feat(previews): add file signature check before opening files

Signed-off-by: Benjamin Gaussorgues <benjamin.gaussorgues@nextcloud.com>

Author: Altahrim

lib/private/Preview/Bitmap.php

@@ -49,6 +49,13 @@ abstract class Bitmap extends ProviderV2 {
 	 */
 	abstract protected function getAllowedMimeTypes(): string;
 
+	/**
+	 * @return list<string>
+	 */
+	abstract protected function getMagicStrings(): array;
+
+	abstract protected function getImagickFormatHint(): string;
+
 	/**
 	 * {@inheritDoc}
 	 */
@@ -104,15 +111,19 @@ public function getThumbnail(File $file, int $maxX, int $maxY): ?IImage {
 	private function getResizedPreview($tmpPath, $maxX, $maxY) {
 		$bp = new Imagick();
 
+		if (!$this->isMagicStringSupported($tmpPath)) {
+			throw new \Exception('Invalid image type: magic string not recognized');
+		}
+
 		// Validate mime type
-		$bp->pingImage($tmpPath . '[0]');
+		$bp->pingImage($this->getImagickFormatHint() . ':' . $tmpPath . '[0]');
 		$mimeType = $bp->getImageMimeType();
 		if (!preg_match($this->getAllowedMimeTypes(), $mimeType)) {
 			throw new \Exception('File mime type does not match the preview provider: ' . $mimeType);
 		}
 
 		// Layer 0 contains either the bitmap or a flat representation of all vector layers
-		$bp->readImage($tmpPath . '[0]');
+		$bp->readImage($this->getImagickFormatHint() . ':' . $tmpPath . '[0]');
 
 		$bp = $this->resize($bp, $maxX, $maxY);
 
@@ -121,6 +132,22 @@ private function getResizedPreview($tmpPath, $maxX, $maxY) {
 		return $bp;
 	}
 
+	private function isMagicStringSupported(string $filepath): bool {
+		$signatures = $this->getMagicStrings();
+		if (empty($signatures)) {
+			return true;
+		}
+		$length = array_reduce($signatures, static fn (int $carry, string $signature) => max($carry, strlen($signature)), 0);
+		$firstBytes = file_get_contents($filepath, false, null, 0, $length);
+		foreach ($signatures as $signature) {
+			if (str_starts_with($firstBytes, $signature)) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
 	/**
 	 * Returns a resized \Imagick object
 	 *

lib/private/Preview/Font.php

@@ -37,4 +37,17 @@ public function getMimeType(): string {
 	protected function getAllowedMimeTypes(): string {
 		return '/(application|image)\/(?:font-sfnt|x-font|x-otf|x-ttf|x-pfb$)/';
 	}
+
+	#[\Override]
+	protected function getMagicStrings(): array {
+		return [
+			"\x00\x01\x00\x00\x00", // TTF
+			'OTTO', // OTF
+		];
+	}
+
+	#[\Override]
+	protected function getImagickFormatHint(): string {
+		return 'ttf';
+	}
 }

lib/private/Preview/HEIC.php

@@ -116,14 +116,14 @@ private function getResizedPreview($tmpPath, $maxX, $maxY) {
 
 		// Some HEIC files just contain (or at least are identified as) other formats
 		// like JPEG. We just need to check if the image is safe to process.
-		$bp->pingImage($tmpPath . '[0]');
+		$bp->pingImage('heic:' . $tmpPath . '[0]');
 		$mimeType = $bp->getImageMimeType();
 		if (!preg_match('/^image\/(x-)?(png|jpeg|gif|bmp|tiff|webp|hei(f|c)|avif)$/', $mimeType)) {
 			throw new \Exception('File mime type does not match the preview provider: ' . $mimeType);
 		}
 
 		// Layer 0 contains either the bitmap or a flat representation of all vector layers
-		$bp->readImage($tmpPath . '[0]');
+		$bp->readImage('heic:' . $tmpPath . '[0]');
 
 		// Fix orientation from EXIF
 		$bp->autoOrient();

lib/private/Preview/Illustrator.php

@@ -38,4 +38,14 @@ public function getMimeType(): string {
 	protected function getAllowedMimeTypes(): string {
 		return '/application\/(illustrator|pdf)/';
 	}
+
+	#[\Override]
+	protected function getMagicStrings(): array {
+		return ["\x25\x50\x44\x46"];
+	}
+
+	#[\Override]
+	protected function getImagickFormatHint(): string {
+		return 'ai';
+	}
 }

lib/private/Preview/PDF.php

@@ -38,4 +38,14 @@ public function getMimeType(): string {
 	protected function getAllowedMimeTypes(): string {
 		return '/application\/pdf/';
 	}
+
+	#[\Override]
+	protected function getMagicStrings(): array {
+		return ['%PDF-'];
+	}
+
+	#[\Override]
+	protected function getImagickFormatHint(): string {
+		return 'pdf';
+	}
 }

lib/private/Preview/Photoshop.php

@@ -38,4 +38,14 @@ public function getMimeType(): string {
 	protected function getAllowedMimeTypes(): string {
 		return '/(application|image)\/(x-photoshop|x-psd)/';
 	}
+
+	#[\Override]
+	protected function getMagicStrings(): array {
+		return ['8BPS'];
+	}
+
+	#[\Override]
+	protected function getImagickFormatHint(): string {
+		return 'psd';
+	}
 }

lib/private/Preview/Postscript.php

@@ -38,4 +38,14 @@ public function getMimeType(): string {
 	protected function getAllowedMimeTypes(): string {
 		return '/application\/postscript/';
 	}
+
+	#[\Override]
+	protected function getMagicStrings(): array {
+		return ['%!PS'];
+	}
+
+	#[\Override]
+	protected function getImagickFormatHint(): string {
+		return 'ps';
+	}
 }

lib/private/Preview/SGI.php

@@ -37,4 +37,14 @@ public function getMimeType(): string {
 	protected function getAllowedMimeTypes(): string {
 		return '/image\/(x-)?sgi/';
 	}
+
+	#[\Override]
+	protected function getMagicStrings(): array {
+		return ["\x01\xDA"];
+	}
+
+	#[\Override]
+	protected function getImagickFormatHint(): string {
+		return 'sgi';
+	}
 }

lib/private/Preview/TGA.php

@@ -37,4 +37,14 @@ public function getMimeType(): string {
 	protected function getAllowedMimeTypes(): string {
 		return '/image\/(x-)?t(ar)?ga/';
 	}
+
+	#[\Override]
+	protected function getMagicStrings(): array {
+		return [];
+	}
+
+	#[\Override]
+	protected function getImagickFormatHint(): string {
+		return 'tga';
+	}
 }

lib/private/Preview/TIFF.php

@@ -38,4 +38,19 @@ public function getMimeType(): string {
 	protected function getAllowedMimeTypes(): string {
 		return '/image\/tiff/';
 	}
+
+	#[\Override]
+	protected function getMagicStrings(): array {
+		return [
+			"II*\x00",
+			"MM\x00*",
+			"II+\x00",
+			"MM\x00+",
+		];
+	}
+
+	#[\Override]
+	protected function getImagickFormatHint(): string {
+		return 'tiff';
+	}
 }