{"componentChunkName":"component---src-templates-post-template-js","path":"/ctf-cybersecurity-rumble-2023-en","result":{"data":{"markdownRemark":{"id":"3a3af57e-c941-5a18-884f-aa1069d9b75f","html":"
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This page has been machine-translated from the original page.

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I participated in Cyber Security Rumble CTF 2023, which started on 7/8, as part of 0nePadding, and we finished 35th out of 622 teams.

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There did not seem to be many Japanese participants this time, so somehow we ended up as the top-ranked team from Japan, which was a surprise haha.

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As usual, I will go through the writeups.

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SHELLCODE-CEPTION(Rev)

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I think I lost my flag in some kind of inception. Can you help me find my flag?

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Decompiling the challenge binary produced the following output.

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\n \n \n \n \n \n \n \n \n

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The program XORs data embedded in the binary, stores it in memory, and then uses mprotect to set attributes such as PROT_EXEC on that region.

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Reference: mprotect(2) - Linux manual page

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From this, I inferred that the program had functionality to execute shellcode decoded at runtime.

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So I used the following script to extract the shellcode into a file.

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data = b'\\x3c\\x21\\xe0\\x8c\\x21\\xd1\\x6b\\x7b\\x7a\\x53\\x58\\x1c\\x4b\\x43\\x21\\xd3\\x41\\x46\\x4f\\x77\\x19\\x46\\x4b\\x1b\\x21\\xe0\\x2c\\xb9\\x21\\xe0\\x3c\\xb1\\x21\\xd1\\x58\\x5c\\x19\\x18\\x46\\x77\\x4b\\x1c\\x21\\xd3\\x46\\x77\\x4a\\x4d\\x77\\x1c\\x46\\x46\\x21\\xe0\\x2c\\x89\\x21\\xe0\\x3c\\x81\\xae\\x2c\\x99\\x47\\x51\\x19\\x46\\x0f\\xae\\x2c\\x9d\\x4f\\x55\\xaf\\x2c\\x9f\\x69\\xae\\x2c\\x95\\x69\\x69\\x69\\x69\\x82\\x75\\xe2\\x2c\\x95\\x21\\xf1\\x66\\xdf\\x2d\\x6c\\xb9\\xea\\x99\\x28\\xe0\\xab\\xe2\\x2c\\x95\\x21\\xf1\\xe1\\x3d\\x6c\\xb9\\xea\\x2c\\x95\\x68\\xea\\x14\\x95\\x4c\\x17\\xb7\\xae\\x2c\\x91\\x69\\x69\\x69\\x69\\x82\\x67\\xe2\\x2c\\x91\\x21\\xf1\\xaf\\x2d\\x6c\\xb9\\x69\\xea\\x2c\\x91\\x68\\xea\\x14\\x91\\x4c\\x17\\x85\\xf9\\x34\\xaa'\ndist = b''\nfor d in data:\n    dist += (d^0x69).to_bytes(1,'big')\nwith open(\"shellcode\", \"wb\") as f:\n    f.write(dist)
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Further analyzing the extracted shellcode in Ghidra produced the following result.

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\n \n \n \n \n \n \n \n \n

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The logic itself was simple, so I wrote the following solver and recovered the flag.

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import struct\n\na = 0x2a2275313a131202\nb = 0x72222f701e262f28\nc = 0x75221e2f71703531\nd = 0x2f2f751e24231e2f\ne = 0x2f70382e\nf = 0x3c26\n\n# https://docs.python.org/ja/3.9/library/struct.html\ndata = struct.pack(\"<qqqqih\", a, b, c, d,e,f)\nfor d in data:\n    print(chr(d^0x41),end=\"\")\n\n# CSR{p4cking_1nc3pt10n_c4n_be_4nnoy1ng}
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LIGHTBULB(Rev)

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I have a very nice light bulb at home, and I found out I can switch with my phone using the app I wrote.

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But the app only works on my phone, so good luck switching my light!

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The challenge binary is provided as an APK file.

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Running it in an emulator showed that it was an app where you log in with a password and then toggle a light switch on and off.

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First, after unpacking the APK with apktool and looking through the files, I found that 583908295080, which is used as the login password, was stored in plaintext.

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Then, to trace what happened after login, I used Smali2Java to decompile the Smali files into Java and inspected the result.

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This showed that LightSwitchingActivity performs the following processing.

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String string = sharedPreferences.getString(\"secret_key\", \"\");\nLog.d(\"LIGHTSWITCH\", \"the sk was \" + string);\nIntrinsics.checkNotNull(string);\nbyte[] bytes = string.getBytes(Charsets.UTF_8);\nIntrinsics.checkNotNullExpressionValue(bytes, \"this as java.lang.String).getBytes(charset)\");\nAPIKeyHash aPIKeyHash = new APIKeyHash(bytes);\nStringBuilder sb = new StringBuilder(\"CSR{\");\nbyte[] bytes2 = \"APIKEY\".getBytes(Charsets.UTF_8);\nIntrinsics.checkNotNullExpressionValue(bytes2, \"this as java.lang.String).getBytes(charset)\");\nthis.apiKey = sb.append(LightSwitchingActivityKt.toHex(aPIKeyHash.hash(bytes2))).append('}').toString();\nRequestQueue newRequestQueue = Volley.newRequestQueue((Context) this);\nIntrinsics.checkNotNullExpressionValue(newRequestQueue, \"newRequestQueue(this)\");\nthis.requestQueue = newRequestQueue;\nSwitchCompat findViewById = findViewById(2131231147);\nIntrinsics.checkNotNullExpressionValue(findViewById, \"findViewById(R.id.switch_light_bulb)\");\nfindViewById.setOnCheckedChangeListener(new LightSwitchingActivity$.ExternalSyntheticLambda0(this));
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Here, it first converts the secret_key string—which is also the login password—into a byte array and then constructs an APIKeyHash object.

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After that, it converts the string APIKEY into a byte array and passes it to the hash method of APIKeyHash.

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I found that feeding the resulting byte array returned by hash into LightSwitchingActivityKt.toHex() produced the flag.

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As before, I used Smali2Java to decompile APIKeyHash and LightSwitchingActivityKt into Java so I could inspect their implementations.

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// LightSwitchingActivityKt \npackage club.redrocket.lightbulb;\n\nimport kotlin.Metadata;\nimport kotlin.jvm.internal.Intrinsics;\n/* compiled from: LightSwitchingActivity.kt */\n@Metadata(d1 = {\"\\u0000\\u0012\\n\\u0000\\n\\u0002\\u0010\\u0019\\n\\u0000\\n\\u0002\\u0010\\u000e\\n\\u0002\\u0010\\u0012\\n\\u0000\\u001a\\n\\u0010\\u0002\\u001a\\u00020\\u0003*\\u00020\\u0004\\\"\\u000e\\u0010\\u0000\\u001a\\u00020\\u0001X\\u0082\\u0004¢\\u0006\\u0002\\n\\u0000¨\\u0006\\u0005\"}, d2 = {\"HEX_CHARS\", \"\", \"toHex\", \"\", \"\", \"app_release\"}, k = 2, mv = {1, 8, 0}, xi = 48)\n/* loaded from: /tmp/jadx-9359415826287627730.dex */\npublic final class LightSwitchingActivityKt {\n    private static final char[] HEX_CHARS;\n\n    static {\n        char[] charArray = \"0123456789ABCDEF\".toCharArray();\n        Intrinsics.checkNotNullExpressionValue(charArray, \"this as java.lang.String).toCharArray()\");\n        HEX_CHARS = charArray;\n    }\n\n    public static final String toHex(byte[] bArr4) {\n        Intrinsics.checkNotNullParameter(bArr4, \"<this>\");\n        StringBuffer stringBuffer = new StringBuffer();\n        for (byte b : bArr4) {\n            char[] cArr = HEX_CHARS;\n            stringBuffer.append(cArr[(b & 240) >>> 4]);\n            stringBuffer.append(cArr[b & 15]);\n        }\n        String stringBuffer2 = stringBuffer.toString();\n        Intrinsics.checkNotNullExpressionValue(stringBuffer2, \"result.toString()\");\n        return stringBuffer2;\n    }\n}\n\n// APIKeyHash \npackage club.redrocket.lightbulb;\n\nimport kotlin.Metadata;\nimport kotlin.jvm.internal.Intrinsics;\n/* compiled from: APIKeyHash.kt */\n@Metadata(d1 = {\"\\u0000\\u001a\\n\\u0002\\u0018\\u0002\\n\\u0002\\u0010\\u0000\\n\\u0000\\n\\u0002\\u0010\\u0012\\n\\u0002\\b\\u0005\\n\\u0002\\u0010\\u0002\\n\\u0002\\b\\u0002\\u0018\\u00002\\u00020\\u0001B\\u000f\\b\\u0000\\u0012\\u0006\\u0010\\u0002\\u001a\\u00020\\u0003¢\\u0006\\u0002\\u0010\\u0004J\\u000e\\u0010\\u0006\\u001a\\u00020\\u00032\\u0006\\u0010\\u0007\\u001a\\u00020\\u0003J\\b\\u0010\\b\\u001a\\u00020\\tH\\u0002J\\u0006\\u0010\\n\\u001a\\u00020\\tR\\u000e\\u0010\\u0002\\u001a\\u00020\\u0003X\\u0082\\u0004¢\\u0006\\u0002\\n\\u0000R\\u000e\\u0010\\u0005\\u001a\\u00020\\u0003X\\u0082\\u0004¢\\u0006\\u0002\\n\\u0000¨\\u0006\\u000b\"}, d2 = {\"Lclub/redrocket/lightbulb/APIKeyHash;\", \"\", \"key\", \"\", \"([B)V\", \"s\", \"hash\", \"plaintext\", \"initializeS\", \"\", \"reset\", \"app_release\"}, k = 1, mv = {1, 8, 0}, xi = 48)\n/* loaded from: /tmp/jadx-2851593408582218842.dex */\npublic final class APIKeyHash {\n    private final byte[] key;\n    private final byte[] s;\n\n    public APIKeyHash(byte[] bArr) {\n        Intrinsics.checkNotNullParameter(bArr, \"key\");\n        this.key = bArr;\n        this.s = new byte[256];\n        if ((bArr.length == 0) || bArr.length > 256) {\n            throw new IllegalArgumentException(\"key length must be between 1 and 256\");\n        }\n        initializeS();\n    }
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