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When burning a DVD, there are two options, close or finalize a disk. According to this link: You can't add any additional files to a disc that has been finalized. When you burn a CD or DVD using Windows Explorer, the disc is not finalized.

I tried, it seems if I burn a DVD on Windows, then I can rewrite content of the disk. If I use another software and finalize the disk, trying to modify content of the disk will report error. Though IMO there is just maybe a simple check, and if software/DVD firmware is hacked, I think even a finalized disk can be rewritten.

I often rely on DVD for protected content, e.g. contents that can not be changed by malware or hacker. After finding this out, I doubt that content on a DVD is as protected as I thought.

The question is: Can finalized DVD be rewritten?

What is the method - software or hardware implementation, close or finalize? Can it be override by hacking, e.g. hack firmware to force DVD drive to burn on a finalized disk?

migrated from security.stackexchange.com Oct 14 '14 at 12:34

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    Was the disc inherently rewritable (DVD-RW or DVD+RW)? For non-rewritable discs it does depend on whether the burning tool or OS actually finalises the disc. – LateralFractal Oct 14 '14 at 5:06
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Assuming you are using a Write-Once media, it does not matter if it is finalized or not. Finalizing just tells the software not to read past this point. Depending on what software that reads/writes, a malicious software can write past this point, and if the target media contains a malicious software that reads past this point, then you are "owned" in hacker's parlance.

I guess you ask this question in the terms "are live CDs really secure when it comes to read-onlyness"

However, burning a CD is a physical process where the actual metallic layer is burnt so it no longer reflects light. Archive media (M-DISC) goes further and actually burns a hole in the reflective layer.

This means it's not physically possible to modify written information. Written information often also have certain check codes (CRC, but it can also be software based MD5/SHA checks) that prevent burning additional marks/holes to modify data.

So a conclusion: If you can trust EITHER the reading device, or writing device, it will not be possible to modify a finalized disk past its finalization. However, if BOTH the writing and reading devices are dishonest, it's possible for the devices to collaborate to access data outside of finalization sector.

Modification of a unfinalized disk is simply adding data to the disk telling the OS to "No, delete the file xxx.txt and add the file yyy.txt [content of file yyy.txt]" at the end. Which also means that deleting a file from a CD-R/DVD-R will actually waste space on the disk (all modifications to a write-once media is additions), and the file will of course not be deleted, it will be hidden like a file is deleted on a regular hard drive - fully accessible by file recovery tools.

If you want to be 100 % sure that no data ever is written past finalization, you should fill up the disk to the brim, preferable with files containing random gibberish. That's the only 100 % sure option. As an additional security measure, you could for example store SHA hashes of the random gibberish and then verify that the random gibberish is in fact unmodified, upon loading whatever software or files you have on disk.

One usage case of this, is for example if you are going to load a disk of unsensitive data from a high security class, then, verify that no sensitive data in addition to the unsensitive data was written, by using a second computer, and then using a third computer in a low security area to read the disk and for example publish this unsensitive data on web.

If you don't fill the disk with gibberish, there's a possibility that malicious software on the sensitive computer writes sensitive data past the finalization. When you verify the disk on the second computer, you see no sensitive data. Upon reading the disk in the low security area, malicious software might read past the finalization to leak sensitive data.

  • A CRC is certainly not enough to prevent modification by burning additional marks. I'm pretty certain even a 128 bit cryptographic hash isn't enough. The interactions between low level error correction used on CDs/DVDs and checksums/hashes are pretty tricky as well. – CodesInChaos Oct 14 '14 at 14:27
  • In normal sense, a CRC is not enough. But with the given constraints (that you only can turn 0's to 1's and not the other way around), and same applies for CRC/hash, it will be very hard to create any meaningful leakage data in the sense that it will validate successfully. – sebastian nielsen Oct 15 '14 at 14:39
  • I'm pretty certain that even with the contains you can fix a CRC/hash if you can write garbage into a small part of the message. I'm also not certain if those constrains even apply, since error correction codes might might "correct" a 1 to a 0. – CodesInChaos Oct 15 '14 at 14:41
  • If you look into the physical encoding scheme used by CDs/DVDs (EFM - en.wikipedia.org/wiki/Eight-to-fourteen_modulation#EFMPlus), I think (but am not 100% sure) that it would be very difficult to insert additional 1's and have the decoder return valid data. The modulation scheme depends on having at least two 0's between 1's so "filling them in" would probably just make the decoder unable to read it. – LawrenceC Apr 7 '17 at 0:25
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If you have a DVD-R disk and when burning something to it you selected finalize disk option - then you should not be able to burn nothing more on it - even if it's not full.

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    Not without modifying the optical drive's firmware. – LateralFractal Oct 14 '14 at 15:13

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