How severe is CVE-2026-55517?

CVE-2026-55517 has a CVSS 3.x base score of 4.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is none, integrity none, and availability low.

Is CVE-2026-55517 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (8th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-55517?

CVE-2026-55517 affects Deno. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-55517?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-55517 have an EU (EUVD) identifier?

Yes. CVE-2026-55517 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-38549.

When was CVE-2026-55517 published?

CVE-2026-55517 was published on 2026-06-23 and last updated on 2026-06-29.

CVE-2026-55517

CVE-2026-55517 is a medium-severity vulnerability in Deno with a CVSS 3.x base score of 4.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-248.

Key facts

Severity: Medium (CVSS 3.x base score 4.3)
EPSS exploit prediction: 0% (8th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-38549
Weakness: CWE-248
Affected product: Deno
Published: 2026-06-23
Last modified: 2026-06-29

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.

Frequently asked questions

What is CVE-2026-55517?: 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.
How severe is CVE-2026-55517?: CVE-2026-55517 has a CVSS 3.x base score of 4.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is none, integrity none, and availability low.
Is CVE-2026-55517 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (8th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-55517?: CVE-2026-55517 affects Deno. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-55517?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-55517 have an EU (EUVD) identifier?: Yes. CVE-2026-55517 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-38549.
When was CVE-2026-55517 published?: CVE-2026-55517 was published on 2026-06-23 and last updated on 2026-06-29.

References

https://github.com/denoland/deno/security/advisories/GHSA-x2qc-cmh9-f4hf

Affected products (1)

cpe:2.3:a:deno:deno:*:*:*:*:*:*:*:*

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