CVE-2026-55517: Deno: Denial of service via non-ASCII bytes in WebSocket response headers

Summary

A Deno program that opens a client WebSocket connection could be crashed by the remote server. While handling the WebSocket handshake response, Deno parsed the Sec-WebSocket-Protocol and Sec-WebSocket-Extensions response headers in a way that assumed their bytes were always printable ASCII. A response header containing non-visible-ASCII bytes (0x80-0xFF) caused a panic that aborted the entire Deno process.

Details

When establishing a client WebSocket connection, Deno read the Sec-WebSocket-Protocol and Sec-WebSocket-Extensions headers from the server's 101 Switching Protocols response and converted them to strings without handling the failure case. HeaderValue::tostr() returns an error for any value containing bytes outside the visible-ASCII range, so a header carrying such bytes triggered an unrecoverable error during conversion.

Because the client initiates the outbound connection, the handshake response is fully controlled by the server. A server that returns bytes such as 0xFF 0xFE in either header could therefore crash any client that connected to it.

This is purely an availability issue. There is no information disclosure and no memory-safety impact; the only effect is termination of the current process.

Impact

Remote denial of service. Any Deno application that establishes WebSocket connections to untrusted or potentially-compromised endpoints could be terminated by the remote peer. Exploitation requires the victim application to initiate the outbound WebSocket connection. An attacker who controls the WebSocket endpoint, or who can man-in-the-middle a plaintext ws:// connection, could trigger the crash. The effect is confined to crashing the process that opened the connection.

Patch

The issue is fixed in Deno 2.7.5. The header values are now parsed with graceful fallbacks: values that cannot be represented as ASCII strings are skipped instead of aborting the process. A regression test covers a server that returns non-ASCII bytes in Sec-WebSocket-Protocol.

Users should upgrade to Deno 2.7.5 or later.

Workarounds

Until you can upgrade, only connect to trusted WebSocket endpoints and prefer wss:// (TLS) over ws://, which prevents a network man-in-the-middle from injecting malicious header bytes into the handshake response.