CVE-2021-37634
LeafKit allows XSS with untrusted user input
Leafkit is a templating language with Swift-inspired syntax. Versions prior to 1.3.0 are susceptible to Cross-site Scripting (XSS) attacks. 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. This has been patched in 1.3.0. As a workaround sanitize any untrusted input before passing it to Leaf and enable a CSP to block inline script and CSS data.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- None
- User interaction
- Required
What an attacker can affect
- Confidentiality
- High
- Integrity
- None
- Availability
- None
Exploit likelihood
What each source reports
CVE List Score used
LeafKit allows XSS with untrusted user input
Leafkit is a templating language with Swift-inspired syntax. Versions prior to 1.3.0 are susceptible to Cross-site Scripting (XSS) attacks. 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. This has been patched in 1.3.0. As a workaround sanitize any untrusted input before passing it to Leaf and enable a CSP to block inline script and CSS data.
- Record
CVE-2021-37634- CVSS 3.1
- 7.4 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N - Weakness
- CWE-79
- Updated
- 2026-11-02
EUVDEUVD-2023-1881
Leafkit is a templating language with Swift-inspired syntax. Versions prior to 1.3.0 are susceptible to Cross-site Scripting (XSS) attacks. 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. This has been patched in 1.3.0. As a workaround sanitize any untrusted input before passing it to Leaf and enable a CSP to block inline script and CSS data.
- Record
EUVD-2023-1881- CVSS 3.1
- 7.4 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N - Updated
- 2026-11-03
GitHub AdvisoriesGHSA-rv3x-xq3r-8j9h No score
LeafKit allows XSS with untrusted user input
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]
- Record
GHSA-rv3x-xq3r-8j9h- Weakness
- CWE-79, CWE-80
- Updated
- 2026-11-02
NVD
Leafkit is a templating language with Swift-inspired syntax. Versions prior to 1.3.0 are susceptible to Cross-site Scripting (XSS) attacks. 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. This has been patched in 1.3.0. As a workaround sanitize any untrusted input before passing it to Leaf and enable a CSP to block inline script and CSS data.
- Record
CVE-2021-37634- CVSS 3.1
- 7.4 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N - CVSS 3.1
- 6.1 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N - CVSS 2.0
- 4.3 Medium
AV:N/AC:M/Au:N/C:N/I:P/A:N - Weakness
- CWE-79
- Updated
- 2026-11-02
OSV
Leafkit is a templating language with Swift-inspired syntax. Versions prior to 1.3.0 are susceptible to Cross-site Scripting (XSS) attacks. 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. This has been patched in 1.3.0. As a workaround sanitize any untrusted input before passing it to Leaf and enable a CSP to block inline script and CSS data.
- Record
CVE-2021-37634- CVSS 3.1
- 6.1 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N - Updated
- 2026-11-02
OSVGHSA-rv3x-xq3r-8j9h No score
LeafKit allows XSS with untrusted user input
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]
- Record
GHSA-rv3x-xq3r-8j9h- Weakness
- CWE-79, CWE-80
- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
github.com/vapor/leaf-kitSwiftURL |
before 1.3.0 | 1.3.0 |
History
| Recorded | Changed | Sources |
|---|---|---|
| aliases | EUVD | |
| first indexed | CVE List, GitHub Advisories, NVD, OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2021-37634 is a high vulnerability (CVSS 7.4) in Vapor Leafkit, published August 9, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.71%, the 53rd percentile. Fixed in github.com/vapor/leaf-kit 1.3.0.