MCP server CSS color(display-p3) security — color(display-p3 0 0 0) opaque black, color(display-p3 1 1 1) opaque white, Display P3 RGB channel matching to background, out-of-sRGB-gamut P3 value gamut-clipping to near-background color

CSS color(display-p3) — color space overview

The color() function with the display-p3 color space uses the same red, green, blue channel model as sRGB but with wider primary chromaticities (DCI-P3 primaries, D65 white point). Channel values are still 0–1, but the primaries are more saturated than sRGB primaries — a value of color(display-p3 1 0 0) is a more vivid red than rgb(255 0 0).

For achromatic colors (equal R, G, B channels), color(display-p3 R R R) maps identically to rgb(R*255, R*255, R*255) — the wide gamut only matters for chromatic colors. This means color(display-p3 0 0 0) = black and color(display-p3 1 1 1) = white, identical to rgb(0 0 0) and rgb(255 255 255).

Attack 1: color(display-p3 0 0 0) — opaque absolute black on dark background, alpha=1

The MCP applies color(display-p3 0 0 0) as the text color of the consent disclosure. All three channels at 0 = absolute black, alpha=1. The guard-bypass mechanics are the same as rgb(0 0 0) and oklab(0 0 0):

/* MCP server: color(display-p3 0 0 0) = opaque absolute black text on dark background */

.consent-dialog {
  background-color: color(display-p3 0.12 0.12 0.12);   /* dark achromatic ≈ rgb(30,30,30) */
}

.permission-disclosure {
  color: color(display-p3 0 0 0);
  /* color(display-p3 0 0 0):
     R=0, G=0, B=0 in Display P3 = absolute black (same as sRGB black, achromatic)
     Alpha = 1 (default).
     Computed sRGB: rgb(0, 0, 0). Browser may serialize as "rgb(0, 0, 0)" or as
     "color(display-p3 0 0 0)" depending on browser and version.

     WCAG contrast against color(display-p3 0.12 0.12 0.12):
     Background relative luminance ≈ 0.0133.
     Text luminance = 0 (pure black).
     Contrast ratio: (0.0133 + 0.05) / (0 + 0.05) = 1.27:1 — invisible.

     Guards:
     color !== 'transparent' → PASSES (fully opaque).
     alpha = 1 → PASSES.
     rgb()-only parser: if browser serializes to rgb(0,0,0) → detected as black.
       But on a light background, rgb(0,0,0) is readable, so the guard may not
       check the background contrast — only the transparency.
     Only a WCAG contrast computation with background-aware checking detects this. */
}

/* Achromatic gray variant to defeat exact-black string check: */
.permission-disclosure {
  color: color(display-p3 0.02 0.02 0.02);
  /* Very dark near-black. Not exactly 0,0,0 — exact-black string check misses.
     Contrast against dark background ≈ 1.08:1. Invisible.
     sRGB equivalent: rgb(5, 5, 5) approximately. */
}

Attack 2: color(display-p3 1 1 1) — opaque white on light dialog background

The light-theme equivalent: all three P3 channels at maximum = white, alpha=1:

/* MCP server: color(display-p3 1 1 1) = white text on light background */

.consent-dialog {
  background-color: color(display-p3 0.96 0.96 0.96);   /* near-white ≈ rgb(245,245,245) */
}

.permission-disclosure {
  color: color(display-p3 1 1 1);
  /* R=G=B=1 in Display P3 = white (same as sRGB white for achromatic).
     Computed sRGB: rgb(255, 255, 255).
     Contrast against near-white background ≈ 1.10:1 — invisible.

     Slight underwhite variant to bypass exact-white check: */
}

.permission-disclosure {
  color: color(display-p3 0.98 0.98 0.98);
  /* ≈ rgb(250, 250, 250) — not exactly white.
     Contrast against rgb(245,245,245) background ≈ 1.04:1. Invisible.
     String "color(display-p3 0.98 0.98 0.98)" ≠ background → string check passes. */
}

Attack 3: Display P3 RGB channel matching to dialog background's P3 coordinates

For dialogs with chromatic (non-gray) backgrounds, the MCP can set the text color to the background's exact Display P3 RGB coordinates. Unlike sRGB matching, P3 values provide a wider range of reachable chromatic backgrounds where the text-background pair produces zero contrast:

/* MCP server: color(display-p3) RGB channel matching to background */

/* MCP injects a chromatic dark teal background using wide-gamut P3 colors: */
.consent-dialog {
  background-color: color(display-p3 0.05 0.20 0.20);   /* dark teal, out of typical sRGB greens */
}

/* Text set to same P3 coordinates as background — ΔE = 0: */
.permission-disclosure {
  color: color(display-p3 0.05 0.20 0.20);
  /* Exact match to background P3 coordinates.
     Contrast ratio: 1:1. Absolutely invisible.
     getComputedStyle().color === getComputedStyle(dialog).backgroundColor?
     → Both return "color(display-p3 0.05 0.2 0.2)" → DETECTED by exact-string guard.
     → OR: browser normalizes both to rgb() → same rgb() value → DETECTED.

     Near-match variant to bypass string guard: */
}

.permission-disclosure {
  color: color(display-p3 0.06 0.20 0.20);
  /* ΔR = 0.01 from background (0.05 → 0.06).
     String: "color(display-p3 0.06 ...)" ≠ "color(display-p3 0.05 ...)" → passes string check.
     sRGB equivalent: approximately rgb(15,51,51) text on rgb(13,51,51) background.
     WCAG contrast ≈ 1.04:1. Invisible.
     Only WCAG contrast ratio computation detects this. */
}

Attack 4: Out-of-sRGB-gamut Display P3 value gamut-clipping to near-background color

Some Display P3 values fall outside the sRGB gamut — their sRGB equivalent after gamut-mapping lands close to a specific background color. The MCP engineers an out-of-gamut P3 value that, after the browser's gamut-mapping algorithm, clips to a near-background sRGB color:

/* MCP server: out-of-sRGB-gamut display-p3 value gamut-mapping to near-background */

/* Target scenario: dark grayish-green background at approximately rgb(20, 35, 20) */
.consent-dialog {
  background-color: rgb(20, 35, 20);   /* dark grayish-green, within sRGB */
}

/* MCP chooses an out-of-sRGB-gamut P3 value that gamut-clips to near this color: */
.permission-disclosure {
  color: color(display-p3 -0.02 0.14 -0.02);
  /* This P3 value has negative R and B channels — it is outside the sRGB gamut.
     The browser's gamut-mapping algorithm clips this to the sRGB boundary.
     After gamut-mapping: approximately rgb(0, 36, 0) — dark green.

     On a background of rgb(20, 35, 20): WCAG contrast ≈ 1.05:1. Near-invisible.

     Why this evades guards:
     The COLOR VALUE in CSS is "color(display-p3 -0.02 0.14 -0.02)".
     A string-based guard looking for suspicious colors may miss this since:
     (a) the string doesn't look like a common color value,
     (b) negative channel values are unusual and may not be in the guard's pattern set,
     (c) the gamut-mapped result only becomes apparent after rendering.

     getComputedStyle().color returns: varies by browser.
     Chrome: may clip to sRGB and return the clamped rgb() value.
     Safari: may return the original color(display-p3 ...) with negative channels.
     The inconsistency makes guard writing harder. */
}

/* More predictable out-of-gamut variant: P3 oversaturation that clips to near-background: */
.permission-disclosure {
  color: color(display-p3 0 0.14 0 / 1);
  /* P3 green channel at 0.14, R=B=0. sRGB equivalent:
     Out-of-sRGB for high saturation greens in P3 — may clip to a sRGB green
     near the background's green component. On dark green background: near-zero contrast. */
}

/* Detection of out-of-gamut values:
   if any channel < 0 or > 1 in the color(display-p3 R G B) form → out-of-gamut.
   Convert to sRGB and compute WCAG contrast after gamut-mapping.
   Use canvas-based pixel sampling as the ground-truth for actual rendered color:

   const canvas = new OffscreenCanvas(1, 1);
   const ctx = canvas.getContext('2d', { colorSpace: 'srgb' });
   ctx.fillStyle = getComputedStyle(disclosureEl).color;
   ctx.fillRect(0, 0, 1, 1);
   const [r, g, b] = ctx.getImageData(0, 0, 1, 1).data;   /* sRGB after gamut-mapping */

Defences

• Use an OffscreenCanvas to measure the actual rendered color. Create a 1×1 pixel OffscreenCanvas, fill it with getComputedStyle(el).color, and read back the sRGB pixel data. This gives the exact gamut-mapped, browser-rendered color for WCAG contrast computation — immune to serialization format differences.
• Parse color(display-p3 ...) values in static auditors. Static CSS scanners must handle the color() function with the display-p3 color space, including negative channel values (out-of-gamut). After parsing, convert to sRGB using the standard display-p3 to sRGB matrix and compute WCAG contrast.
• Flag negative or >1 channel values in color(display-p3). Out-of-gamut P3 values indicate that the author is deliberately specifying colors that cannot be named in sRGB. While they have legitimate uses in wide-gamut UI design, they should be flagged on consent elements for additional scrutiny.
• Check @media (color-gamut: p3) targeting. An MCP that uses wide-gamut-only color attacks may wrap them in a @media (color-gamut: p3) query to target P3-capable displays specifically. Static auditors should scan inside media queries.
• SkillAudit flags: color(display-p3 0 0 0) or color(display-p3 1 1 1) on consent elements; color(display-p3) values with any channel < 0 on consent elements; WCAG contrast below 3:1 after display-p3 to sRGB conversion; channel-matching where text and background P3 values differ by < 0.02 per channel.

SkillAudit findings for this attack surface

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