#How to replace a string at runtime in a compiled .jar

44 messages · Page 1 of 1 (latest)

oblique dock
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Hi guys, i want to replace a string for another string in a compiled .jar, anyone can guide me how to do this? Tried ChatGPT but didn't got any luck lol, just problems

grand cairnBOT
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oblique dock
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For example, when I try this:

import java.io.*;
import java.util.*;
import java.util.jar.*;

public class PlaceholderReplacer {
    public static void main(String[] args) {
        String inputJarPath = "path/to/input.jar";
        String outputJarPath = "path/to/output.jar";
        String placeholder = "your_placeholder";
        String replacement = "your_replacement";

        try {
            replacePlaceholderInJar(inputJarPath, outputJarPath, placeholder, replacement);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static void replacePlaceholderInJar(String inputJarPath, String outputJarPath, String placeholder, String replacement) throws IOException {
        try (JarInputStream jarInputStream = new JarInputStream(new FileInputStream(inputJarPath));
             JarOutputStream jarOutputStream = new JarOutputStream(new FileOutputStream(outputJarPath))) {

            byte[] buffer = new byte[4096];
            JarEntry entry;

            while ((entry = jarInputStream.getNextJarEntry()) != null) {
                String entryName = entry.getName();

                if (entryName.endsWith(".class")) {
                    ByteArrayOutputStream baos = new ByteArrayOutputStream();
                    int bytesRead;
                    while ((bytesRead = jarInputStream.read(buffer)) != -1) {
                        baos.write(buffer, 0, bytesRead);
                    }

                    byte[] classBytes = baos.toByteArray();

                    String modifiedContent = new String(classBytes).replace(placeholder, replacement);
                    byte[] modifiedBytes = modifiedContent.getBytes();
# 
                    JarEntry modifiedEntry = new JarEntry(entryName);
                    modifiedEntry.setSize(modifiedBytes.length);
                    jarOutputStream.putNextEntry(modifiedEntry);
                    jarOutputStream.write(modifiedBytes);
                    jarOutputStream.closeEntry();
                } else {
                    jarOutputStream.putNextEntry(entry);

                    int bytesRead;
                    while ((bytesRead = jarInputStream.read(buffer)) != -1) {
                        jarOutputStream.write(buffer, 0, bytesRead);
                    }

                    jarOutputStream.closeEntry();
                }
            }
        }
    }
}
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My class that got replaced is now when I try to decompile: "java.lang.IllegalStateException: Wrong magic number: 3216895935
at com.strobel.assembler.metadata.ClassFileReader.readClass(ClassFileReader.java:338)
at com.strobel.assembler.metadata.MetadataSystem.resolveType(MetadataSystem.java:129)
at com.strobel.assembler.metadata.MetadataSystem.lookupTypeCore(MetadataSystem.java:86)
at com.strobel.assembler.metadata.MetadataResolver.lookupType(MetadataResolver.java:46)
at us.deathmarine.luyten.Model.openEntryByTreePath(Model.java:338)
at us.deathmarine.luyten.Model$TreeListener$1.run(Model.java:266)
"

mystic oyster
oblique dock
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Not sure if the repo you are sending me will do that but let me know

mystic oyster
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as in, load the jar into your program, modify the class, spit it back out

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instead of modifying it manually

oblique dock
oblique dock
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Basically people will download from my website

mystic oyster
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the link i sent is to a bytecode manipulation tool, that you can use to modify it manually, among other things

oblique dock
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I will inject their identifier

mystic oyster
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in your case you'd have to use asm

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the asm library

oblique dock
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the problem is that he is changing the size of the files

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So it is actually replacing

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but the file get corrupted or smth

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because when I tried he gets me error and there are other files losing some bytes

mystic oyster
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usage is a bit complicated and i wont be able to explain it all, so all i can really do is link you to https://asm.ow2.io/ and wish you good luck

oblique dock
mystic oyster
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strings are funny, especially so with encodings

oblique dock
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Yeah I know, in that case actually I was doing new String() without nothing

mystic oyster
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class files are practically random bytes, the chance that an invalid utf8 sequence got into there is not small

oblique dock
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But when I mention "ISO-8859-1"

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it seems it preserves the actual size

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but can't replace nothing on the file

mystic oyster
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you should replace it on the byte array directly itself

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and even then, the size of the string is stored along with it

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if you change the length of the string in this way, the resulting file will be invalid

oblique dock
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Yeah that make sense

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But the question is, how can i do that lol

mystic oyster
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asm

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that handles everything for you

oblique dock
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yeah but I already mentioned, I'm getting an error, let me get the code

mystic oyster
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you find the instruction with the string you want to replace, you replace the string and export the class file again

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asm handles everything else

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its a pretty based library