CVE-2026-55517: Deno is a JavaScript, TypeScript, and WebAssembly runtime

Q: How do I fix CVE-2026-55517?

deno: upgrade to 2.7.5. Exact commands for each OS are on this page.

Q: Is CVE-2026-55517 being exploited?

CVE-2026-55517 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-55517 affect?

CVE-2026-55517 affects deno. Severity is medium (CVSS 4.3). 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-W

Q: How urgent is CVE-2026-55517?

Priority 21/100. No elevated risk signals yet.

Routine

MEDIUM 4.3UPGRADE

CVE-2026-55517 is a medium-severity flaw (CVSS 4.3) affecting deno. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 tha

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	4.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.3% (top 76%)
CISA exploited list	No
Weakness	CWE-248
Published	2026-06-23
Last modified	2026-10-25

Publisher (CNA): GitHub, Inc. · Vendor: deno

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-23 18:18 UTCPublished CVE record published
2026-10-25 23:10 UTCUpdated Record last modified

deno deno UPGRADE

Affected: < 2.7.5

Fixed in: 2.7.5

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy deno || rpm -q deno || deno --version

2. UPGRADE to 2.7.5 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade deno

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade deno

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-55517?

deno: upgrade to 2.7.5. Exact commands for each OS are on this page.

Is CVE-2026-55517 being exploited?

CVE-2026-55517 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-55517 affect?

CVE-2026-55517 affects deno. Severity is medium (CVSS 4.3). 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-W

How urgent is CVE-2026-55517?

Priority 21/100. No elevated risk signals yet.

Related CVEs

CVE-2026-49402 HIGH 8.1 UPGRADE Deno is a JavaScript, TypeScript, and WebAssembly runtime
CVE-2026-44726 HIGH 7.4 UPGRADE Deno is a JavaScript, TypeScript, and WebAssembly runtime
CVE-2026-49440 HIGH 7.4 UPGRADE Deno is a JavaScript, TypeScript, and WebAssembly runtime
CVE-2026-49401 HIGH 7.3 UPGRADE Deno is a JavaScript, TypeScript, and WebAssembly runtime
CVE-2026-49411 MEDIUM 6.5 UPGRADE Deno is a JavaScript, TypeScript, and WebAssembly runtime

References

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