The matchesSingleElement() function of the InternalSchemaBinDataEncryptedTypeExpression match expression checks an FLE1-encrypted BinData field to determine whether the first byte of the encrypted blob has the correct value (either 0x01 for 'deterministic', or 0x02 for 'random'). Then, it performs an unsafe cast of the BinData to a FleBlobHeader structure, without first checking the size, before reading and verifying the originalBsonType field of the header, which could potentially be outside the actual binary data buffer. If the BinData input is somehow malformed such that it is shorter than the size of FleBlobHeader, and the subsequent bytes in the BSON object have the correct values so as to pass validation of the type, then the match expression could allow this malformed document to pass schema validation of FLE1 fields, and therefore allow it to be inserted.