{"id":"CVE-2026-55517","aliases":["EUVD-2026-38549","GHSA-x2qc-cmh9-f4hf","JLSEC-2026-1167"],"status":"published","title":"Deno: Denial of service via non-ASCII bytes in WebSocket response headers","description":"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.","severity":{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-11","score":0.00315,"percentile":0.24737,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-248"],"published":"2026-06-17T18:48:18Z","modified":"2026-10-28T02:30:52Z","fixes":[{"ecosystem":"crates.io","package":"deno","fixed_in":["2.7.5"],"sources":["ghsa","osv"]},{"ecosystem":"Julia","package":"Deno_jll","fixed_in":["2.8.1+0"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"crates.io","package":"deno","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"2.7.4"},{"fixed":"2.7.5"}]}]},{"source":"osv","ecosystem":"crates.io","package":"deno","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"2.7.5"}]}]},{"source":"osv","ecosystem":"Julia","package":"Deno_jll","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"2.8.1+0"}]}]}],"products":[{"vendor":"deno","product":"deno"},{"vendor":"denoland","product":"deno"}],"references":[{"url":"https://github.com/denoland/deno/security/advisories/GHSA-x2qc-cmh9-f4hf","tags":["x_refsource_CONFIRM","Mitigation","Vendor Advisory","advisory","web"]},{"url":"https://github.com/advisories/GHSA-x2qc-cmh9-f4hf","tags":["web"]},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55517.json","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55517","tags":["advisory"]},{"url":"https://github.com/denoland/deno","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-55517","title":"Deno: Denial of service via non-ASCII bytes in WebSocket response headers","description":"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.","status":"published","published":"2026-06-23T17:24:59.498Z","modified":"2026-06-23T17:52:56.96Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","level":"medium","assessor":"cna"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T14:20:12.260247Z"}],"euvd":[{"id":"EUVD-2026-38549","description":"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.","status":"published","published":"2026-06-23T17:24:59Z","modified":"2026-06-23T17:52:56Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","level":"medium"}],"updated_at":"2026-11-02T18:20:31.51905Z"}],"ghsa":[{"id":"GHSA-x2qc-cmh9-f4hf","title":"Deno: Denial of service via non-ASCII bytes in WebSocket response headers","description":"## Summary\n\nA Deno program that opens a client `WebSocket` connection could be crashed by\nthe remote server. While handling the WebSocket handshake response, Deno parsed\nthe `Sec-WebSocket-Protocol` and `Sec-WebSocket-Extensions` response headers in\na way that assumed their bytes were always printable ASCII. A response header\ncontaining non-visible-ASCII bytes (`0x80`-`0xFF`) caused a panic that aborted\nthe entire Deno process.\n\n## Details\n\nWhen establishing a client WebSocket connection, Deno read the\n`Sec-WebSocket-Protocol` and `Sec-WebSocket-Extensions` headers from the\nserver's `101 Switching Protocols` response and converted them to strings\nwithout handling the failure case. `HeaderValue::to_str()` returns an error for\nany value containing bytes outside the visible-ASCII range, so a header carrying\nsuch bytes triggered an unrecoverable error during conversion.\n\nBecause the client initiates the outbound connection, the handshake response is\nfully controlled by the server. A server that returns bytes such as `0xFF 0xFE`\nin either header could therefore crash any client that connected to it.\n\nThis is purely an availability issue. There is no information disclosure and no\nmemory-safety impact; the only effect is termination of the current process.\n\n## Impact\n\nRemote denial of service. Any Deno application that establishes WebSocket\nconnections to untrusted or potentially-compromised endpoints could be\nterminated by the remote peer. Exploitation requires the victim application to\ninitiate the outbound WebSocket connection. An attacker who controls the\nWebSocket endpoint, or who can man-in-the-middle a plaintext `ws://` connection,\ncould trigger the crash. The effect is confined to crashing the process that\nopened the connection.\n\n## Patch\n\nThe issue is fixed in Deno `2.7.5`. The header values are now parsed with\ngraceful fallbacks: values that cannot be represented as ASCII strings are\nskipped instead of aborting the process. A regression test covers a server that\nreturns non-ASCII bytes in `Sec-WebSocket-Protocol`.\n\nUsers should upgrade to Deno `2.7.5` or later.\n\n## Workarounds\n\nUntil you can upgrade, only connect to trusted WebSocket endpoints and prefer\n`wss://` (TLS) over `ws://`, which prevents a network man-in-the-middle from\ninjecting malicious header bytes into the handshake response.","status":"published","published":"2026-06-17T18:48:18Z","modified":"2026-06-17T18:48:18Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","level":"medium"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T14:40:28.438722Z"}],"nvd":[{"id":"CVE-2026-55517","description":"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.","status":"published","published":"2026-06-23T18:18:09.787Z","modified":"2026-10-25T23:10:00.167Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","level":"medium","assessor":"secondary"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T14:12:25.288279Z"}],"osv":[{"id":"CVE-2026-55517","title":"Deno: Denial of service via non-ASCII bytes in WebSocket response headers","description":"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.","status":"published","published":"2026-06-23T17:24:59.498Z","modified":"2026-10-28T02:30:52.434874494Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","level":"medium"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T15:45:11.22012Z"},{"id":"GHSA-x2qc-cmh9-f4hf","title":"Deno: Denial of service via non-ASCII bytes in WebSocket response headers","description":"## Summary\n\nA Deno program that opens a client `WebSocket` connection could be crashed by\nthe remote server. While handling the WebSocket handshake response, Deno parsed\nthe `Sec-WebSocket-Protocol` and `Sec-WebSocket-Extensions` response headers in\na way that assumed their bytes were always printable ASCII. A response header\ncontaining non-visible-ASCII bytes (`0x80`-`0xFF`) caused a panic that aborted\nthe entire Deno process.\n\n## Details\n\nWhen establishing a client WebSocket connection, Deno read the\n`Sec-WebSocket-Protocol` and `Sec-WebSocket-Extensions` headers from the\nserver's `101 Switching Protocols` response and converted them to strings\nwithout handling the failure case. `HeaderValue::to_str()` returns an error for\nany value containing bytes outside the visible-ASCII range, so a header carrying\nsuch bytes triggered an unrecoverable error during conversion.\n\nBecause the client initiates the outbound connection, the handshake response is\nfully controlled by the server. A server that returns bytes such as `0xFF 0xFE`\nin either header could therefore crash any client that connected to it.\n\nThis is purely an availability issue. There is no information disclosure and no\nmemory-safety impact; the only effect is termination of the current process.\n\n## Impact\n\nRemote denial of service. Any Deno application that establishes WebSocket\nconnections to untrusted or potentially-compromised endpoints could be\nterminated by the remote peer. Exploitation requires the victim application to\ninitiate the outbound WebSocket connection. An attacker who controls the\nWebSocket endpoint, or who can man-in-the-middle a plaintext `ws://` connection,\ncould trigger the crash. The effect is confined to crashing the process that\nopened the connection.\n\n## Patch\n\nThe issue is fixed in Deno `2.7.5`. The header values are now parsed with\ngraceful fallbacks: values that cannot be represented as ASCII strings are\nskipped instead of aborting the process. A regression test covers a server that\nreturns non-ASCII bytes in `Sec-WebSocket-Protocol`.\n\nUsers should upgrade to Deno `2.7.5` or later.\n\n## Workarounds\n\nUntil you can upgrade, only connect to trusted WebSocket endpoints and prefer\n`wss://` (TLS) over `ws://`, which prevents a network man-in-the-middle from\ninjecting malicious header bytes into the handshake response.","status":"published","published":"2026-06-17T18:48:18Z","modified":"2026-06-17T19:11:27.308501Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","level":"medium"}],"cwes":["CWE-248"],"updated_at":"2026-11-02T15:46:59.484835Z"},{"id":"JLSEC-2026-1167","title":"Deno: Denial of service via non-ASCII bytes in WebSocket response headers","description":"## Summary\n\nA Deno program that opens a client `WebSocket` connection could be crashed by\nthe remote server. While handling the WebSocket handshake response, Deno parsed\nthe `Sec-WebSocket-Protocol` and `Sec-WebSocket-Extensions` response headers in\na way that assumed their bytes were always printable ASCII. A response header\ncontaining non-visible-ASCII bytes (`0x80`-`0xFF`) caused a panic that aborted\nthe entire Deno process.\n\n## Details\n\nWhen establishing a client WebSocket connection, Deno read the\n`Sec-WebSocket-Protocol` and `Sec-WebSocket-Extensions` headers from the\nserver's `101 Switching Protocols` response and converted them to strings\nwithout handling the failure case. `HeaderValue::to_str()` returns an error for\nany value containing bytes outside the visible-ASCII range, so a header carrying\nsuch bytes triggered an unrecoverable error during conversion.\n\nBecause the client initiates the outbound connection, the handshake response is\nfully controlled by the server. A server that returns bytes such as `0xFF 0xFE`\nin either header could therefore crash any client that connected to it.\n\nThis is purely an availability issue. There is no information disclosure and no\nmemory-safety impact; the only effect is termination of the current process.\n\n## Impact\n\nRemote denial of service. Any Deno application that establishes WebSocket\nconnections to untrusted or potentially-compromised endpoints could be\nterminated by the remote peer. Exploitation requires the victim application to\ninitiate the outbound WebSocket connection. An attacker who controls the\nWebSocket endpoint, or who can man-in-the-middle a plaintext `ws://` connection,\ncould trigger the crash. The effect is confined to crashing the process that\nopened the connection.\n\n## Patch\n\nThe issue is fixed in Deno `2.7.5`. The header values are now parsed with\ngraceful fallbacks: values that cannot be represented as ASCII strings are\nskipped instead of aborting the process. A regression test covers a server that\nreturns non-ASCII bytes in `Sec-WebSocket-Protocol`.\n\nUsers should upgrade to Deno `2.7.5` or later.\n\n## Workarounds\n\nUntil you can upgrade, only connect to trusted WebSocket endpoints and prefer\n`wss://` (TLS) over `ws://`, which prevents a network man-in-the-middle from\ninjecting malicious header bytes into the handshake response.","status":"published","published":"2026-08-05T18:20:40.259Z","modified":"2026-08-05T18:36:54.154086364Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","level":"medium"}],"updated_at":"2026-11-02T15:47:18.159338Z"}]}}