Summary
Workarounds
Sanitise any untrusted input before passing it to Leaf and enable a CSP to block inline script and CSS data.
References
https://github.com/vapor/leaf-kit-ghsa-rv3x-xq3r-8j9h/pull/1
For more information
If you have any questions or comments about this advisory:
- Open an issue in Leaf Kit
- Email us at [email protected]
Impact
This affects anyone passing unsanitised data to Leaf's variable tags. Before this fix, Leaf would not escape any strings passed to tags as variables. If an attacker managed to find a variable that was rendered with their unsanitised data, they could inject scripts into a generated Leaf page, which could enable XSS attacks if other mitigations such as a Content Security Policy were not enabled.
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.
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
This has been patched in 1.3.0
Frequently Asked Questions
- What is CVE-2021-37634? CVE-2021-37634 is a medium-severity cross-site scripting (XSS) vulnerability in github.com/vapor/leaf-kit (swift), affecting versions < 1.3.0. It is fixed in 1.3.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- Which versions of github.com/vapor/leaf-kit are affected by CVE-2021-37634? github.com/vapor/leaf-kit (swift) versions < 1.3.0 is affected.
- Is there a fix for CVE-2021-37634? Yes. CVE-2021-37634 is fixed in 1.3.0. Upgrade to this version or later.
- Is CVE-2021-37634 exploitable, and should I be worried? Whether CVE-2021-37634 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 CVE-2021-37634 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 CVE-2021-37634? Upgrade
github.com/vapor/leaf-kitto 1.3.0 or later.