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Pixel owners have struggled with editing their screenshots using the default markup tool. Paint inputs aren’t great at retouching anything, even if you brush a spot vigorously, and the cropping tool lacks any preset aspect ratios. But another reason not to use markup is why you want to look at where or who you sent your images to.
Researchers Simon Aarons and David Buchanan have publicly announced that they’re calling it “aCropalypse” which, in essence, allows anyone to take a screenshot of a cropped PNG in Android’s default markup tool and undo at least some edits to produce parts of the image that weren’t intended for rendering. . While the exploit was reported to Google and patched in the March security update for the Pixel (see CVE-2023-21036), redacted images were submitted on certain platforms — including, but not limited to, Discord before mid-January — during Pictures last several years could be at risk of exposure.
You can see the exploit in action with your images with this demo tool provided by the researchers of acropalypse. We also obtained more information about the detection and debugging of this exploit by researchers before it was published on this web page.
What’s wrong, the human-friendly version
The technical aspects of the exploit seem to stem from an API change in Android 10 (see an IssueTracker thread from 2021 and a general explanation from Redditor OatmealDome). Before changing, the application tasked with writing new data to an existing file truncates said file by default if the amount of that new data is less than what the original file holds. With the change, this truncation behavior is no longer the default.
Therefore, if the amount of new data is less than what the current file holds, because writing data occurs sequentially, the back end of the current file will remain, as it is, as part of the new file. This change provided some negative consequences: if the old data is not needed for the new file, it at least takes up precious storage space; If the data is of a sensitive nature, those with the right tools can read it and extract it on their own for malicious uses.
While the issue was eventually deemed “fixed” – as far as we can tell, the guidelines have changed on which writing mode implementations you should use – the encoder still uses this uncut writing mode.
Courtesy: acropalypse
Aarons consulted Buchanan about this vulnerability regarding PNG screenshots (not JPEG images) on January 2 and Buchanan was quickly able to develop a proof of concept for the current exploit. The two reported the error to Google on the same day. The company acknowledged the error on January 3. The fix was finalized internally on January 24th, but it didn’t start rolling out to Pixel devices until March 13th with the month’s security patch.
The engineer posted on his personal blog about the actual goings-on, but from a high-level perspective, here’s what’s going on: PNG file format compresses data through certain operations across a chain of blocks. Any given block of compressed data maybe They contain references to the previous data block, which theoretically paves the way for the previous data blocks to be decompressed one by one. Thus, Buchanan was able to develop a decompression method that combines a look-back aspect with some cryptographic investigative work that focuses on intact file data and stage to get context on what needs to be uncovered.
impact
As you know by now, the upshot is that an image edited in the encoder can contain parts of the original unedited image after the fact with some knowledge. In addition to Pixels, some non-Pixel Android devices and custom ROMs use coding.
While most online platforms do their own processing (such as further compression or metadata stripping) on user-uploaded images, Discord wasn’t processing image uploads sufficiently to prevent this exploit from working. aCropalypse researchers say the instant chat app began stripping redundant data from files on January 17. But this means that tagged images dating back to late 2019 can be decompressed to reveal information not intended for public viewing.
While Discord has rich search tools to help users find image files they may have shared, you might have a lot of uncapped content from screenshots to sift through, and every screenshot you sent might not be exploitable – we tested a screenshot on a Pixel 6 a against the demo tool and we couldn’t get a result, but we did notice other users reporting hands-on results online.
Buchanan said on his blog that he wrote a script to scour his Discord uploads for any vulnerable images and found “a lot of them.” While most of them were generally harmless, he did have a screenshot of the eBay order and was able to extract his full mailing address from it.
