Description

CommandStart(deep_link=True, deep_link_encoded=True) only catches UnicodeDecodeError when it decodes the payload. If the payload is not valid base64url, urlsafe_b64decode raises binascii.Error instead, and that error escapes the filter. This happens, for example, when the payload length is 4k+1: /start a, /start abcde, or a referral link that was cut short when copied.

Because the filter raised, the observer stops at that handler and the update never reaches the handlers registered after it, such as a plain CommandStart() fallback. The user gets no reply and the dispatcher logs "Cause exception while process update". Anyone can trigger this by typing /start a.

@router.message(CommandStart(deep_link=True, deep_link_encoded=True))
async def referral(message: Message): ...

@router.message(CommandStart())
async def plain_start(message: Message): ...

• /start dGVzdA -> referral (as expected)
• /start a -> binascii.Error: Invalid base64-encoded string: number of data characters (1) cannot be 1 more than a multiple of 4, and plain_start is never called

The fix also catches binascii.Error and turns it into CommandException, the same way an undecodable UTF-8 payload is already handled. The filter now returns False and propagation continues.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Added /start a and /start abcde cases to TestCommandFilter.test_parse_command. Both fail with binascii.Error without the fix and pass with it.
• pytest tests: 2327 passed, 56 skipped (redis/mongo not configured)
• ruff format --check and ruff check --preview on the changed files, and mypy aiogram

Test Configuration:

• Operating System: Windows 10
• Python version: 3.11.9

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes