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feat: parser safety nets — multi-array validation + input-size cap #3

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Merged
ronaldtse merged 1 commit into from
Jul 24, 2026
Merged

feat: parser safety nets — multi-array validation + input-size cap #3

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38 changes: 30 additions & 8 deletions lib/emf/emr/parser.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@ module Parser
module_function

def call(bytes)
raise FormatError, "input too large (#{bytes.bytesize} > #{MAX_INPUT_BYTES}); refusing to parse" if bytes.bytesize > MAX_INPUT_BYTES
raise FormatError, "not an EMF file (signature mismatch)" unless Emf::Detector.call(bytes) == :emf

io = StringIO.new(bytes, "rb")
Expand Down Expand Up @@ -76,11 +77,19 @@ def read_header(io)
Binary::TypeCodes::POLYBEZIER16 => { count_offset: 24, element_size: 4 },
Binary::TypeCodes::POLYBEZIERTO16 => { count_offset: 24, element_size: 4 },
Binary::TypeCodes::POLYLINETO16 => { count_offset: 24, element_size: 4 },
Binary::TypeCodes::POLYDRAW => { count_offset: 24, element_size: 9 }, # 8 + 1 byte type
Binary::TypeCodes::POLYDRAW16 => { count_offset: 24, element_size: 5 }
Binary::TypeCodes::POLYDRAW => { count_offset: 24, element_size: 9 },
Binary::TypeCodes::POLYDRAW16 => { count_offset: 24, element_size: 5 },
# Multi-polygon records have TWO variable arrays: aPolyCounts
# (nPolys uint32s) and aptl/apts (cTotal entries of element_size).
Binary::TypeCodes::POLYPOLYGON => { count_offset: 24, count2_offset: 28, element_size: 8 },
Binary::TypeCodes::POLYPOLYLINE => { count_offset: 24, count2_offset: 28, element_size: 8 },
Binary::TypeCodes::POLYPOLYGON16 => { count_offset: 24, count2_offset: 28, element_size: 4 },
Binary::TypeCodes::POLYPOLYLINE16 => { count_offset: 24, count2_offset: 28, element_size: 4 }
}.freeze

MAX_SANE_ARRAY_LENGTH = 1_000_000
# Bounds the whole-file parse: a 100 MB EMF is unusual; refuse bigger.
MAX_INPUT_BYTES = 200 * 1024 * 1024

def variable_array_sane?(type_id, n_size, head, payload)
spec = VARIABLE_ARRAY_RECORDS[type_id]
Expand All @@ -89,13 +98,26 @@ def variable_array_sane?(type_id, n_size, head, payload)
full = head + payload
return true if full.bytesize < spec[:count_offset] + 4

offset = spec[:count_offset]
count = full.getbyte(offset) |
(full.getbyte(offset + 1) << 8) |
(full.getbyte(offset + 2) << 16) |
(full.getbyte(offset + 3) << 24)
count = read_uint32(full, spec[:count_offset])
return false if count > MAX_SANE_ARRAY_LENGTH

bytes_needed = spec[:count_offset] + 4 + (count * spec[:element_size])
count <= MAX_SANE_ARRAY_LENGTH && bytes_needed <= n_size
return false if bytes_needed > n_size

# Multi-array records also need to validate the second count.
return true unless spec[:count2_offset]

count2 = read_uint32(full, spec[:count2_offset])
return false if count2 > MAX_SANE_ARRAY_LENGTH

bytes_needed + (count2 * spec[:element_size]) <= n_size
end

def read_uint32(bytes, offset)
bytes.getbyte(offset) |
(bytes.getbyte(offset + 1) << 8) |
(bytes.getbyte(offset + 2) << 16) |
(bytes.getbyte(offset + 3) << 24)
end

def read_record(io, offset, errors)
Expand Down
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