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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Deno is a JavaScript, TypeScript, and WebAssembly runtime. Prior to 2.7.5, 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. This vulnerability is fixed in 2.7.5.

Source published

23 Jun 2026 17:24:59 UTC

Source modified

23 Jun 2026 17:52:56 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 4.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L

Deno is a JavaScript, TypeScript, and WebAssembly runtime. Prior to 2.7.5, 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. This vulnerability is fixed in 2.7.5.

Source published

Jun 23, 2026, 5:24:59 PM (timezone not stated)

Source modified

Jun 23, 2026, 5:52:56 PM (timezone not stated)

Observed here

7 Nov 2026 12:32:23 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 4.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L

Deno is a JavaScript, TypeScript, and WebAssembly runtime. Prior to 2.7.5, 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. This vulnerability is fixed in 2.7.5.

Source published

23 Jun 2026 18:18:09 UTC

Source modified

25 Oct 2026 23:10:00 UTC

Observed here

27 Oct 2026 15:15:14 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 4.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L

## 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::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. ## 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.

Source published

17 Jun 2026 18:48:18 UTC

Source modified

17 Jun 2026 19:11:27 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L

Publisher rating · MODERATE