CVE-2026-42567Network attack vector

Application hang via exponential tag validation

Published Jun 9, 2026 · Updated Jun 9, 2026

Regular expression denial of service in Svelte 5.51.5 through 5.55.6 allows remote authenticated users to hang application processing. The runtime validation for a dynamic svelte:element tag applies an internal regular expression whose evaluation time grows exponentially on crafted input. Exposure requires an application to accept unconstrained-length dynamic tag names; predetermined tag lists or prior length limits prevent the CPU-exhaustion path.

CVSS severity5.9
Medium
EPSS probability0.45%
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 Svelte 5.51.5 through 5.55.6 allows remote authenticated users to hang application processing. The runtime validation for a dynamic svelte:element tag applies an internal regular expression whose evaluation time grows exponentially on crafted input. Exposure requires an application to accept unconstrained-length dynamic tag names; predetermined tag lists or prior length limits prevent the CPU-exhaustion path.

The record

CVE
CVE-2026-42567
Published
Jun 9, 2026
Updated
Jun 9, 2026
Vendor
Svelte
Product
Svelte
Classifications
CWE-1333, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jun 9, 2026CVE publishedPublication date reported by the CVE source.
  2. Jun 9, 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=>= 5.51.5, < 5.55.7

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Svelte · Svelteversion=>= 5.51.5, < 5.55.7

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.9GitHub, Inc.CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-1333Inefficient Regular Expression Complexity
T1499.004Application or System Exploitation Flood

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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