CVE-2026-27121Network attack vector

Browser script execution via SSR spread attributes

Published Feb 20, 2026 · Updated Feb 20, 2026

Cross-site scripting in Svelte before 5.51.5 allows remote authenticated users to execute JavaScript through server-rendered spread attributes. During server-side rendering, spread syntax emits event-handler properties from untrusted attribute objects into HTML without excluding executable handlers. Exposure requires an application to spread attacker-controlled or external data onto an element; a rendered handler then runs in a victim's browser.

CVSS severity5.1
Medium
EPSS probability0.19%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site scripting in Svelte before 5.51.5 allows remote authenticated users to execute JavaScript through server-rendered spread attributes. During server-side rendering, spread syntax emits event-handler properties from untrusted attribute objects into HTML without excluding executable handlers. Exposure requires an application to spread attacker-controlled or external data onto an element; a rendered handler then runs in a victim's browser.

The record

CVE
CVE-2026-27121
Published
Feb 20, 2026
Updated
Feb 20, 2026
Vendor
Svelte
Product
Svelte
Classifications
CWE-79, T1059.007
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Feb 20, 2026CVE publishedPublication date reported by the CVE source.
  2. Feb 20, 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

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Svelte · Svelteversion=< 5.51.5

Published CVSS scores

5.1GitHub, Inc.CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:N/VC:L/VI:N/VA:N/SC:H/SI:H/SA:N
5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A: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-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1059.007JavaScript

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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