Description#

A JPEG image file that appears to be corrupted and cannot be opened.

Analysis#

Examining the file header reveals the corruption:

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$ xxd sigmatour-image.jpg | head -3
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00000000: ff00 0000 0000 0000 4946 0001 0100 0001  ........IF......
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00000010: 0001 0000 ffdb 0043 0002 0101 0101 0102  .......C........
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00000020: 0101 0102 0202 0202 0403 0101 0102 0504  ................

The file starts with FF 00 00 00 00 00 00 00 but a valid JPEG/JFIF file should start with:

• FF D8 - JPEG SOI (Start of Image) marker

• FF E0 - APP0 marker (JFIF)

• 00 10 - Length of APP0 segment (16 bytes)

• 4A 46 49 46 - “JFIF” identifier

Notice that 49 46 (“IF” from “JFIF”) is still present at offset 8, confirming this is a corrupted JFIF header.

Solution#

Restore the correct JPEG/JFIF file signature:

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# Method 1: Using printf and dd
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$ cp sigmatour-image.jpg fixed.jpg
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$ printf '\xff\xd8\xff\xe0\x00\x10\x4a\x46' | dd of=fixed.jpg bs=1 count=8 conv=notrunc
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# Method 2: Using Python
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$ python3 -c "
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with open('sigmatour-image.jpg', 'rb') as f:
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    data = bytearray(f.read())
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# Fix JFIF header (bytes 0-7)
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data[0:8] = b'\xff\xd8\xff\xe0\x00\x10\x4a\x46'
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with open('fixed.jpg', 'wb') as f:
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    f.write(data)
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"
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# Verify the fix
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$ file fixed.jpg
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fixed.jpg: JPEG image data, JFIF standard 1.01, aspect ratio, density 1x1, segment length 16, baseline, precision 8, 2848x1600, components 3

After fixing the header, open the image - the flag is displayed visually within the image itself.

File Signature Reference#

Format Magic Bytes (Hex) ASCII

JPEG/JFIF FF D8 FF E0 xx xx 4A 46 49 46 ÿØÿà..JFIF

JPEG/EXIF FF D8 FF E1 xx xx 45 78 69 66 ÿØÿá..Exif

PNG 89 50 4E 47 0D 0A 1A 0A .PNG…

GIF 47 49 46 38 GIF8