Miggo at BlackHat 2026!
Easily map the attack path and prioritize which CVEs are a threat to your organization
Miggo AIGenerate vendor-ready rules for the observed attack patterns, plus reasoning and safe deployment guidance
Miggo AIRoot Cause AnalysisThe vulnerability is a DOM Clobbering issue in Svelte, which can lead to Cross-Site Scripting (XSS). The root cause was Svelte's internal mechanism for managing component state, which relied on storing data in properties directly on DOM elements using predictable string names (e.g., __attributes, __className, __style).
An attacker could exploit this by using Svelte's attribute spreading feature (<element {...user_controlled_data}>) to supply attributes with names that match Svelte's internal properties, such as { name: '__attributes', value: '...' }. This would "clobber" or overwrite the framework's internal state, leading to unpredictable behavior. In the worst case, this could be escalated to XSS by tricking the attribute_effect function into setting dangerous HTML attributes like innerHTML.
The patch addresses this vulnerability comprehensively:
Symbols. Since Symbols are unique and cannot be created from user-provided strings in attributes, this effectively isolates Svelte's internal state from the DOM and prevents it from being overwritten.get_setters function was updated to explicitly block dangerous attributes (innerHTML, textContent, innerText) from being set via attribute spreading, closing a direct XSS vector.The identified vulnerable functions (attribute_effect, get_setters, set_class, set_style, set_text, and attributes) are the exact locations in the codebase where this vulnerable pattern was present and subsequently fixed. During an exploit, these functions would be on the call stack as they process the malicious, clobbering attributes.
attribute_effectpackages/svelte/src/internal/client/dom/elements/attributes.js
get_setterspackages/svelte/src/internal/client/dom/elements/attributes.js
set_classpackages/svelte/src/internal/client/dom/elements/class.js
set_stylepackages/svelte/src/internal/client/dom/elements/style.js
set_textpackages/svelte/src/internal/client/render.js
attributespackages/svelte/src/internal/server/index.js
| Package Name | Ecosystem | Vulnerable Versions | First Patched Version |
|---|---|---|---|
| svelte | npm | <= 5.55.6 | 5.55.7 |