magic_bytes.py
python
sha256:5667a3e21bf16fd2e6d6bd4a769bd1c0cf7634afa12cef6450cc77573196b7f9
asyncpg caps query parameters
Human
patch
8 days ago
| 1 | """Magic-bytes file type validation. |
| 2 | |
| 3 | Prevents polyglot file attacks where a file is served under a trusted |
| 4 | extension (e.g. ``.jpg``) but its actual content is something else (e.g. |
| 5 | a PHP script, a ZIP archive, or an HTML file containing JavaScript). |
| 6 | |
| 7 | ``check_magic_bytes(path, content)`` returns the detected type string and |
| 8 | raises ``PolyglotFileError`` when the declared extension does not match |
| 9 | the content's magic bytes. |
| 10 | |
| 11 | Supported type detection (first-bytes signatures): |
| 12 | MIDI — MThd header |
| 13 | MP3 — ID3 tag or MPEG sync word |
| 14 | WebP — RIFF….WEBP container |
| 15 | PNG — PNG signature |
| 16 | JPEG — JPEG SOI marker |
| 17 | ZIP — PK signature (also covers .muse objects if ever zipped) |
| 18 | PDF — %PDF header |
| 19 | HTML — starts with <!DOCTYPE html or <html |
| 20 | Script — starts with #! (shebang) |
| 21 | JSON / XML are validated by structure, not magic bytes. |
| 22 | |
| 23 | Files whose extension is not in the known-type map are allowed through |
| 24 | without a magic-bytes check (content-addressed storage of arbitrary |
| 25 | source files must not be blocked). |
| 26 | """ |
| 27 | |
| 28 | from collections.abc import Callable |
| 29 | from pathlib import Path |
| 30 | |
| 31 | type _Checker = Callable[[bytes], bool] |
| 32 | type _CheckerEntry = tuple[str, _Checker] |
| 33 | |
| 34 | class PolyglotFileError(ValueError): |
| 35 | """Raised when declared file extension does not match content magic bytes.""" |
| 36 | |
| 37 | # ── Magic byte signatures ───────────────────────────────────────────────────── |
| 38 | |
| 39 | # (offset, expected_bytes) tuples. offset is the byte position to match at. |
| 40 | _MIDI_MAGIC = [(0, b"MThd")] |
| 41 | _MP3_ID3 = [(0, b"ID3")] |
| 42 | _MP3_SYNC_1 = [(0, bytes([0xFF, 0xFB]))] |
| 43 | _MP3_SYNC_2 = [(0, bytes([0xFF, 0xF3]))] |
| 44 | _MP3_SYNC_3 = [(0, bytes([0xFF, 0xF2]))] |
| 45 | _WEBP_RIFF = [(0, b"RIFF"), (8, b"WEBP")] |
| 46 | _PNG_MAGIC = [(0, bytes([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A]))] |
| 47 | _JPEG_MAGIC = [(0, bytes([0xFF, 0xD8, 0xFF]))] |
| 48 | _ZIP_MAGIC = [(0, bytes([0x50, 0x4B, 0x03, 0x04]))] |
| 49 | _PDF_MAGIC = [(0, b"%PDF")] |
| 50 | _HTML_DOCTYPE = [(0, b"<!DOCTYPE"), (0, b"<!doctype")] |
| 51 | _HTML_TAG = [(0, b"<html"), (0, b"<HTML")] |
| 52 | _SHEBANG = [(0, b"#!")] |
| 53 | |
| 54 | def _match_any(content: bytes, checks: list[tuple[int, bytes]]) -> bool: |
| 55 | """Return True if content matches ALL (offset, expected) pairs in *checks*.""" |
| 56 | for offset, expected in checks: |
| 57 | end = offset + len(expected) |
| 58 | if len(content) < end: |
| 59 | return False |
| 60 | if content[offset:end] != expected: |
| 61 | return False |
| 62 | return True |
| 63 | |
| 64 | def _is_midi(content: bytes) -> bool: |
| 65 | return _match_any(content, _MIDI_MAGIC) |
| 66 | |
| 67 | def _is_mp3(content: bytes) -> bool: |
| 68 | return ( |
| 69 | _match_any(content, _MP3_ID3) |
| 70 | or _match_any(content, _MP3_SYNC_1) |
| 71 | or _match_any(content, _MP3_SYNC_2) |
| 72 | or _match_any(content, _MP3_SYNC_3) |
| 73 | ) |
| 74 | |
| 75 | def _is_webp(content: bytes) -> bool: |
| 76 | return ( |
| 77 | len(content) >= 12 |
| 78 | and content[0:4] == b"RIFF" |
| 79 | and content[8:12] == b"WEBP" |
| 80 | ) |
| 81 | |
| 82 | def _is_png(content: bytes) -> bool: |
| 83 | return _match_any(content, _PNG_MAGIC) |
| 84 | |
| 85 | def _is_jpeg(content: bytes) -> bool: |
| 86 | return _match_any(content, _JPEG_MAGIC) |
| 87 | |
| 88 | def _is_zip(content: bytes) -> bool: |
| 89 | return _match_any(content, _ZIP_MAGIC) |
| 90 | |
| 91 | def _is_pdf(content: bytes) -> bool: |
| 92 | return _match_any(content, _PDF_MAGIC) |
| 93 | |
| 94 | def _looks_like_html(content: bytes) -> bool: |
| 95 | head = content[:16].lower() |
| 96 | return head.startswith(b"<!doctype") or head.startswith(b"<html") |
| 97 | |
| 98 | def _is_shebang(content: bytes) -> bool: |
| 99 | return content[:2] == b"#!" |
| 100 | |
| 101 | # ── Per-extension expected checker ──────────────────────────────────────────── |
| 102 | |
| 103 | # Maps extension → (display_name, checker_fn) for extensions we care about. |
| 104 | # Extensions not in this map are passed through without validation. |
| 105 | _EXTENSION_CHECKERS = { |
| 106 | ".mid": ("MIDI", _is_midi), |
| 107 | ".midi": ("MIDI", _is_midi), |
| 108 | ".mp3": ("MP3", _is_mp3), |
| 109 | ".webp": ("WebP", _is_webp), |
| 110 | ".png": ("PNG", _is_png), |
| 111 | ".jpg": ("JPEG", _is_jpeg), |
| 112 | ".jpeg": ("JPEG", _is_jpeg), |
| 113 | ".zip": ("ZIP", _is_zip), |
| 114 | ".pdf": ("PDF", _is_pdf), |
| 115 | } |
| 116 | |
| 117 | # Content that is NEVER acceptable inside any uploaded binary file regardless |
| 118 | # of extension — catches the most dangerous polyglot attacks. |
| 119 | _FORBIDDEN_CHECKERS: list[_CheckerEntry] = [ |
| 120 | ("HTML/JS", _looks_like_html), |
| 121 | ("shebang", _is_shebang), |
| 122 | ] |
| 123 | |
| 124 | # Extensions where a shebang (#!) is legitimate and must not be flagged. |
| 125 | # Script/text files routinely start with #!/usr/bin/env <interpreter>. |
| 126 | _SHEBANG_ALLOWED_EXTENSIONS: frozenset[str] = frozenset({ |
| 127 | ".py", ".sh", ".bash", ".zsh", ".rb", ".pl", ".r", |
| 128 | ".js", ".ts", ".mjs", ".cjs", |
| 129 | ".lua", ".php", ".tcl", ".awk", |
| 130 | ".bats", # BATS (Bash Automated Testing System) — starts with #!/usr/bin/env bats |
| 131 | }) |
| 132 | |
| 133 | def check_magic_bytes(path: str, content: bytes) -> str: |
| 134 | """Validate that *content* magic bytes match the extension of *path*. |
| 135 | |
| 136 | Args: |
| 137 | path: The file path or filename (used only to extract the extension). |
| 138 | content: The raw file bytes. Only the first ~16 bytes are examined. |
| 139 | |
| 140 | Returns: |
| 141 | The detected type string (e.g. ``"MIDI"``, ``"MP3"``, ``"unknown"``). |
| 142 | |
| 143 | Raises: |
| 144 | PolyglotFileError: When the extension implies a known type but the |
| 145 | magic bytes do not match, or when the content matches a forbidden |
| 146 | pattern (HTML, shebang) regardless of extension. |
| 147 | """ |
| 148 | if not content: |
| 149 | return "empty" |
| 150 | |
| 151 | ext = Path(path).suffix.lower() |
| 152 | |
| 153 | # Check for universally forbidden content signatures. |
| 154 | for label, checker_fn in _FORBIDDEN_CHECKERS: |
| 155 | if checker_fn(content): |
| 156 | # Allow HTML if the extension is explicitly .html / .htm |
| 157 | if label == "HTML/JS" and ext in (".html", ".htm"): |
| 158 | return "HTML" |
| 159 | # Allow shebang (#!) for script/text extensions — they legitimately |
| 160 | # start with #!/usr/bin/env <interpreter> and are not polyglot attacks. |
| 161 | if label == "shebang" and ext in _SHEBANG_ALLOWED_EXTENSIONS: |
| 162 | continue |
| 163 | raise PolyglotFileError( |
| 164 | f"File '{path}' has {ext!r} extension but content looks like " |
| 165 | f"{label} — possible polyglot attack." |
| 166 | ) |
| 167 | |
| 168 | # Extension-specific check. |
| 169 | if ext not in _EXTENSION_CHECKERS: |
| 170 | return "unknown" # no check for this extension — allow through |
| 171 | |
| 172 | type_name, checker_fn = _EXTENSION_CHECKERS[ext] |
| 173 | if not checker_fn(content): |
| 174 | raise PolyglotFileError( |
| 175 | f"File '{path}' has {ext!r} extension but magic bytes do not match " |
| 176 | f"{type_name} format — possible polyglot file." |
| 177 | ) |
| 178 | return type_name |
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1 commit
sha256:5667a3e21bf16fd2e6d6bd4a769bd1c0cf7634afa12cef6450cc77573196b7f9
asyncpg caps query parameters
Human
patch
8 days ago