CVE-2026-66062Network attack vector

Unauthenticated CPU exhaustion via unanchored header regex

Published Aug 7, 2026 · Updated Aug 7, 2026

Regular expression denial of service in SvelteKit before 2.70.2 allows remote attackers to exhaust CPU through crafted Accept headers. The negotiate function in packages/kit/src/utils/http.js applies an unanchored media-type regular expression to each comma-delimited header segment, forcing quadratic scanning when a long segment contains no slash. No authentication or user interaction is required, but default header-length limits on most platforms constrain exposure; raised or absent limits permit service degradation or denial.

CVSS severity5.3
Medium
EPSS probability0.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in SvelteKit before 2.70.2 allows remote attackers to exhaust CPU through crafted Accept headers. The negotiate function in packages/kit/src/utils/http.js applies an unanchored media-type regular expression to each comma-delimited header segment, forcing quadratic scanning when a long segment contains no slash. No authentication or user interaction is required, but default header-length limits on most platforms constrain exposure; raised or absent limits permit service degradation or denial.

The record

CVE
CVE-2026-66062
Published
Aug 7, 2026
Updated
Aug 7, 2026
Vendor
Svelte
Product
SvelteKit
Classifications
CWE-1333, CAPEC-492, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 7, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 7, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2.70.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Svelte · SvelteKitversion=< 2.70.2

Published CVSS scores

5.3GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-1333Inefficient Regular Expression Complexity
CAPEC-492Regular Expression Exponential Blowup
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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