Deno TLS Plaintext Injection Vulnerability

Deno, a modern runtime for JavaScript and TypeScript, contains a flaw within its Node.js tls compatibility layer that can lead to plaintext transmission of sensitive data. Specifically, when using the node:tls or node:https APIs with the default autoSelectFamily option enabled, a failed initial connection attempt can cause a subsequent retry to occur without proper TLS negotiation. This occurs because the socket reinitialization process reuses a stale TLS upgrade hook associated with the original, unsuccessful handle. An attacker who can manipulate network conditions to induce this initial failure can then observe or modify the data transmitted by the client application. This vulnerability affects Deno versions 2.0.0 through 2.7.7 and poses a significant risk to applications relying on TLS for secure communication. The vulnerability is tracked as CVE-2026-44726.

Attack Chain

1. The victim application initiates a TLS connection using node:tls or node:https with autoSelectFamily enabled.
2. The application resolves the target hostname to multiple IP addresses, including an unreachable address (e.g., IPv6 address).
3. The initial connection attempt to the unreachable address fails (e.g., due to dropped IPv6 traffic).
4. Deno’s tls compatibility layer attempts to retry the connection using a different resolved IP address (e.g., IPv4).
5. The socket reinitialization process reuses a stale TLS upgrade hook from the failed connection attempt.
6. The subsequent TCP connection is established without being upgraded to TLS.
7. The victim application writes data to the socket before the secureConnect event is triggered. This data includes sensitive information like API keys and card numbers.
8. The attacker intercepts the plaintext data transmitted over the unencrypted TCP connection.

Impact

Successful exploitation of this vulnerability allows a network attacker to observe and potentially tamper with data that the victim application believes is protected by TLS. This can lead to the disclosure of sensitive information such as API keys, authentication tokens, and financial data like credit card numbers. The proof-of-concept demonstrates the exposure of an Authorization header containing a secret bearer token and card details. Applications that transmit sensitive data over TLS using vulnerable versions of Deno are at risk. The number of potential victims is difficult to estimate, but any application using the affected Deno versions with node:tls or node:https is susceptible.

Recommendation

• Upgrade Deno to version 2.7.8 or later to patch CVE-2026-44726.
• Monitor network traffic for connections to unexpected destinations without TLS negotiation to identify potential exploitation attempts based on the attack chain described above.
• Consider disabling autoSelectFamily in node:tls and node:https if upgrading is not immediately feasible. This will prevent the vulnerable connection retry behavior, although it may impact connectivity in certain network environments.
• Implement the “Detect Deno Plaintext TLS Communication” Sigma rule to identify potentially vulnerable Deno processes attempting to communicate without proper TLS encryption.