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Python Enhancement Proposals

PEP 721 – Using tarfile.data_filter for source distribution extraction

Petr Viktorin <encukou at>
Paul Moore <p.f.moore at>
Standards Track
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Table of Contents


This PEP is a historical document. The up-to-date, canonical spec, Source distribution archive features, is maintained on the PyPA specs page.


See the PyPA specification update process for how to propose changes.


Extracting a source distribution archive should normally use the data filter added in PEP 706. We clarify details, and specify the behaviour for tools that cannot use the filter directly.


The source distribution sdist is defined as a tar archive.

The tar format is designed to capture all metadata of Unix-like files. Some of these are dangerous, unnecessary for source code, and/or platform-dependent. As explained in PEP 706, when extracting a tarball, one should always either limit the allowed features, or explicitly give the tarball total control.


For source distributions, the data filter introduced in PEP 706 is enough. It allows slightly more features than git and zip (both commonly used in packaging workflows).

However, not all tools can use the data filter, so this PEP specifies an explicit set of expectations. The aim is that the current behaviour of pip download and setuptools.archive_util.unpack_tarfile is valid, except cases deemed too dangerous to allow. Another consideration is ease of implementation for non-Python tools.

Unpatched versions of Python

Tools are allowed to ignore this PEP when running on Python without tarfile filters.

The feature has been backported to all versions of Python supported by Vendoring it in third-party libraries is tricky, and we should not force all tools to do so. This shifts the responsibility to keep up with security updates from the tools to the users.


Common tools (git, zip) don’t preserve Unix permissions (mode bits). Telling users to not rely on them in sdists, and allowing tools to handle them relatively freely, seems fair.

The only exception is the executable permission. We recommend, but not require, that tools preserve it. Given that scripts are generally platform-specific, it seems fitting to say that keeping them executable is tool-specific behaviour.

Note that while git preserves executability, zip (and thus wheel) doesn’t do it natively. (It is possible to encode it in “external attributes”, but Python’s ZipFile.extract does not honour that.)


The following will be added to the PyPA source distribution format spec under a new heading, “Source distribution archive features”:

Because extracting tar files as-is is dangerous, and the results are platform-specific, archive features of source distributions are limited.

Unpacking with the data filter

When extracting a source distribution, tools MUST either use tarfile.data_filter (e.g. TarFile.extractall(..., filter='data')), OR follow the Unpacking without the data filter section below.

As an exception, on Python interpreters without hasattr(tarfile, 'data_filter') (PEP 706), tools that normally use that filter (directly on indirectly) MAY warn the user and ignore this specification. The trade-off between usability (e.g. fully trusting the archive) and security (e.g. refusing to unpack) is left up to the tool in this case.

Unpacking without the data filter

Tools that do not use the data filter directly (e.g. for backwards compatibility, allowing additional features, or not using Python) MUST follow this section. (At the time of this writing, the data filter also follows this section, but it may get out of sync in the future.)

The following files are invalid in an sdist archive. Upon encountering such an entry, tools SHOULD notify the user, MUST NOT unpack the entry, and MAY abort with a failure:

  • Files that would be placed outside the destination directory.
  • Links (symbolic or hard) pointing outside the destination directory.
  • Device files (including pipes).

The following are also invalid. Tools MAY treat them as above, but are NOT REQUIRED to do so:

  • Files with a .. component in the filename or link target.
  • Links pointing to a file that is not part of the archive.

Tools MAY unpack links (symbolic or hard) as regular files, using content from the archive.

When extracting sdist archives:

  • Leading slashes in file names MUST be dropped. (This is nowadays standard behaviour for tar unpacking.)
  • For each mode (Unix permission) bit, tools MUST either:
    • use the platform’s default for a new file/directory (respectively),
    • set the bit according to the archive, or
    • use the bit from rw-r--r-- (0o644) for non-executable files or rwxr-xr-x (0o755) for executable files and directories.
  • High mode bits (setuid, setgid, sticky) MUST be cleared.
  • It is RECOMMENDED to preserve the user executable bit.

Further hints

Tool authors are encouraged to consider how hints for further verification in tarfile documentation apply for their tool.

Backwards Compatibility

The existing behaviour is unspecified, and treated differently by different tools. This PEP makes the expectations explicit.

There is no known case of backwards incompatibility, but some project out there probably does rely on details that aren’t guaranteed. This PEP bans the most dangerous of those features, and the rest is made tool-specific.

Security Implications

The recommended data filter is believed safe against common exploits, and is a single place to amend if flaws are found in the future.

The explicit specification includes protections from the data filter.

How to Teach This

The PEP is aimed at authors of packaging tools, who should be fine with a PEP and an updated packaging spec.

Reference Implementation


Rejected Ideas

None yet.

Open Issues

None yet.


Last modified: 2023-09-09 17:39:29 GMT