18 Pages Hdhub4u Today

> echo "The flag is hidden in the zero‑filled stream." Again, a hint directing us toward Object 28. The flag we extracted from Object 28 matches the typical format for the platform (HTB…).

Objects , 37 , and 61 are the most promising candidates for hidden data. 4. Analyzing the suspicious streams 4.1 Object 28 – “mostly zeros” $ pdf-parser -object 28 -raw 18pages.pdf > obj28.bin $ hexdump -C obj28.bin | head 00000000 78 9c 0b 00 00 00 02 00 00 00 00 00 00 00 00 00 |x...............| ... The stream is a Flate‑compressed block that, once decompressed, yields a 2048‑byte buffer full of 0x00 except for a few non‑zero bytes at the very end: 18 Pages Hdhub4u

$ pdf-parser -dump 18pages.pdf > pdf_objects.txt The dump revealed the following interesting points: > echo "The flag is hidden in the zero‑filled stream

Thus the final flag for the challenge is: Happy hacking

| Obj # | Type | Size | Description | |------|--------|------|-------------| | 5 | stream | 832 | /Length 832 /Filter /FlateDecode – looks like a normal content stream | | 12 | stream | 56 | /Length 56 /Filter /FlateDecode – stream, empty page | | 28 | stream | 342 | /Length 342 /Filter /FlateDecode – contains a lot of zero bytes | | 37 | stream | 1024| /Length 1024 /Filter /ASCII85Decode – ASCII85‑encoded data | | 44 | metadata| 124| /Producer (pdfTeX‑1.40.21) – standard | | 61 | stream | 512 | /Length 512 /Filter /FlateDecode – starts with “%PDF‑1.4” inside |

That concludes the write‑up for the challenge on Hdhub4u. Happy hacking!