479 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.53.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-40479? CVE-2026-40479 is a medium-severity cross-site scripting (XSS) vulnerability in kimai/kimai (composer), affecting versions < 2.53.0. It is fixed in 2.53.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-40479? CVE-2026-40479 has a CVSS score of 5.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of kimai/kimai are affected by CVE-2026-40479? kimai/kimai (composer) versions < 2.53.0 is affected.
4. Is there a fix for CVE-2026-40479? Yes. CVE-2026-40479 is fixed in 2.53.0. Upgrade to this version or later.
5. Is CVE-2026-40479 exploitable, and should I be worried? Whether CVE-2026-40479 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-40479 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-40479? Upgrade kimai/kimai to 2.53.0 or later.