CVE-2026-55517

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::to_str() 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.

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.

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.

CVE-2026-55517 has a CVSS score of 4.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.7.5); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2026-55517? CVE-2026-55517 is a medium-severity security vulnerability in deno (rust), affecting versions <= 2.7.4. It is fixed in 2.7.5.
2. How severe is CVE-2026-55517? CVE-2026-55517 has a CVSS score of 4.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of deno are affected by CVE-2026-55517? deno (rust) versions <= 2.7.4 is affected.
4. Is there a fix for CVE-2026-55517? Yes. CVE-2026-55517 is fixed in 2.7.5. Upgrade to this version or later.
5. Is CVE-2026-55517 exploitable, and should I be worried? Whether CVE-2026-55517 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-55517 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-55517? Upgrade deno to 2.7.5 or later.