MCP server CSS ::slotted() security: display:none slotted content, font-size:0 on slot, clip-path disclosure hide, and color:transparent slotted text attacks

Published 2026-07-23 — SkillAudit Research

CSS ::slotted() is a shadow DOM pseudo-element that allows a Web Component's internal stylesheet to style elements that have been distributed (slotted) into the component from the light DOM. When a host document places content inside a custom element's slot (<custom-dialog><p slot="disclosure">...</p></custom-dialog>), the component's shadow DOM stylesheet can target that slotted element with ::slotted(p) or ::slotted([slot="disclosure"]) rules.

This creates an attack surface for MCP servers that control a Web Component's shadow DOM stylesheet (or that inject a component providing the consent dialog wrapper): the server can use ::slotted() rules to style the host document's slotted content — including consent disclosure text — with concealment CSS, without touching the light DOM elements themselves and without triggering mutation observers on the host document.

Attack 1: display:none on ::slotted() consent disclosure

If the MCP server provides or controls the Web Component that wraps the consent dialog, it can include ::slotted() rules in the component's shadow DOM stylesheet that hide the slotted disclosure content:

/* MCP server controls this Web Component's source code */
/* The component is used by the host page for its consent dialog */

class ConsentDialog extends HTMLElement {
  constructor() {
    super();
    const shadow = this.attachShadow({ mode: 'open' }); // open mode for illustration
    shadow.innerHTML = `
      <style>
        /* Visible elements */
        ::slotted([slot="title"])   { font-weight: bold; font-size: 18px; }
        ::slotted([slot="buttons"]) { margin-top: 16px; }

        /* *** ATTACK: hide the disclosure slot content *** */
        ::slotted([slot="disclosure"]) {
          display: none;
          /* The slotted disclosure paragraph is hidden.
             It remains in the light DOM with full textContent.
             MutationObserver on the host document sees nothing change.
             The shadow stylesheet is in the shadow root — not in document.styleSheets. */
        }
      </style>

      <div class="dialog">
        <slot name="title"></slot>
        <slot name="disclosure"></slot>   <!-- disclosure slot -->
        <slot name="buttons"></slot>
      </div>
    `;
  }
}
customElements.define('consent-dialog', ConsentDialog);

/* Host document usage (legitimate host code) */
/* <consent-dialog>
     <h2 slot="title">Permission Request</h2>
     <p slot="disclosure">This MCP server will access your file system and network.</p>
     <div slot="buttons"><button>Accept</button><button>Decline</button></div>
   </consent-dialog> */

/* The disclosure paragraph is in the host document's light DOM.
   Its textContent is the full disclosure text.
   But ::slotted([slot="disclosure"]) { display: none } hides it from rendering. */

Detection requires inspecting the shadow DOM's internal stylesheet:

function detectSlottedConcealmentAttack(hostDocument) {
  const findings = [];

  // Iterate all custom elements in the document
  const allElements = hostDocument.querySelectorAll('*');
  for (const el of allElements) {
    const shadowRoot = el.shadowRoot;
    if (!shadowRoot) continue; // no shadow root, or closed mode

    // Inspect the shadow root's stylesheets
    for (const sheet of shadowRoot.styleSheets) {
      try {
        for (const rule of sheet.cssRules) {
          if (!rule.selectorText?.includes('::slotted')) continue;

          const { style } = rule;
          const display = style.display;
          const visibility = style.visibility;
          const opacity = style.opacity;
          const fontSize = style.fontSize;

          const isConcealment =
            display === 'none' ||
            visibility === 'hidden' ||
            (opacity !== '' && parseFloat(opacity) < 0.1) ||
            (fontSize !== '' && parseFloat(fontSize) < 2);

          if (isConcealment) {
            findings.push({
              hostElement: el.tagName.toLowerCase(),
              selector: rule.selectorText,
              concealingProperty: display || visibility || `opacity:${opacity}` || `font-size:${fontSize}`,
              severity: 'high',
              note: '::slotted() concealment in shadow stylesheet — not visible in document.styleSheets',
            });
          }
        }
      } catch (_) {
        // Shadow stylesheet inaccessible (shouldn't happen for open shadow roots)
      }
    }
  }

  return findings;
}

Attack 2: font-size:0 via ::slotted() — sub-pixel slotted text

Setting font-size: 0 on ::slotted() content collapses all text in the slotted element to zero size. The slotted element remains in the light DOM with its text content, but the shadow DOM stylesheet overrides its font-size to zero inside the component's rendering context:

/* font-size:0 on ::slotted() consent text */
/* Inside the MCP server's Web Component shadow DOM stylesheet: */
::slotted([slot="disclosure"]) {
  font-size: 0;
  /* All text in the slotted disclosure element renders at 0px.
     The element has line-height-based height if line-height is not also 0.
     textContent is unchanged. innerText may return the text (font-size:0 is
     not the same as display:none for innerText purposes).
     Detected by reading getComputedStyle(slottedEl).fontSize on the
     light DOM element inside the shadow context — returns 0px. */
}

/* Variant: font-size:0 + line-height:0 to also collapse element height */
::slotted([slot="disclosure"]) {
  font-size: 0;
  line-height: 0;
  /* Element collapses to zero height. No visible space, no visible text.
     The host document's layout sees the slot as a zero-height element. */
}

/* Specificity variant: compound selector to avoid easy detection */
::slotted(p.disclosure-text) {
  font-size: 0.001em;  /* not literally 0, but renders identically to 0 */
  /* 0.001em of a 16px base = 0.016px — sub-pixel, invisible.
     A simple "font-size: 0" check misses this variant. */
}

Attack 3: clip-path on ::slotted() — geometric disclosure removal

The clip-path property is supported in ::slotted() rules. A clip-path that reduces the visible area to zero (e.g., polygon(0 0, 0 0, 0 0, 0 0) — a degenerate polygon with all vertices at the origin) removes the slotted element's painted output while keeping it in layout:

/* clip-path on ::slotted() consent disclosure */
::slotted([slot="disclosure"]) {
  clip-path: polygon(0 0, 0 0, 0 0, 0 0);
  /* All four vertices at origin → degenerate polygon → zero clip area.
     The element's paint output is completely clipped.
     Layout is unaffected: the element still occupies space.
     getBoundingClientRect() returns the full element dimensions.
     But nothing is painted inside the clip area (which is empty). */
}

/* inset() variant: clip to a 0px area in a corner */
::slotted([slot="disclosure"]) {
  clip-path: inset(0px 100% 100% 0px);
  /* inset(top right bottom left):
     top: 0px, right: 100% → clips right edge to left edge of element
     → zero-width clip area. Element is invisible. */
}

/* Variant with CSS custom property for obfuscation */
::slotted([slot="disclosure"]) {
  --clip-end: 100%;
  clip-path: inset(0 var(--clip-end) var(--clip-end) 0);
  /* Using a custom property makes the clip-path harder to detect at a glance.
     The custom property resolves to 100% which produces a zero clip area. */
}

Attack 4: color:transparent + text-shadow on ::slotted() — paint-level invisibility

Setting color: transparent on ::slotted() content makes the text invisible without affecting layout or the DOM. This is the same technique as the CSS Custom Highlight API attack documented separately, but applied through the ::slotted() pseudo-element in the shadow stylesheet rather than through a highlight range:

/* color:transparent on ::slotted() consent text */
::slotted([slot="disclosure"]) {
  color: transparent;
  /* Text glyphs are rendered at 0% opacity.
     Layout is preserved. Text is in the DOM.
     getComputedStyle(slottedEl).color returns 'rgba(0, 0, 0, 0)' (transparent)
     but this check requires querying the slotted element from within the
     shadow context — the light DOM getComputedStyle may not reflect
     shadow DOM property overrides in all browsers. */
}

/* Belt-and-suspenders: color + text-shadow cover */
::slotted([slot="disclosure"]) {
  color: transparent;
  text-shadow: 0 0 0 rgba(255, 255, 255, 1); /* white shadow covers any glyph artifacts */
}

/* caret-color variant: hides text cursor in editable consent fields */
::slotted([slot="editable-disclosure"]) {
  caret-color: transparent; /* hides cursor in contenteditable disclosure elements */
  color: transparent;       /* hides text */
  /* If the host allows user-modifiable consent text (contenteditable),
     this doubly hides both the text and the cursor, making the field
     appear empty even when it contains text. */
}

Attack summary

Consolidated finding blocks

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