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wdormann's profile
Will Dormann
Will Dormann
Will Dormann
@wdormann

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Will Dormann

@wdormann

Vulnerability Analyst at the CERT/CC. My thoughts are my own, not my employer's.

Joined August 2012

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    1. Will Dormann‏ @wdormann Jan 10
      • Report Tweet
      • Report NetzDG Violation

      Now that Twitter has changed how it handles uploaded images, this unexpected behavior is perhaps more important now than before. Your challenge: Tell me what I've redacted from this image. (Anybody I've talked to about this so far is ineligible to play) It can be done w/o tools.pic.twitter.com/Gg4WXFBljR

      53 replies 520 retweets 1,264 likes
      Show this thread
    2. Will Dormann‏ @wdormann Jan 10
      • Report Tweet
      • Report NetzDG Violation

      Answer: Several apps (e.g. @GIMP_Official, @Apple Preview) do not actually delete content from images with an alpha channel. They simply create an alpha-channel tunnel through the content you think that you're removing. You may think you've removed content, but it's just hidden.pic.twitter.com/vCWi80CH33

      5 replies 48 retweets 150 likes
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    3. Will Dormann‏ @wdormann Jan 10
      • Report Tweet
      • Report NetzDG Violation

      If you remove the alpha channel, you now can see what's behind it. You can do this with ImageMagick, e.g. convert input.png -alpha off output.png You now have an image that doesn't have the alpha channel, so therefore is unredacted. But it's actually even easier than this!pic.twitter.com/wOuA8gJ7g5

      4 replies 27 retweets 191 likes
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    4. Will Dormann‏ @wdormann Jan 10
      • Report Tweet
      • Report NetzDG Violation

      When you open an image in Twitter, there are URI arguments. For example, for the image for this thread, it's: https://pbs.twimg.com/media/EN8zPZeXsAMynMK?format=png&name=360x360 … What if you change "format=png" to "format=jpg" ? https://pbs.twimg.com/media/EN8zPZeXsAMynMK?format=jpg&name=360x360 … You how get the image, but Twitter removes the alpha channel for you!

      6 replies 41 retweets 221 likes
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      Will Dormann‏ @wdormann Jan 10
      • Report Tweet
      • Report NetzDG Violation

      Similarly, even if you export directly to something that doesn't support alpha channels in Gimp (e.g. JPEG), by default you'll get a thumbnail in the EXIF data. Any guesses as to what that thumbnail contains?pic.twitter.com/7zAEjpXuHV

      5:35 PM - 10 Jan 2020
      • 9 Retweets
      • 94 Likes
      • FractalEncrypt Asad Khan قەزوان Simen Nishant Bhandari Nico stays at Home Alejandro Liberman Charge Lane, Kueng, & Thao as Accessories 😷 Mithgol the Webmaster
      3 replies 9 retweets 94 likes
        1. New conversation
        2. Will Dormann‏ @wdormann Jan 11
          • Report Tweet
          • Report NetzDG Violation

          I've found Apple Preview and GIMP to not remove image data properly. If you wish to test your own app, the easiest way is to: 1) Open pic with an alpha channel 2) Cut out content 3) Save as PNG 4) Send it to https://compress-or-die.com/analyze pic.twitter.com/o1OoIBpBqG

          3 replies 8 retweets 53 likes
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        3. Will Dormann‏ @wdormann Jan 11
          • Report Tweet
          • Report NetzDG Violation

          And actually, as I verify this further, I believe that I was mistaken to say that Apple Preview is affected. I cannot seem to reproduce what I thought I originally saw with Preview. So perhaps just GIMP for now. I'm curious if anybody can confirm another misbehaving application.

          3 replies 2 retweets 19 likes
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        4. Will Dormann‏ @wdormann Jan 11
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          And apparently this is "expected behavior" for GIMP users? That is, when a user deletes pixels (e.g. Some are selected and the "Delete" key is pressed on the keyboard), the user should expect that the pixels aren't deleted. They are just made transparent. Neat.pic.twitter.com/s5DDnZ0uPq

          5 replies 8 retweets 40 likes
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        5. Will Dormann‏ @wdormann Jan 11
          • Report Tweet
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          I can't help but think that this is the developer equivalent of "I reject your reality and substitute my own."pic.twitter.com/BUeVnU0v4z

          7 replies 4 retweets 60 likes
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        6. End of conversation
        1. New conversation
        2. GIMP‏ @GIMP_Official Jan 11
          • Report Tweet
          • Report NetzDG Violation
          Replying to @wdormann

          That last tweet reads like GIMP doesn't support alpha channel in JPEG. But JPEG itself doesn't support alpha channel :) The only hackaround we know of is embedding PNG and then extracting it in a special way.

          1 reply 0 retweets 3 likes
        3. Will Dormann‏ @wdormann Jan 11
          • Report Tweet
          • Report NetzDG Violation
          Replying to @GIMP_Official

          Yeah, I'm aware that JPEG doesn't support alpha channels. But it does support image thumbnails (via EXIF)! The problem that GIMP has is that the the unredacted image is the thumbnail. Presumably due to GIMP removing the alpha channel to create it. I'd consider that unexpected.

          1 reply 0 retweets 1 like
        4. 1 more reply
        1. Daniel Ruf‏ @DanielRufde Jan 11
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          Replying to @wdormann

          People in InfoSec and others probably remove metadata (can be easily done and checked).

          0 replies 0 retweets 0 likes
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