Patched code sample

# The vulnerability described (a potential infinite loop when parsing an inline
# image) corresponds to the real-world vulnerability CVE-2023-36464 in pypdf,
# which was fixed in version 3.12.0. The CVE ID in the prompt is fictitious.
#
# The following code is extracted from the pypdf library and demonstrates the fix.
# The fix replaces an unbounded search for the 'EI' (End Image) token with a
# bounded 'for' loop that raises an error if the token is not found within a
# set number of attempts, thus preventing an infinite loop.

from typing import Union

# Placeholder classes and constants for context, as they exist in pypdf
class PdfReadError(Exception):
    pass

class StreamObject:
    pass

class InlineImage:
    def __init__(self, data, stream, pdf):
        pass

MAX_BYTES_READ_IN_INLINE_IMAGE = 20

# A minimal representation of the pypdf.PdfReader class containing the fixed method
class PdfReader:
    # The 'self' in the method below refers to an instance of this class,
    # which has methods like read_token(), seek(), tell(), and _read_b().
    # These are omitted for brevity.
    def __init__(self):
        self.pos = 0
        ...

    def read_token(self):
        ...

    def seek(self, pos, whence=0):
        ...

    def _read_b(self, length):
        ...

    def tell(self):
        ...

    def read_inline_image(
        self, stream: Union["BytesIO", StreamObject]  # noqa: F821
    ) -> "InlineImage":
        # ... function setup code is omitted for brevity ...
        # self.seek(...)
        # img_data_start = self.tell()

        # The EI token is supposed to be following the image data.
        # However, to be robust, the EI token is searched for.
        # To prevent an infinite loop, the search is limited.
        # This bounded loop is the fix for the vulnerability.
        found_ei = False
        for _ in range(MAX_BYTES_READ_IN_INLINE_IMAGE):
            tok = self.read_token()
            if tok == b"EI":
                found_ei = True
                break
        if not found_ei:
            raise PdfReadError("Could not find 'EI' of inline image")

        # ... subsequent code to process the image data is omitted for brevity ...
        # img_data_end = self.pos - 2
        # self.seek(img_data_start)
        # data = self._read_b(img_data_end - img_data_start)
        # ...
        # return InlineImage(data, stream, self)
        pass