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URI malformed

Fix "URI malformed" when decoding Base64

The Base64 decoded, but the bytes are not valid UTF-8, so turning them into text failed. The payload is probably binary or uses another encoding.

Input that triggers it

/9j/4AAQSkZJRgABAQEAYABgAAD/2wBDAAgGBgcGBQgHBwcJCQgKDA==
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What the error means

Decoding Base64 has two steps. First the characters become bytes; that part succeeded. Then, to show you text, the bytes are interpreted as UTF-8. URI malformed is the message JavaScript's decodeURIComponent throws when a byte sequence is not valid UTF-8, and it surfaces here because that function is the usual way to turn decoded bytes into a string. In short: the data is fine, but it is not text.

The example is the first few dozen bytes of a JPEG file. /9j/4AAQSkZJRg is the Base64 spelling of the JPEG signature and the JFIF marker, and no image's bytes form valid UTF-8.

Why it happens

  • The payload is binary. Images, PDFs, ZIP archives, protobuf messages, and encrypted blobs are routinely Base64-encoded for transport. They decode to bytes, never to text.
  • The text is in another encoding. Latin-1, Windows-1252, Shift JIS, and UTF-16 all produce byte sequences that UTF-8 rejects. An é in Latin-1 is a single byte 0xE9, which on its own is invalid UTF-8.
  • Compressed text. Gzip or deflate output is binary even when the original was plain text.
  • A truncated multi-byte character. Valid UTF-8 cut off in the middle of a 3- or 4-byte sequence fails at the boundary.

How to fix it

  1. Decode to hex with Text to Hex or look at the first bytes in the tool above. FF D8 FF is JPEG, 89 50 4E 47 is PNG, 25 50 44 46 is PDF, 1F 8B is gzip, 50 4B is ZIP. A recognisable signature means you should not be decoding to text at all.
  2. For an image, use Base64 Image Converter, which renders the bytes as a picture and offers a download.
  3. For text in a legacy encoding, decode to bytes and convert from that encoding (TextDecoder("windows-1252") in JavaScript, .decode("latin-1") in Python) rather than assuming UTF-8.
  4. For compressed data, decompress first, then decode the text.

If it still fails

  • If only the end of the output is affected, the input was truncated mid-character. Check the source length.
  • decodeURIComponent throws the same URI malformed for a % sequence that is not valid percent-encoding, which is a different problem with the same name; if the input was a URL rather than Base64, look for a stray %.
  • Nothing pasted here leaves your browser, so a payload from a production message queue can be inspected safely.

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