UMass CTF 2026 - Take a Slice

Posted on In Views: Disqus:

It’s in the name!

Initial Analysis

We are given a file cake. Binary STL files, introduced in 1987, do not have a mandatory magic number. The first 80 bytes are reserved for a header (comments/description), which in this case are all zeros.

1
2
3
4
5
6
7
8
9
10
11
❯ xxd cake.stl | head
00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000050: 2a99 0000 59a9 053f 24a9 05bf 78a4 2cbf  *...Y..?$...x.,.
00000060: f669 d740 6242 6140 2fd6 8b40 2a71 d940  .i.@bBa@/..@*q.@
00000070: 5a24 6540 63e7 8b40 bbb3 d940 3d3c 6440  Z$e@c..@...@=<d@
00000080: c774 8c40 0000 11a9 053f efa8 05bf d9a4  .t.@.....?......
00000090: 2cbf 3c49 d940 2cc4 5e40 4840 8e40 c748  ,.<I.@,.^@H@.@.H

Because of this lack of a fixed signature, tools like file may simply identify it as data.

Solution

Using ImHex with an STL file pattern, we can confirm the file structure. The triangleCount field (2a99 at offset 0x50) reveals that there are 39,210 triangles in the model.

1
triangleCount -> 2a99 -> 39210

By importing the model into Blender and zooming into its interior, we can find the flag model floating in the center.

Flag

UMASS{SL1C3_&_D1C3}