How severe is CVE-2021-37634?

CVE-2021-37634 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-37634?

CVE-2021-37634 affects the following packages: github.com/vapor/leaf-kit (SwiftURL). Ecosystems affected: SwiftURL.

How do I fix CVE-2021-37634?

Update github.com/vapor/leaf-kit to 1.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-37634 is resolved across your whole dependency graph.

How do I detect CVE-2021-37634 in my SwiftURL dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/vapor/leaf-kit, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-37634 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2021-37634 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-37634 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-37634?

CVE-2021-37634 has an EPSS (Exploit Prediction Scoring System) score of 0.7%, placing it in the 54th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-37634?

CVE-2021-37634 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-37634 published, and has it been updated?

CVE-2021-37634 was published on August 9, 2021 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 SwiftURL

Not in CISA KEV

MEDIUM severity

CVE-2021-37634 — leaf-kit

MEDIUMFix: vapor/leaf-kit-ghsa-rv3x-xq3r-8j9h#1

CVE-2021-37634 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in github.com/vapor/leaf-kit. A fix is available for github.com/vapor/leaf-kit — see the affected versions and patch details below.

LeafKit allows XSS with untrusted user input

Also known asGHSA-rv3x-xq3r-8j9h

Published

Aug 9, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 5, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.7%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs54th percentile — riskier than 54% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

📦github.com/vapor/leaf-kit

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects SwiftURL packages — download data is not available via public APIs for these ecosystems.

Description

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.

Patches

This has been patched in 1.3.0

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]

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦SwiftURL	`github.com/vapor/leaf-kit`	all versions	1.3.0

Affected Products

1 product · 1 configurations

Application

leafkitvapor

< 1.3.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/vapor/leaf-kit, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/vapor/leaf-kit to 1.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-37634 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### 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. ### Patches This has been patched in 1.3.0 ### Workarounds Sanitise any untrusted input before passing it to Leaf and enable a CSP to block inline script and CSS data. ### References https: