Security readout for executives and security teams
Plain-English summary
CVE-2021-37634 is an XSS issue in Vapor LeafKit before 1.3.0. If an application renders attacker-controlled data through Leaf variable tags without sanitization, malicious script could run in a user’s browser. The vendor states this is patched in 1.3.0.
Executive priority
Treat this as a high-priority dependency update for Swift/Vapor applications using LeafKit. The business risk is user browser compromise through affected pages, but urgency should be scoped to services actually using vulnerable LeafKit rendering paths.
Technical view
LeafKit before 1.3.0 did not escape strings passed to variable tags. The CVSS 3.1 vector is network-accessible, low complexity, no privileges required, user interaction required, and scope-changing, with high confidentiality impact and no integrity or availability impact recorded.
Likely exposure
Exposure is limited to applications using vapor leaf-kit versions earlier than 1.3.0 that pass unsanitized user input into Leaf variable tags. Risk is higher where generated pages lack a Content Security Policy that blocks inline script and CSS data.
Exploitation context
The provided sources do not report active exploitation, and the CVE is not marked as CISA KEV. Exploitation depends on an attacker-controlled value reaching a rendered Leaf variable and a victim interacting with the affected page.
Researcher notes
Key evidence is the GitHub advisory: affected versions are below 1.3.0, patched in 1.3.0, with sanitization and CSP as workarounds. Evidence does not identify specific downstream applications, exploit campaigns, or broader affected products.
Mitigation direction
- Upgrade vapor leaf-kit to version 1.3.0 or later.
- Sanitize untrusted input before passing it to Leaf templates.
- Enable a Content Security Policy blocking inline script and CSS data.
- Review vendor advisory and project guidance before deploying changes.
Validation and detection
- Inventory applications using vapor leaf-kit and record installed versions.
- Confirm no deployed service uses leaf-kit earlier than 1.3.0.
- Review Leaf templates for variables populated from user-controlled data.
- Verify input sanitization exists before rendering untrusted values.
- Check production pages enforce an appropriate Content Security Policy.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-79: User-session and phishing behavior lookup
Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-37634 mapping review
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Open ATT&CK lookup- Severity
- High
- CVSS
- 7.4 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N2.84Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.4HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/vapor/leaf-kit/security/advisories/GHSA-rv3x-xq3r-8j9hCVE reference · x_refsource_CONFIRM
- https://github.com/vapor/leaf-kit-ghsa-rv3x-xq3r-8j9h/pull/1CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.