obsidian-local-rest-api: Authenticated path traversal via URL-encoded %2F in /vault/{path} — arbitrary host file read/write/delete

Summary

### Summary The Local REST API's `/vault/{path}` endpoints (GET/PUT/PATCH/POST/DELETE) percent-decode the request path *inside the handler — after* Express has already routed and normalized it, then hand it to the Obsidian vault adapter with no confinement check. A literal `../` is resolved/rejected at the routing layer (→ 404), but `%2F` is not a separator there, so `..%2F..%2F` survives routing and is only turned into a real `/` by the handler's `decodeURIComponent`, reconstituting a `../` traversal that walks out of the vault. An **authenticated** client can read, write, or delete **arbitrary files on the host** with the Obsidian process's privileges.

### Details **Framework:** Express (`import express from "express"`; routes registered via `this.api.route("/vault/*")…`).

**The vulnerable line** — in `src/requestHandler.ts`, every vault handler (`vaultGet`, `vaultPut`, `vaultPatch`, `vaultPost`, `vaultDelete`) derives the path like this: ```ts const rawPath = decodeURIComponent( req.path.slice(req.path.indexOf("/", 1) + 1), ); ``` The path is `decodeURIComponent`'d **after** Express routing. A literal `../` is collapsed/rejected at the routing layer, but `%2F` isn't a separator there — so `..%2F..%2F` reaches the handler intact and this `decodeURIComponent` turns it into a real `../../`. **The string routing saw (`…%2F…`) is not the string the handler uses (`…/…`)**, and `%2e%2e` behaves the same way.

**No confinement on the decoded path.** The handlers pass `rawPath` straight to the vault adapter — e.g. `this.app.vault.adapter.readBinary(filePath)` / `this.app.vault.getAbstractFileByPath(filePath)` — with no `path.resolve` + vault-root prefix check, so the reconstituted `../../` escapes.

**The fix already exists in your code — for MOVE only.** `vaultMove` confines correctly: ```ts const syntheticRoot = "/vault"; const resolved = posix.resolve(syntheticRoot, normalized); if (resolved !== syntheticRoot && !resolved.startsWith(syntheticRoot + "/")) { this.returnCannedResponse(res, { errorCode: ErrorCode.PathTraversalNotAllowed }); return; } ``` GET/PUT/PATCH/POST/DELETE lack this guard. **Apply the same `posix.resolve(syntheticRoot, …)` + `startsWith` check to the decoded path in every vault handler**, and reject any segment that decodes to `..`.

### PoC Prereq: a running Obsidian with the Local REST API plugin enabled and its configured API key (`$API_KEY`). Targets below are Unix; adjust per OS (e.g. `..%2F..%2FWindows%2Fwin.ini` on Windows).

```bash # READ outside the vault (returns 200 + the target file's real bytes): curl --path-as-is -k -H "Authorization: Bearer $API_KEY" \ "https://127.0.0.1:27124/vault/..%2F..%2F..%2F..%2Fetc%2Fpasswd"

# WRITE outside the vault (creates a file on disk outside the vault root): curl --path-as-is -k -X PUT -H "Authorization: Bearer $API_KEY" --data "pwned" \ "https://127.0.0.1:27124/vault/..%2F..%2F..%2Ftmp%2Fcanary.txt" ```

`--path-as-is` stops curl from collapsing `..` client-side. A plain `../` (unencoded) request returns 404 — only the `%2F`/`%2e%2e` encoded form bypasses, confirming the decode-after-routing gap.

### Impact Authenticated arbitrary file **read / write / delete** outside the Obsidian vault, with the OS privileges of the Obsidian process — typically the user's home directory (SSH keys, browser profiles, dotfiles, credentials). Amplified in MCP/LLM-agent deployments: this API is widely used as an MCP server, so a prompt-injection in vault content (or a malicious MCP client) can make an agent emit a `%2F` path — turning "the agent can edit my notes" into "the agent can read/write any file on the host," with no user intent to grant filesystem access beyond the vault.

This vulnerability was reported by Caleb Brisbin through responsible disclosure.