diff --git a/src/lib/image.ts b/src/lib/image.ts
index 6603c37..7fe8b59 100644
--- a/src/lib/image.ts
+++ b/src/lib/image.ts
@@ -20,6 +20,18 @@ function isAllowedMimeType(mimeType: string): mimeType is AllowedMimeType {
 	return (ALLOWED_MIME_TYPES as readonly string[]).includes(mimeType);
 }
 
+const SHARP_FORMAT_TO_MIME: Record<string, AllowedMimeType> = {
+	jpeg: "image/jpeg",
+	png: "image/png",
+	gif: "image/gif",
+	webp: "image/webp",
+};
+
+/** GIF magic bytes: "GIF87a" or "GIF89a" */
+function isGifData(data: Buffer): boolean {
+	return data.length >= 6 && data.toString("ascii", 0, 3) === "GIF";
+}
+
 export async function processImage(
 	data: Buffer,
 	mimeType: string,
@@ -37,13 +49,28 @@ export async function processImage(
 		);
 	}
 
-	// GIFs: pass through (sharp's animated GIF support is limited, GIFs rarely have EXIF)
+	// GIFs: validate magic bytes, then pass through
+	// (sharp's animated GIF support is limited, GIFs rarely have EXIF)
 	if (mimeType === "image/gif") {
+		if (!isGifData(data)) {
+			throw new ImageValidationError("File content is not a valid GIF");
+		}
 		return { data, mimeType };
 	}
 
-	// Auto-orient (applies EXIF rotation) and strip metadata
+	// Auto-orient (applies EXIF rotation) and strip metadata.
+	// sharp validates magic bytes internally -- throws on non-image data.
 	const { default: sharp } = await import("sharp");
-	const processed = await sharp(data).rotate().toBuffer();
+	const instance = sharp(data);
+	const meta = await instance.metadata();
+	const detectedMime = meta.format
+		? SHARP_FORMAT_TO_MIME[meta.format]
+		: undefined;
+	if (detectedMime && detectedMime !== mimeType) {
+		throw new ImageValidationError(
+			`MIME type mismatch: claimed ${mimeType} but content is ${detectedMime}`,
+		);
+	}
+	const processed = await instance.rotate().toBuffer();
 	return { data: processed, mimeType };
 }