Full Report
Uncovering the validity of a PDF by utilizing some of the tools and methods to detect changes made to a PDF, and understand the limitations in proving PDF integrity.
Analysis Summary
# Tool/Technique: PDF Forensic Integrity Analysis
## Overview
PDF Forensic Integrity Analysis is a collection of techniques and tools used to verify the validity of a PDF document. Its primary purpose is to detect unauthorized modifications, identify obscured content (redaction failures), and validate the authenticity of digital signatures to ensure a document has not been tampered with post-signing.
## Technical Details
- **Type**: Technique / Forensic Analysis
- **Platform**: Cross-platform (Windows, macOS, Linux)
- **Capabilities**: Version extraction, signature verification, object inspection, and hidden layer identification.
- **First Seen**: N/A (Methodology based on the PDF ISO 32000 specification)
## MITRE ATT&CK Mapping
- **[TA0005 - Defense Evasion]**
- **[T1027 - Obfuscated Files or Information]**: Using PDF layers or annotations to hide sensitive data or malicious code.
- **[T1553.002 - Sub-vert Trust Controls: Code Signing]**: Tampering with signed documents to bypass integrity checks.
- **[TA0007 - Discovery]**
- **[T1083 - File and Directory Discovery]**: Extracting metadata and hidden objects to understand document history.
## Functionality
### Core Capabilities
- **Signature Validation**: Checking the cryptographic integrity of digital signatures to confirm authorship and detect post-signature changes.
- **Version Comparison**: Utilizing the PDF "Incremental Update" feature to view the document as it existed at the time of signing compared to its current state.
- **Metadata Inspection**: Analyzing modification times, author fields, and producer strings (though these are noted as easily spoofable).
### Advanced Features
- **Object Extraction**: Identifying stamp annotations, ink annotations, and text overlays used to manually redact or obscure content.
- **Layer Analysis**: Revealing hidden image objects or "overlay" images that may be invisible during standard viewing but persist in the file structure.
## Indicators of Compromise
- **Behavioral Indicators**:
- Adobe Acrobat warning: “There have been subsequent changes to the document.”
- Discrepancy between "Creation Date" and "Modification Date" in file properties.
- Presence of multiple `%%EOF` markers, indicating incremental updates or appended data.
- Presence of "Redaction" boxes that are actually just black rectangles (annotations) rather than removed text.
## Associated Threat Actors
- While not attributed to a specific group in this context, these techniques are used to counter:
- **Insider Threats**: Employees altering legal or financial documents.
- **Social Engineering Actors**: Forging invoices or official communications.
## Detection Methods
- **Signature-based detection**: Using PDF readers (like Adobe Acrobat Pro) to verify the validity of the certificate chain.
- **Behavioral/Structural detection**:
- Scanning for specific PDF objects like `/Annots` (Annotations) or `/XObject` (External Objects) that overlay original text.
- **YARA rules**: Can be used to flag PDFs containing multiple versions or suspicious incremental updates.
## Mitigation Strategies
- **Prevention measures**:
- Apply "Finalize" or "Flatten" operations to documents before distribution to merge all layers and annotations.
- Use high-assurance digital signatures (AATL) for official documents.
- **Hardening recommendations**:
- Implement automated PDF sanitization to strip metadata and hidden layers.
- Require multi-factor authentication for document signing processes.
## Related Tools/Techniques
- **Adobe Acrobat Pro**: Used for signature inspection and viewing previous versions.
- **PDF-Processing Tool**: Used to extract specific versions and embedded image objects.
- **pdfimages (Poppler-utils)**: Used to identify, locate, and extract all image objects within a PDF to find hidden overlays.
- **Redaction**: The technique of permanently removing data, which, if failed, leads to the need for these forensic techniques.