Disallow Lists

Disallow lists let you restrict which image formats and transforms PreviewProxy will accept. This reduces attack surface by preventing processing of formats or operations that your application does not need.

PP_INPUT_DISALLOW_LIST

Blocks requests where the source image is of a specific format.

PP_INPUT_DISALLOW_LIST=pdf,psd,video

Accepted tokens:

Token	Blocks
`jpeg`	JPEG source images
`png`	PNG source images
`gif`	GIF source images
`webp`	WebP source images
`avif`	AVIF source images
`jxl`	JPEG XL source images
`bmp`	BMP source images
`tiff`	TIFF source images
`pdf`	PDF source documents
`psd`	Photoshop PSD files
`video`	Video file inputs

PP_OUTPUT_DISALLOW_LIST

Blocks requests that ask for a specific output format via the format parameter.

PP_OUTPUT_DISALLOW_LIST=bmp,tiff,ico

Accepted tokens:

Token	Blocks
`jpeg`	JPEG output
`png`	PNG output
`gif`	GIF output
`webp`	WebP output
`avif`	AVIF output
`jxl`	JPEG XL output
`bmp`	BMP output
`tiff`	TIFF output
`ico`	ICO output

PP_TRANSFORM_DISALLOW_LIST

Blocks requests that use specific transform operations.

PP_TRANSFORM_DISALLOW_LIST=watermark,gif_anim

Accepted tokens:

Token	Blocks
`resize`	Width/height resize
`rotate`	Image rotation
`flip`	Horizontal/vertical flip
`grayscale`	Grayscale conversion
`brightness`	Brightness adjustment
`contrast`	Contrast adjustment
`blur`	Blur filter
`watermark`	Watermark overlay
`gif_anim`	Animated GIF processing

Behavior

Multiple tokens are comma-separated with no spaces
Unknown tokens are silently ignored (a warning is logged)
Requests that use a disallowed format or transform receive an error response