[P2][Security/Privacy] Raster image tools should not accept raw SVG through generic image policies

[baixiangcpp]

Summary

Generic raster image policies currently allow SVG files in image-standard, image-compact, and image-logo. Those policies are used by tools such as Image Resizer, Photo Censor, Image Cropper, Image Filters, Color Picker, and Caption Generator.

The project already treats SVG as a special security surface in dedicated SVG/Markdown guards and sanitizers. Generic raster image tools should not ingest raw SVG through unsanitized object URLs unless the SVG path has an explicit sanitizer/rasterization boundary.

Why this matters

SVG is not the same risk class as PNG/JPEG/WebP/GIF/AVIF:

• SVG is XML/markup, not a pure raster image.
• SVG can contain links, embedded references, styles, metadata, and other markup-like surfaces.
• Browser behavior differs depending on whether SVG is loaded as <img>, object URL, data URL, canvas source, or parsed text.
• The site's privacy claim depends on avoiding accidental external-resource loads and avoiding ambiguous rendering boundaries.

Dedicated SVG tools already use explicit sanitization paths. Generic raster tools should either reject SVG or route it through an explicit sanitized SVG-to-raster path.

Current behavior

Relevant policy:

"image-standard": {
  accept: "image/*",
  allowedMimePrefixes: ["image/"],
  allowedExtensions: [".png", ".jpg", ".jpeg", ".webp", ".gif", ".svg", ".avif"],
}

Similar .svg entries exist in image-compact and image-logo.

Example path:

• src/features/tools/image-resizer/browser-actions.ts
  • validates with FILE_INPUT_POLICIES["image-standard"];
  • creates URL.createObjectURL(file);
  • loads it into an Image() via loadImageElement();
  • later renders it to canvas/worker.

There is no SVG sanitizer in this generic image path.

Expected behavior

Choose one of these approaches.

Preferred option: exclude SVG from generic raster image policies

• Remove .svg and image/svg+xml acceptance from image-standard, image-compact, and image-logo.
• Keep SVG only in the dedicated svg policy.
• Point users to SVG Optimizer / SVG to PNG / SVG converter tools for SVG workflows.

Alternative option: explicit sanitized SVG raster path

• Detect SVG uploads in generic image tools.
• Sanitize SVG using the shared sanitizer before preview/rasterization.
• Prevent external references from loading.
• Rasterize through a controlled data URL/blob path.
• Add tests proving dangerous SVG payloads are sanitized and no external references remain.

Suggested implementation plan

1. Audit every consumer of:
   • FILE_INPUT_POLICIES["image-standard"]
   • FILE_INPUT_POLICIES["image-compact"]
   • FILE_INPUT_POLICIES["image-logo"]
2. Remove .svg from generic image policies unless a consumer has an explicit sanitizer/raster path.
3. Keep FILE_INPUT_POLICIES.svg for SVG-specific tools.
4. Update upload copy so users understand SVG should use SVG tools.
5. Add guard tests:
   • generic raster policies do not include .svg;
   • dedicated SVG policy still accepts .svg;
   • image-resizer/photo-censor/cropper/filter tools do not silently accept SVG through raster policy.
6. If supporting SVG in raster tools is intentionally required, add sanitizer tests with malicious SVG fixtures.

Acceptance criteria

• Generic raster image policies do not accept raw SVG unless an explicit sanitized SVG raster path exists.
• Dedicated SVG tools continue to accept and sanitize SVG.
• Upload UI copy points SVG users to SVG-specific tools or describes the sanitized path.
• Tests cover policy separation between raster image inputs and SVG inputs.
• Existing image tool tests still pass.
• npm test -- --run tests/unit/file-input-policy.test.ts tests/component/bf-025-bf-031-image-file-safety.test.tsx tests/guards/html-injection-surface-guard.test.ts passes if updated.
• npm run check:types and npm run lint pass.

Related code pointers

• src/core/files/file-input-policy.ts
• src/core/utils/image-canvas-utils.ts
• src/features/tools/image-resizer/browser-actions.ts
• src/core/security/sanitize.ts
• tests/guards/html-injection-surface-guard.test.ts

[baixiangcpp]

Reopening after follow-up source review: the explicit .svg entries were removed, but the acceptance criterion can still be bypassed by conflicting extension/MIME metadata.

Current bypass

hasAllowedFileType() treats an allowed extension and an allowed MIME type as independent alternatives:

if (extension && policy.allowedExtensions?.length && !policy.allowedExtensions.includes(extension)) return false
if (mime && policy.allowedMimePrefixes?.some((prefix) => mime.startsWith(prefix))) return true
if (mime && policy.allowedMimeTypes?.includes(mime)) return true
if (extension && policy.allowedExtensions?.includes(extension)) return true

This file passes image-standard today:

const disguisedSvg = new File(
  ["<svg xmlns='http://www.w3.org/2000/svg'></svg>"],
  "unsafe.png",
  { type: "image/svg+xml" },
)

expect(validateFileAgainstPolicy(
  disguisedSvg,
  FILE_INPUT_POLICIES["image-standard"],
)).toMatchObject({ ok: true })

The .png extension is allowlisted, the SVG MIME is not, but the final extension fallback still returns true.

loadResizeImageFile() then creates an object URL from the original File. The Blob response retains image/svg+xml, so the generic raster path can still receive SVG markup without the dedicated SVG sanitizer.

Test gap

The current policy tests cover:

• unsafe.svg + image/svg+xml;
• unsafe.svg + image/png.

They do not cover the inverse mismatch: an allowed raster extension with an SVG MIME type. That is the reachable bypass under the current OR-style validation.

Required follow-up

• When both extension and MIME are present, require them to agree with the same policy instead of accepting either independently.
• Keep extension fallback only for browsers that provide an empty MIME type.
• For raster-security boundaries, add lightweight file-signature validation before creating object URLs/decoding; extension and caller-controlled MIME metadata are not content validation.
• Audit scan-image, whose image/ MIME-prefix rule can create the same class of mismatch.
• Add regression cases for unsafe.png/image/svg+xml, unsafe.jpg/image/svg+xml, empty MIME with valid raster signature, and spoofed raster MIME with SVG bytes.

The original acceptance criterion — generic raster policies do not accept raw SVG — is therefore not fully met yet.