Path traversal via double percent-encoded slash bypasses directory containment check

Summary

isFileInDirectory()'s containment check uses a naive string-prefix test with no trailing-separator boundary, so a sibling directory whose name happens to share a prefix with the served directory (e.g. _site-leak next to _site) is incorrectly treated as "inside" it. This is normally unreachable over HTTP because new URL()'s dot-segment removal neutralizes literal ..//%2e%2e/ sequences — but if the path separator itself is also percent-encoded (%2f), the whole segment survives new URL() untouched as one opaque path component, and a later decodeURIComponent() call materializes a real ../ traversal after the containment check's path has already been constructed.

CWE: CWE-22 (Path Traversal)
Severity: Medium
CVSS: 5.3 — CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N (AC:H because exploitation depends on a sibling directory/file existing whose name happens to share the served directory's name as a literal prefix)

Root Cause

server.js:282-284:

isFileInDirectory(dir, file) {
  let absoluteDir = TemplatePath.absolutePath(dir);
  let absoluteFile = TemplatePath.absolutePath(file);
  return absoluteFile.startsWith(absoluteDir);   // no trailing separator check
}

Reproduction

mkdir -p /tmp/poc/_site /tmp/poc/_site-leak
echo "SIBLING-DIR-SECRET" > /tmp/poc/_site-leak/leak.txt
# start server on /tmp/poc/_site as in issue #150

$ curl http://localhost:PORT/%2e%2e%2f_site-leak%2fleak.txt
SIBLING-DIR-SECRET

Unit-level confirmation of the underlying primitive:

isFileInDirectory("/tmp/poc/_site", "/tmp/poc/_site-leak/leak.txt") // → true (wrong)

A single-level escape to a path that does not share the served directory's name prefix (e.g. /%2e%2e%2fsecret.txt reaching straight outside with no naming coincidence) is correctly rejected — this bug's impact is bounded to prefix-sharing siblings, not arbitrary filesystem read.

Recommended Fix

isFileInDirectory(dir, file) {
  let absoluteDir = TemplatePath.absolutePath(dir);
  let absoluteFile = TemplatePath.absolutePath(file);
  return absoluteFile === absoluteDir || absoluteFile.startsWith(absoluteDir + path.sep);
}

Verification

Dynamically confirmed on v3.0.0-alpha.11 against a real running server instance with real curl requests, as shown above.

Start at server.js:282-284 and inspect isFileInDirectory(), then reproduce the reported sibling-directory case with the provided /tmp/poc setup and curl request. Confirm that the containment check rejects /tmp/poc/_site-leak while preserving valid paths inside /tmp/poc/_site; the issue's unit-level call provides an additional verification point.