How to sanitize PDFs and images before sharing sensitive content
Sharing documents or images without proper sanitization risks leaking credentials, PII, or intellectual property. This guide gives concrete steps and verification techniques to ensure exports are safe to distribute.
- Understand what to protect and where secrets hide.
- Sanitize metadata/embedded objects, rasterize layered content, and redact at the pixel level.
- Verify with automated extraction/OCR and manual checks; automate in pipelines and enforce via DLP and CI.
Quick answer — Remove metadata/embedded objects, rasterize or flatten layered content, and perform robust pixel-based redaction; then run automated extraction/OCR and manual checks before sharing. For PDFs: strip metadata, attachments, form data, hidden layers, and XObjects; for images: strip EXIF, remove steganographic payloads, and ensure no readable text remains. Integrate DLP, CI checks, and access controls so sanitized exports are the only distributable artifacts.
Quick answer (featured snippet): Before sharing, strip metadata and embedded objects, flatten or rasterize layered content, perform pixel-level redaction (not character deletion), then validate with OCR, extraction tools, and steganography checks; automate these steps in CI/DLP so only sanitized files can be exported.
Identify sensitive content to protect
Start by listing categories of sensitive content that may be present in documents and images. Use this list to guide targeted sanitization rules and tests.
- Personal data: names, SSNs, birthdates, addresses, phone numbers.
- Credentials & secrets: API keys, tokens, passwords, private keys.
- Business IP: designs, source code snippets, financials, contract clauses.
- Hidden/artifacts: comments, revision history, form fields, annotations, attachments.
- Steganographic payloads and EXIF/location metadata in images.
Sanitize metadata and embedded objects
Metadata and embedded content often persist through normal exports. Remove or overwrite these fields before distribution.
- Strip document metadata: title, author, producer, creation/mod dates, custom XMP fields.
- Remove embedded files/attachments and embedded fonts not required for display.
- Clear form field values and flatten or remove interactive form objects.
- Delete comments, annotations, and revision histories.
- For images, remove EXIF, XMP, and GPS coordinates.
| File type | Metadata locations | Tools/commands |
|---|---|---|
| XMP, Info dict, attachments, /Annots, AcroForm | qpdf –strip and exiftool, pdfcpu | |
| JPEG/PNG | EXIF, XMP, PNG tEXt chunks | exiftool -all=, pngcrush –text |
Flatten, rasterize and manage layers
Layered content (PDF Optional Content Groups, Photoshop layers, SVG groups) can conceal data that remains accessible. Convert to a single visual layer or raster image where appropriate.
- Flatten PDFs: remove Optional Content Groups (OCG) and merge form/annotation layers into the visible page.
- Rasterize vector text and objects to pixels at a resolution matching intended use to prevent re-extraction of selectable text.
- For multi-page/vector art, export to high-quality images (PNG/TIFF) then rebuild a new PDF from those images if necessary.
- Retain originals in secured archives; only export flattened/rasterized copies for distribution.
Redact pixels, not characters; verify edits
Text-based redaction (removing characters) can leave information in underlying content streams. Always perform redaction that changes pixel data.
- Use drawing/painting tools or export-to-image + paint over the sensitive area, then re-encode as PDF/image.
- Avoid overlay-only redaction (black rectangles on top of text streams) — this can be reversed or searchable.
- After redaction, re-save at a format that does not preserve editable text (e.g., flattened PDF, PNG).
Example workflow for a PDF with a visible SSN:
- Export page as 300–600 DPI PNG.
- Paint over SSN with opaque color, anti-aliased if needed to match background.
- Recreate PDF page from the edited PNG and replace original page.
Test with extraction, OCR and steganography checks
Verification is as important as sanitization. Run multiple extraction techniques and manual inspection to ensure nothing remains recoverable.
- Text extraction: run pdftotext, pdfminer, or similar to confirm no selectable text remains.
- OCR: run Tesseract or commercial OCR to detect readable content that survived rasterization or low-contrast remnants.
- Attachment scan: list and attempt to extract embedded files/objects (qpdf, pdf-parser).
- Steganography checks: run stego detectors (stegseek variants, statistical analysis) and inspect LSB patterns if images may contain payloads.
- Binary inspection: search raw file bytes for secrets patterns (API token regexes, key headers, base64 sequences).
| Purpose | Example command/tool |
|---|---|
| Extract text | pdftotext file.pdf – | grep -i “password” |
| List objects | qpdf –qdf in.pdf out.pdf; inspect with pdf-parser |
| OCR | tesseract page.png stdout |
| Strip EXIF | exiftool -all= image.jpg |
Automate safety in your processing pipeline
Manual steps are error-prone. Integrate sanitization and verification into automated CI pipelines and DLP gates so only approved artifacts are exportable.
- Enforce pre-export hooks: require sanitized export artifacts and signed attestations from the pipeline.
- Integrate static/dynamic scanning: pattern detection, regex checks for secrets, OCR verification, stego detectors.
- Block exports that fail checks and provide actionable error messages to users.
- Log and audit exports with file hashes and verification results; keep originals in restricted storage.
- Combine access controls: role-based export rights, two-person review for high-sensitivity materials.
Common pitfalls and how to avoid them
- Pitfall: Using black overlay instead of pixel-editing — Remedy: Rasterize and paint; rebuild the file from pixels.
- Pitfall: Leaving metadata or attachments — Remedy: Run metadata stripping tools and verify attachment lists.
- Pitfall: Low-resolution rasterization that makes content still machine-readable — Remedy: Choose resolution matched to OCR thresholds or remove text entirely.
- Pitfall: Not checking steganography — Remedy: Run stego detectors and analyze LSBs when images come from untrusted sources.
- Pitfall: Relying only on manual review — Remedy: Enforce automated checks in CI/DLP and require failing-proof attestations before export.
Implementation checklist
- Inventory sensitive content types and create rulesets.
- Strip metadata/XMP/EXIF and remove embedded objects/attachments.
- Flatten layers and rasterize vector/text where appropriate.
- Apply pixel-level redaction and rebuild file from sanitized pixels.
- Run extraction, OCR, stego and byte-level scans; fail on matches.
- Automate in CI/DLP and enforce access controls for exports.
- Log, audit, and store originals securely with restricted access.
FAQ
- Q: Is adding a black box over text sufficient?
- A: No — overlays can be removed or text may remain searchable. Use pixel-based edits and re-encode so text streams are gone.
- Q: Will stripping EXIF remove hidden payloads?
- A: Stripping EXIF/XMP removes metadata but not steganographic payloads embedded in pixel data; run stego checks and/or re-encode images.
- Q: How can I balance quality and security when rasterizing?
- A: Choose DPI high enough for visual fidelity but low enough to prevent reliable OCR if text must be removed; or remove text entirely via painting.
- Q: Can automation produce false negatives?
- A: Yes — combine automated checks (regex/OCR/stego) with targeted manual review for high-risk exports.
- Q: Should originals be deleted after sanitization?
- A: No — retain originals in a secure, access-controlled archive for records and incident response; only sanitized copies should be distributable.
