PDF security issues¶
OCRmyPDF should only be used on PDFs you trust. It is not designed to protect you against malware.
Recognizing that many users have an interest in handling PDFs and applying OCR to PDFs they did not generate themselves, this article discusses the security implications of PDFs and how users can protect themselves.
The disclaimer applies: this software has no warranties of any kind.
PDFs may contain malware¶
In short, PDFs may contain viruses.
This article describes a high-paranoia method which allows potentially hostile PDFs to be viewed and rasterized safely in a disposable virtual machine. A trusted PDF created in this manner is converted to images and loses all information making it searchable and losing all compression. OCRmyPDF could be used restore searchability.
How OCRmyPDF processes PDFs¶
OCRmyPDF must open and interpret your PDF in order to insert an OCR layer. First, it runs all PDFs through pikepdf, a library based on qpdf, a program that repairs PDFs with syntax errors. This is done because, in the author’s experience, a significant number of PDFs in the wild especially those created by scanners are not well-formed files. qpdf makes it more likely that OCRmyPDF will succeed, but offers no security guarantees. qpdf is also used to split the PDF into single page PDFs.
Finally, OCRmyPDF rasterizes each page of the PDF using Ghostscript in
Depending on the options specified, OCRmyPDF may graft the OCR layer into the existing PDF or it may essentially reconstruct (“re-fry”) a visually identical PDF that may be quite different at the binary level. That said, OCRmyPDF is not a tool designed for sanitizing PDFs.
Using OCRmyPDF online or as a service¶
OCRmyPDF is not designed for use as a public web service where a malicious user could upload a chosen PDF. In particular, it is not necessarily secure against PDF malware or PDFs that cause denial of service. OCRmyPDF relies on Ghostscript, and therefore, if deployed online one should be prepared to comply with Ghostscript’s Affero GPL license, OCRmyPDF’s GPL license, and any other licenses.
Setting aside these concerns, a side effect of OCRmyPDF is it may incidentally sanitize PDFs that contain certain types of malware. It runs
qpdf to repair the PDF, which could correct malformed PDF structures that are part of an attack. When PDF/A output is selected (the default), the input PDF is partially reconstructed by Ghostscript. When
--force-ocr is used, all pages are rasterized and reconverted to PDF, which could remove malware in embedded images.
OCRmyPDF should be relatively safe to use in a trusted intranet, with some considerations:
Limiting CPU usage¶
OCRmyPDF will attempt to use all available CPUs and storage, so executing
nice ocrmypdf or limiting the number of jobs with the
-j argument may ensure the server remains available. Another option would be run OCRmyPDF jobs inside a Docker container, a virtual machine, or a cloud instance, which can impose its own limits on CPU usage and be terminated “from orbit” if it fails to complete.
Temporary storage requirements¶
OCRmyPDF will use a large amount of temporary storage for its work, proportional to the total number of pixels needed to rasterize the PDF. The raster image of a 8.5×11” color page at 300 DPI takes 25 MB uncompressed; OCRmyPDF saves its intermediates as PNG, but that still means it requires about 9 MB per intermediate based on average compression ratios. Multiple intermediates per page are also required, depending on the command line given. A rule of thumb would be to allow 100 MB of temporary storage per page in a file – meaning that a small cloud servers or small VM partitions should be provisioned with plenty of extra space, if say, a 500 page file might be sent.
To check temporary storage usage on actual files, run
ocrmypdf -k ... which will preserve and print the path to temporary storage when the job is done.
To change where temporary files are stored, change the
TMPDIR environment variable for ocrmypdf’s environment. (Python’s
tempfile.gettempdir() returns the root directory in which temporary files will be stored.) For example, one could redirect
TMPDIR to a large RAM disk to avoid wear on HDD/SSD and potentially improve performance. On Amazon Web Services,
TMPDIR can be set to empheral storage.
To prevent excessively long OCR jobs consider setting
--skip-big is particularly helpful if your PDFs include documents such as reports on standard page sizes with large images attached - often large images are not worth OCR’ing anyway.
The author also provides professional services that include OCR and building databases around PDFs, and is happy to provide consultation.
Abbyy Cloud OCR is a viable commercial alternative with a web services API.
Password protection, digital signatures and certification¶
Password protected PDFs usually have two passwords, and owner and user password. When the user password is set to empty, PDF readers will open the file automatically and marked it as “(SECURED)”. While not as reliable as a digital signature, this indicates that whoever set the password approved of the file at that time. When the user password is set, the document cannot be viewed without the password.
Either way, OCRmyPDF does not remove passwords from PDFs and exits with an error on encountering them.
qpdf, one of OCRmyPDF’s dependencies, can remove passwords. If the owner and user password are set, a password is required for
qpdf. If only the owner password is set, then the password can be stripped, even if one does not have the owner password.
After OCR is applied, password protection is not permitted on PDF/A documents but the file can be converted to regular PDF.
Many programs exist which are capable of inserting an image of someone’s signature. On its own, this offers no security guarantees. It is trivial to remove the signature image and apply it to other files. This practice offers no real security.
Important documents can be digitally signed and certified to attest to their authorship. OCRmyPDF cannot do this. Open source tools such as pdfbox (Java) have this capability as does Adobe Acrobat.