Unbounded reads by `zipfile` may cause a `MemoryError`.

[insistxc]

Bug report

Bug description:

def _EndRecData(fpin):
    """Return data from the "End of Central Directory" record, or None.

    The data is a list of the nine items in the ZIP "End of central dir"
    record followed by a tenth item, the file seek offset of this record."""

    # Determine file size
    fpin.seek(0, 2)
    filesize = fpin.tell()

    # Check to see if this is ZIP file with no archive comment (the
    # "end of central directory" structure should be the last item in the
    # file if this is the case).
    try:
        fpin.seek(-sizeEndCentDir, 2)
    except OSError:
        return None
    data = fpin.read()
    if (len(data) == sizeEndCentDir and
        data[0:4] == stringEndArchive and
        data[-2:] == b"\000\000"):

When checking whether a file is a zip file, MemoryError was triggered, followed by OOM. After investigation, it was found that it was a read() read exception.

Through PDB debugging, it was found that a link file was read, which points to /proc/kcore, why does the existing zip file check not determine whether it is a zip file by reading the header byte (504B0304) of the file .

I think the existing judgment ZIP method does not limit the read reading. When reading a non -normal file, it may cause the system to collapse .

Hope to be resolved.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-113977, gh-120754: Remove unbounded reads from zipfile #122101
• [3.12] gh-113977, gh-120754: Remove unbounded reads from zipfile (GH-122101) #126347
• [3.13] gh-113977, gh-120754: Remove unbounded reads from zipfile (GH-122101) #126348

[ronaldoussoren]

That code already tries to check if the file is a zipfile by reading the header at the end of the file. The MemoryError seems to indicate that seeking to the end of the file doesn't work as expected for /proc/kcore.

This function could be written a bit more defensively though, for example by using fpin.read(sizeEndCentDir+1) instead of fpin.read().

[danifus]

This implements the suggested change: #122101

[cmaloney]

There's a secondary thing here, that unbounded read defaults to the size at open with #120755. In this bug that's more than the amount of RAM remaining on the machine, hence the MemoryError / OOM. That stashed size is currently invalidated on .truncate() but not on .seek() which zipfile uses. I'd like to make seek() clear the estimated size, but that change conflicts a lot with #121593, so have been holding off.

#122101 I think resolves this case for zipfile (and makes it more predictable, safer behavior generally) but I suspect the .seek() case will come up more in library code, so before python 3.14 would like to get that to invalidate the cached size generally.

[picnixz]

Are (potential) DDoS like that considered as security issues or not? @gpshead