MCP server SVG feSpecularLighting specularConstant & specularExponent consent security

The feSpecularLighting filter primitive renders specular reflection using the Phong model. The output intensity at each pixel is: I_s = ks × (N·H)^exp, where N is the surface normal derived from the alpha channel (via the surfaceScale bump map), H is the half-vector between the light direction and the view direction, ks is the specularConstant attribute (reflection amplitude), and exp is the specularExponent attribute (angular width of the highlight). The output is then scaled by the light color. The result is blended with the source element — typically via feComposite or feBlend — to create a lighting effect. specularConstant and specularExponent are orthogonal attack vectors: ks controls amplitude (how bright the highlight is), while exponent controls angular spread (how wide the highlight area is).

SA-CSS-KSP-001: specularConstant=100 white-out with overhead light

With specularConstant="100" and an overhead feDistantLight (elevation=90), the half-vector H equals the surface normal N for a flat surface — so (N·H) = 1.0 at every pixel. The output is I_s = 100 × 1.0^exp = 100, which clamps to 1.0 (maximum brightness) after SVG channel clamping. The feSpecularLighting output is a solid white layer covering the entire filter region. When this white layer is composited over the source element via feBlend mode="screen" (screen(A, white) = white), the consent element renders as a solid white rectangle. The consent text DOM, textContent, BCR, and getComputedStyle remain unchanged.

<filter id="ksp1">
  <feSpecularLighting surfaceScale="1"
    specularConstant="100"
    specularExponent="10"
    result="spec">
    <feDistantLight azimuth="90" elevation="90"/>
  </feSpecularLighting>
  <feBlend in="SourceGraphic" in2="spec" mode="screen"/>
</filter>
<!-- ks=100 × (N·H=1)^10 = 100 → clamped to 1 → white; screen(source, white)=white-out -->

SA-CSS-KSP-002: specularExponent=0.001 — flat specular covering entire surface

As specularExponent approaches 0, (N·H)^exp → 1 for all surface normals — including edge-on normals where the half-vector is nearly perpendicular to the normal. The physical interpretation: an exponent near 0 produces a perfectly diffuse specular distribution; the highlight spreads over the entire hemisphere. For specularExponent="0.001" and any light direction, the specular output approximates ks × 1.0 × L_color at every pixel — a flat constant proportional to ks and light brightness. With ks=1 and a white light source, the entire consent element renders with near-maximum specular brightness. Combined with feBlend screen, this produces white-out regardless of the light direction — the attack does not require a specific azimuth or elevation, making it light-direction-agnostic and harder to detect via light parameter analysis.

<filter id="ksp2">
  <feSpecularLighting surfaceScale="1"
    specularConstant="1"
    specularExponent="0.001"
    result="spec">
    <feDistantLight azimuth="0" elevation="45"/> <!-- any direction works -->
  </feSpecularLighting>
  <feBlend in="SourceGraphic" in2="spec" mode="screen"/>
</filter>
<!-- exp=0.001: (N·H)^0.001 ≈ 1 for all N; full-surface specular; direction-agnostic white-out -->

SA-CSS-KSP-003: specularExponent=200 + grazing light — dark-out

At specularExponent="200", the specular lobe is extremely narrow — a near-perfect mirror. With a grazing feDistantLight (azimuth=0, elevation=5), the light comes nearly horizontal. The half-vector H is tilted far from the surface normal N (which is approximately vertical for a flat surface). The dot product (N·H) at most pixels ≈ cos(87.5°) ≈ 0.044. Raising this to the 200th power: 0.044^200 ≈ 10^{-273} ≈ 0. The feSpecularLighting output is near-zero (black) at virtually all pixels. When this near-black is used with feBlend mode="multiply" (multiply(source, 0)=0), the consent element renders as a solid black box. On a dark background, the contrast is 1:1; on a white background, it presents as a redacted bar.

<filter id="ksp3">
  <feSpecularLighting surfaceScale="1"
    specularConstant="5"
    specularExponent="200"
    result="spec">
    <feDistantLight azimuth="0" elevation="5"/> <!-- grazing horizontal light -->
  </feSpecularLighting>
  <feBlend in="SourceGraphic" in2="spec" mode="multiply"/>
</filter>
<!-- exp=200 + grazing: (N·H)^200 ≈ 0; spec≈black; multiply(source, 0)=0 → dark-out -->

SA-CSS-KSP-004: JS mousedown specularConstant injection (1 → 100)

Initially, specularConstant="1" with an overhead light produces a moderate specular highlight only at the exact center-overhead pixel (a small bright point). The consent text is readable around the highlight. On mousedown, the script changes specularConstant to "100". The 100× amplification instantly converts the moderate point highlight to a full-surface white-out. The transition from a small decorative highlight (ks=1) to a white panel (ks=100) is abrupt and instantaneous, occurring at the exact moment the user's click begins.

consentEl.addEventListener("mousedown", () => {
  document.querySelector("#consentFilter feSpecularLighting")
    .setAttribute("specularConstant", "100"); // amplify to white-out
});

Attack summary

Defences

• Flag extreme specularConstant: specularConstant > 10 guarantees white-out for any overhead or near-overhead light direction. Flag specularConstant > 10 on any feSpecularLighting in a consent ancestor filter chain.
• Flag near-zero specularExponent: specularExponent < 0.1 degenerates the Phong model to flat full-surface illumination. Flag specularExponent < 0.1 regardless of specularConstant value, as even ks=1 with exp=0.001 produces white-out.
• Evaluate blending mode after feSpecularLighting: The specular output itself is the white overlay — it requires compositing with the source to produce the final white-out. Check the feBlend or feComposite following the feSpecularLighting result. mode=screen with a bright specular always produces white-out; mode=multiply with a dark specular produces dark-out.
• Simulate rendered output: Apply the full filter chain to a representative dark-text input and compute WCAG contrast ratio. Flag if < 4.5:1. This catches both KSP-001 and KSP-003 without reasoning about Phong model geometry.
• MutationObserver on specularConstant: SA-CSS-KSP-004 changes specularConstant at runtime. Observe this attribute on feSpecularLighting elements in consent ancestor filters. Any post-load change to a value > 10 is a critical flag.

SkillAudit findings for this attack surface

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