$ pdfinfo 18pages.pdf Title: 18 Pages Creator: LaTeX with hyperref Producer: pdfTeX-1.40.21 CreationDate: D:20260312123456-04'00' ModDate: D:20260312123500-04'00' Tagged: no Pages: 18 Encrypted: no Page size: 595.276 x 841.89 pts (A4) The file looks like an ordinary PDF with (as the title hints).
That concludes the write‑up for the challenge on Hdhub4u. Happy hacking! 18 Pages Hdhub4u
To be thorough, we also checked whether any other objects contained additional base‑64 or XOR‑encoded data, but none yielded a flag. $ pdfinfo 18pages
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: To be thorough, we also checked whether any
> 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…).