fix(export): exclude symlinks from the build export archive (Closes #104)

[SakethSumanBathini]

Closes #104

I've left a comment on #104 with the detail. Short version: the remediation the issue asks for is correct, but I could not reproduce the impact as stated, so this PR describes what I could actually demonstrate.

What the issue predicts, and what actually happens

The issue expects archiver to follow symlinks and pull host file contents into the zip. I installed archiver@7.0.1 — the version pinned in backend/package.json — and reproduced exportZip exactly, same constructor options and same archive.directory(outputPath, false) call, against an output directory containing a link to a file outside it and a link to /etc.

Archiver 7.0.1 does not dereference symlinks. It stored both as symlink entries (mode 0o120000) and rewrote the absolute targets into relative ones (/etc became ../../../../etc). The archive contained the link paths, not the contents of the linked files. I don't believe the backend arbitrary-file-read described in the issue is reachable on this version.

The real exposure points the other way

Extracting that archive with unzip recreated both symlinks, still resolving outside the extraction directory — reading through one returned content from outside the extraction root.

So the risk isn't the backend host; it's whoever downloads the export. A cloned repository's contents are attacker-controlled and git records symlinks, so a malicious PreTeXt repo can commit links that survive into the exported zip. Anyone extracting it with a standard tool (unzip, bsdtar, macOS Archive Utility) materialises links pointing outside their extraction directory — the zip symlink traversal pattern (CWE-59), landing on the user rather than the server.

Change

archive.directory() now takes an entry filter that drops symlinks:

archive.directory(outputPath, false, (entry: any) => (
  entry?.stats?.isSymbolicLink?.() ? false : entry
));

entry.stats comes from an lstat, so this identifies the link itself rather than whatever it points at.

Verification

• npx tsc --noEmit in backend/ clean.
• Against the same fixture, the filter excluded both symlink entries and archived the regular files normally.

An alternative, if you'd prefer it

This drops symlinks outright. If you'd rather resolve and validate them — keeping links whose targets stay inside outputPath — resolveContainedPath in backend/src/utils/pathContainment.ts already performs exactly that realpath-based containment check, and updateFile uses it for the same class of problem. Say the word and I'll switch this to reuse it.

Note: npm test is red on clean main independently of this branch.

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