cpio filename too long error

[drsn00ker]

I'm looking at some SerComm firmware. Neither unblob nor binwalk successfully extract them. The unblob log shows a CPIO filename too long error. As that an easy fix, or is that a symptom that these firmware are not that easy to extract?

[drsn00ker]

RT-FL-1_SERCOMM_RTRU_1006.zip

[drsn00ker]

I'm guessing just limiting filename length does not fix this error?

[kissgyorgy]

I don't think that's a CPIO file. Isn't it encrypted?

[drsn00ker]

I have no idea if it is encrypted. How would I check?

[kissgyorgy]

Here is the entropy of the .img file inside the zip (when you run unblob with -vvv):

                                         Entropy distribution
   ┌───────────────────────────────────────────────────────────────────────────────────────────────┐
100┤ •• Shannon entropy (%)        •••••♰♰♰••••••••••••••••••••••••••••••••••••••••••••••••••••••• │
 90┤ ♰♰ Chi square probability (%)     ♰  ♰    ♰        ♰                ♰           ♰♰          • │
 80┤      ♰♰     ♰♰    ♰    ♰  ♰ ♰     ♰  ♰ ♰ ♰ ♰♰  ♰♰ ♰♰        ♰   ♰♰ ♰ ♰♰        ♰ ♰          • │
 70┤      ♰♰ ♰  ♰  ♰♰ ♰♰   ♰♰  ♰ ♰     ♰  ♰♰ ♰♰  ♰ ♰ ♰ ♰♰     ♰ ♰♰  ♰ ♰ ♰  ♰       ♰♰ ♰  ♰        •│
 60┤      ♰♰♰ ♰ ♰    ♰♰♰  ♰ ♰  ♰ ♰    ♰   ♰♰ ♰♰  ♰ ♰  ♰♰ ♰   ♰♰ ♰♰  ♰ ♰ ♰   ♰♰♰♰  ♰♰♰ ♰ ♰ ♰        │
 50┤      ♰ ♰  ♰♰      ♰ ♰   ♰ ♰  ♰   ♰   ♰♰ ♰♰  ♰ ♰  ♰♰ ♰  ♰ ♰ ♰♰  ♰  ♰♰      ♰ ♰  ♰ ♰ ♰  ♰ ♰     │
 40┤     ♰♰     ♰      ♰ ♰   ♰ ♰  ♰ ♰♰♰   ♰♰ ♰♰  ♰ ♰   ♰ ♰ ♰   ♰♰♰ ♰    ♰      ♰ ♰    ♰♰   ♰♰♰     │
 30┤♰ ♰♰♰ ♰             ♰     ♰   ♰♰  ♰   ♰♰ ♰♰  ♰ ♰     ♰ ♰    ♰ ♰♰           ♰♰     ♰♰   ♰♰♰    ♰│
 20┤♰♰                             ♰      ♰♰  ♰  ♰♰       ♰       ♰♰           ♰♰      ♰   ♰♰♰   ♰ │
 10┤ ♰                                     ♰      ♰                ♰            ♰          ♰♰ ♰♰♰  │
  0┤                                                                                        ♰      │
   └┬─┬─┬─┬─┬──┬──┬─────┬────┬──┬──┬────┬──┬──┬──┬──┬──┬───┬────┬──┬───┬───┬────┬──┬──┬───┬──┬──┬──┘
    1 3 4 6 8 10 13    18   22 25 27   32 34 37 39 42 44  48   52 55  58  61   66 68 71  74 77 79
                                             173906 bytes

Which means it's encrypted or compressed, or as the Chi probability shows some structure, it also might be some unknown format we don't have a handler for.

If you can't provide some information what kind of format is this, we probably can't do much about it.

[kissgyorgy]

I poked around with Claude and it found that

It's a firmware image file for a SERCOMM router (RTRU_1006 model). The file contains version "1.0.06". Based on the hex dump, it appears to be a router firmware image with embedded version information and likely contains compressed filesystem data
and bootloader code.
The firmware image contains MTD (Memory Technology Device) references

[drsn00ker]

Thanks for looking @kissgyorgy. I just downloaded all the firmware from 4pda forums and ran them through unblob, looking for something interesting. Based on feedback from local users, Sercomm is very common brand, so there was more interest in examining its particular firmware.