[storage] upload hardening

2026-10-28 00:18:58 -07:00 · 2026-10-21 22:50:07 -04:00
parent ae360e1e83
commit 7abef376b6
11 changed files with 361 additions and 59 deletions
@@ -0,0 +1,63 @@
+/**
+ * Recognising an uploaded image by its bytes.
+ *
+ * Every image upload (`api`'s saved images and custom avatar item files, `storage`'s
+ * FileType 3) is served back by the `img` worker, which plays the stored content-type
+ * straight to the browser. Nothing about the upload is trustworthy: the multipart part's
+ * `type` and filename are whatever the client chose to send, so a `text/html` body under a
+ * `.jpg` name is an HTML document on the image origin unless something looks at the bytes.
+ * This does — the stored type and extension come from the signature, never the claim.
+ */
+
+/** What an image's leading bytes say it is. */
+export interface SniffedImage {
+	/** The media type to store and serve it under. */
+	contentType: string
+	/** The extension to key it with, dot included. */
+	extension: string
+}
+
+interface Signature extends SniffedImage {
+	/** Byte sequences that must appear at the given offsets. */
+	parts: Array<{ offset: number; bytes: number[] }>
+}
+
+const ascii = (s: string): number[] => Array.from(s, (ch) => ch.charCodeAt(0))
+
+/** The formats an upload may be — the set the `img` worker decodes and resizes. */
+const SIGNATURES: Signature[] = [
+	{
+		contentType: 'image/png',
+		extension: '.png',
+		parts: [{ offset: 0, bytes: [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a] }],
+	},
+	{
+		contentType: 'image/jpeg',
+		extension: '.jpg',
+		parts: [{ offset: 0, bytes: [0xff, 0xd8, 0xff] }],
+	},
+	{ contentType: 'image/gif', extension: '.gif', parts: [{ offset: 0, bytes: ascii('GIF8') }] },
+	{
+		contentType: 'image/webp',
+		extension: '.webp',
+		parts: [
+			{ offset: 0, bytes: ascii('RIFF') },
+			{ offset: 8, bytes: ascii('WEBP') },
+		],
+	},
+	{ contentType: 'image/bmp', extension: '.bmp', parts: [{ offset: 0, bytes: ascii('BM') }] },
+]
+
+/** The formats `sniffImage` accepts, for an error message. */
+export const SNIFFED_IMAGE_FORMATS = 'PNG, JPEG, GIF, WebP or BMP'
+
+/**
+ * Identify an image by its signature, or `undefined` when the bytes are not one of the
+ * supported formats. Operates on the leading bytes only; it does not validate the rest.
+ */
+export function sniffImage(bytes: Uint8Array): SniffedImage | undefined {
+	const match = SIGNATURES.find(({ parts }) =>
+		parts.every(({ offset, bytes: expected }) => expected.every((b, i) => bytes[offset + i] === b))
+	)
+	return match ? { contentType: match.contentType, extension: match.extension } : undefined
+}