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VulnerabilityPublished Updated

CVE-2026-27125: sveltejs svelte · Mass Assignment

Affects sveltejs svelteSource CVE Database V5

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Not known to be exploited · CVSS 5.3 Medium : upgrade to 5.51.5 when convenient.

Data as of 29 Oct 2026, 19:04 UTC

CVSS 4.0 base score
5.3Medium
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.4% chance of exploitation in 30 days – higher than 37% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 5.51.5
CVSS:4.0/AV:N/AC:H/AT:P/PR:L/UI:N/VC:L/VI:L/VA:N/SC:H/SI:H/SA:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 20 Feb 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

What to do

Upgrade sveltejs svelte to 5.51.5.

Affected software

ProductAffectedFixed in
sveltejssvelteAffected
  • <5.51.5
Fixed in
  • 5.51.5

AI-suggested

The primary mitigation is to upgrade all Svelte instances to version 5.51.5 or later, where the vulnerability is fixed by restricting attribute spreading to own properties only. Additionally, organizations should audit their JavaScript environments to prevent pollution of Object.prototype, which is a prerequisite for exploitation. Implementing strict Content Security Policies (CSP) can help mitigate the impact of unexpected attributes if they lead to injection attacks. Developers should review SSR code for usage of attribute spreading and consider sanitizing or validating input objects before spreading attributes. Monitoring server logs for SSR errors or unexpected HTML attributes can help detect exploitation attempts. Finally, applying runtime protections that detect prototype pollution attempts can reduce the risk of this vulnerability being exploited.

Description

svelte performance oriented web framework. Prior to 5.51.5, in server-side rendering, attribute spreading on elements (e.g. <div {...attrs}>) enumerates inherited properties from the object's prototype chain rather than only own properties. In environments where Object.prototype has already been polluted — a precondition outside of Svelte's control — this can cause unexpected attributes to appear in SSR output or cause SSR to throw errors. Client-side rendering is not affected. This vulnerability is fixed in 5.51.5.

References

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