CVE-2026-48157

Slim has Reflected XSS in the HtmlErrorRenderer

Workarounds

Without upgrading, applications can:

• Avoid passing untrusted/request-derived data into HttpException::setTitle() and setDescription(). Use static, plain-text error copy instead.
• Register a custom error renderer (an ErrorRendererInterface implementation, or a subclass of HtmlErrorRenderer that escapes the title and description) for the HTML media type.

Acknowledgments

Slim is grateful to and thanks GitHub user 0xEr3n for reporting this issue.

Resources

• CWE-79: https://cwe.mitre.org/data/definitions/79.html

If an application uses HttpException::setTitle() and/or setDescription() to include untrusted/request-derived data in the error title or description (e.g. "No products found matching '{$query}'."), an attacker could inject arbitrary HTML/JavaScript that executes in the victim's browser when they encounter an HTML error page generated by Slim.

The vulnerability is present even with displayErrorDetails = false as the unescaped title and description are rendered on this error path.

Built-in exceptions (HttpNotFoundException, HttpBadRequestException, etc.) ship plain-text defaults, so a vanilla Slim app with no user code is not exploitable. Only applications that feed untrusted data into setTitle() and/or setDescription() are affected.

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-2026-48157 has a CVSS score of 6.1 (Medium). 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. A fixed version is available (4.15.2); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

The issue is fixed in 4.15.2.

1. What is CVE-2026-48157? CVE-2026-48157 is a medium-severity cross-site scripting (XSS) vulnerability in slim/slim (composer), affecting versions >= 4.4.0, <= 4.15.1. It is fixed in 4.15.2. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-48157? CVE-2026-48157 has a CVSS score of 6.1 (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 slim/slim are affected by CVE-2026-48157? slim/slim (composer) versions >= 4.4.0, <= 4.15.1 is affected.
4. Is there a fix for CVE-2026-48157? Yes. CVE-2026-48157 is fixed in 4.15.2. Upgrade to this version or later.
5. Is CVE-2026-48157 exploitable, and should I be worried? Whether CVE-2026-48157 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-2026-48157 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-2026-48157? Upgrade slim/slim to 4.15.2 or later.

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