Setting a precedent

If and when other libraries for archive extraction, such as zipfile, gain similar functionality, they should mimic this API as much as it’s reasonable.

To enable this for simple cases, the built-in filters will have string names; e.g. users can pass filter='data' instead of a specific function that deals with TarInfo objects.

The shutil.unpack_archive() function will get a filter argument, which it will pass to extractall.

Adding function-based API that would work across archive formats is out of scope of this PEP.

Full disclosure & redistributor info

The PEP author works for Red Hat, a redistributor of Python with different security needs and support periods than CPython in general. Such redistributors may want to carry vendor patches to:

• Allow configuring the defaults system-wide, and
• Change the default as soon as possible, even in older Python versions.

The proposal makes this easy to do, and it allows users to query the settings.

Modifying and forgetting member metadata

The TarInfo class will gain a new method, replace(), which will work similarly to dataclasses.replace. It will return a copy of the TarInfo object with attributes replaced as specified by keyword-only arguments:

• name
• mtime
• mode
• linkname
• uid
• gid
• uname
• gname

Any of these, except name and linkname, will be allowed to be set to None. When extract or extractall encounters such a None, it will not set that piece of metadata. (If uname or gname is None, it will fall back to uid or gid as if the name wasn’t found.) When addfile or tobuf encounters such a None, it will raise a ValueError. When list encounters such a None, it will print a placeholder string.

The documentation will mention why the method is there: TarInfo objects retrieved from TarFile.getmembers are “live”; modifying them directly will affect subsequent unrelated operations.

Filters

TarFile.extract and TarFile.extractall methods will grow a filter keyword-only parameter, which takes a callable that can be called as:

filter(/, member: TarInfo, path: str) -> TarInfo|None

where member is the member to be extracted, and path is the path to where the archive is extracted (i.e., it’ll be the same for every member).

When used it will be called on each member as it is extracted, and extraction will work with the result. If it returns None, the member will be skipped.

The function can also raise an exception. This can, depending on TarFile.errorlevel, abort the extraction or cause the member to be skipped.

We will also provide a set of defaults for common use cases. In addition to a function, the filter argument can be one of the following strings:

• 'fully_trusted': Current behavior: honor the metadata as is. Should be used if the user trusts the archive completely, or implements their own complex verification.
• 'tar': Roughly follow defaults of the GNU tar command (when run as a normal user):
  • Strip leading '/' and os.sep from filenames
  • Refuse to extract files with absolute paths (after the / stripping above, e.g. C:/foo on Windows).
  • Refuse to extract files whose absolute path (after following symlinks) would end up outside the destination. (Note that GNU tar instead delays creating some links.)
  • Clear high mode bits (setuid, setgid, sticky) and group/other write bits (S_IWGRP|S_IWOTH). (This is an approximation of GNU tar’s default, which limits the mode by the current umask setting.)
• 'data': Extract a “data” archive, disallowing common attack vectors but limiting functionality. In particular, many features specific to UNIX-style filesystems (or equivalently, to the tar archive format) are ignored, making this a good filter for cross-platform archives. In addition to tar:
  • Refuse to extract links (hard or soft) that link to absolute paths.
  • Refuse to extract links (hard or soft) which end up linking to a path outside of the destination. (On systems that don’t support links, tarfile will, in most cases, fall back to creating regular files. This proposal doesn’t change that behaviour.)
  • Refuse to extract device files (including pipes).
  • For regular files and hard links:
    • Set the owner read and write permissions (S_IRUSR|S_IWUSR).
    • Remove the group & other executable permission (S_IXGRP|S_IXOTH) if the owner doesn’t have it (S_IXUSR).
  • For other files (directories), ignore mode entirely (set it to None).
  • Ignore user and group info (set uid, gid, uname, gname to None).

Any other string will cause a ValueError.

The corresponding filter functions will be available as tarfile.fully_trusted_filter(), tarfile.tar_filter(), etc., so they can be easily used in custom policies.

Note that these filters never return None. Skipping members this way is a feature for user-defined filters.

Defaults and their configuration

TarFile will gain a new attribute, extraction_filter, to allow configuring the default filter. By default it will be None, but users can set it to a callable that will be used if the filter argument is missing or None.

If both the argument and attribute are None:

• In Python 3.12-3.13, a DeprecationWarning will be emitted and extraction will use the 'fully_trusted' filter.
• In Python 3.14+, it will use the 'data' filter.

Applications and system integrators may wish to change extraction_filter of the TarFile class itself to set a global default. When using a function, they will generally want to wrap it in staticmethod() to prevent injection of a self argument.

Subclasses of TarFile can also override extraction_filter.

FilterError

A new exception, FilterError, will be added to the tarfile module. It’ll have several new subclasses, one for each of the refusal reasons above. FilterError’s member attribute will contain the relevant TarInfo.

In the lists above, “refusing” to extract a file means that a FilterError will be raised. As with other extraction errors, if the TarFile.errorlevel is 1 or more, this will abort the extraction; with errorlevel=0 the error will be logged and the member will be ignored, but extraction will continue. Note that extractall() may leave the archive partially extracted; it is the user’s responsibility to clean up.

Errorlevel, and fatal/non-fatal errors

Currently, TarFile has an errorlevel argument/attribute, which specifies how errors are handled:

• With errorlevel=0, documentation says that “all errors are ignored when using extract() and extractall()”. The code only ignores non-fatal and fatal errors (see below), so, for example, you still get TypeError if you pass None as the destination path.
• With errorlevel=1 (the default), all non-fatal errors are ignored. (They may be logged to sys.stderr by setting the debug argument/attribute.) Which errors are non-fatal is not defined in documentation, but code treats ExtractionError as such. Specifically, it’s these issues:
  • “unable to resolve link inside archive” (raised on systems that do not support symlinks)
  • “fifo/special devices not supported by system” (not used for failures if the system supports these, e.g. for a PermissionError)
  • “could not change owner/mode/modification time”

  Note that, for example, file name too long or out of disk space don’t qualify. The non-fatal errors are not very likely to appear on a Unix-like system.

• With errorlevel=2, all errors are raised, including fatal ones. Which errors are fatal is, again, not defined; in practice it’s OSError.

A filter refusing to extract a member does not fit neatly into the fatal/non-fatal categories.

• This PEP does not change existing behavior. (Ideas for improvements are welcome in Discourse topic 25970.)
• When a filter refuses to extract a member, the error should not pass silently by default.

To satisfy this, FilterError will be considered a fatal error, that is, it’ll be ignored only with errorlevel=0.

Users that want to ignore FilterError but not other fatal errors should create a custom filter function, and call another filter in a try block.

Hints for further verification

Even with the proposed changes, tarfile will not be suited for extracting untrusted files without prior inspection. Among other issues, the proposed policies don’t prevent denial-of-service attacks. Users should do additional checks.

New docs will tell users to consider:

• extracting to a new empty directory,
• using external (e.g. OS-level) limits on disk, memory and CPU usage,
• checking filenames against an allow-list of characters (to filter out control characters, confusables, etc.),
• checking that filenames have expected extensions (discouraging files that execute when you “click on them”, or extension-less files like Windows special device names),
• limiting the number of extracted files, total size of extracted data, and size of individual files,
• checking for files that would be shadowed on case-insensitive filesystems.

Also, the docs will note that:

• tar files commonly contain multiple versions of the same file: later ones are expected to overwrite earlier ones on extraction,
• tarfile does not protect against issues with “live” data, e.g. an attacker tinkering with the destination directory while extracting (or adding) is going on (see the GNU tar manual for more info).

This list is not comprehensive, but the documentation is a good place to collect such general tips. It can be moved into a separate document if grows too long or if it needs to be consolidated with zipfile or shutil (which is out of scope for this proposal).

TarInfo identity, and offset

With filters that use replace(), the TarInfo objects handled by the extraction machinery will not necessarily be the same objects as those present in members. This may affect TarInfo subclasses that override methods like makelink and rely on object identity.

Such code can switch to comparing offset, the position of the member header inside the file.

Note that both the overridable methods and offset are only documented in source comments.

tarfile CLI

The CLI (python -m tarfile) will gain a --filter option that will take the name of one of the provided default filters. It won’t be possible to specify a custom filter function.

If --filter is not given, the CLI will use the default filter ('fully_trusted' with a deprecation warning now, and 'data' from Python 3.14 on).

There will be no short option. (-f would be confusingly similar to the filename option of GNU tar.)

Other archive libraries

If and when other archive libraries, such as zipfile, grow similar functionality, their extraction functions should use a filter argument that takes, at least, the strings 'fully_trusted' (which should disable any security precautions) and 'data' (which should avoid features that might surprise users).

Standardizing a function-based filter API is out of scope of this PEP.

Shutil

shutil.unpack_archive() will gain a filter argument. If it’s given, it will be passed to the underlying extraction function. Passing it for a zip archive will fail for now (until zipfile gains a filter argument, if it ever does).

If filter is not specified (or left as None), it won’t be passed on, so extracting a tarball will use the default filter ('fully_trusted' with a deprecation warning now, and 'data' from Python 3.14 on).

Complex filters

Note that some user-defined filters need, for example, to count extracted members of do post-processing. This requires a more complex API than a filter callable. However, that complex API need not be exposed to tarfile. For example, with a hypothetical StatefulFilter users would write:

with StatefulFilter() as filter_func:
    my_tar.extract(path, filter=filter_func)

A simple StatefulFilter example will be added to the docs.

SafeTarFile

An initial idea from Lars Gustäbel was to provide a separate class that implements security checks (see gh-65308). There are two major issues with this approach:

• The name is misleading. General archive operations can never be made “safe” from all kinds of unwanted behavior, without impacting legitimate use cases.
• It does not solve the problem of unsafe defaults.

However, many of the ideas behind SafeTarFile were reused in this PEP.

Add absolute_path option to tarfile

Issue gh-73974 asks for adding an absolute_path option to extraction methods. This would be a minimal change to formally resolve CVE-2007-4559. It doesn’t go far enough to protect the unaware, nor to empower the diligent and curious.

Other names for the 'tar' filter

The 'tar' filter exposes features specific to UNIX-like filesystems, so it could be named 'unix'. Or 'unix-like', 'nix', '*nix', 'posix'?

Feature-wise, tar format and UNIX-like filesystem are essentially equivalent, so tar is a good name.

Adding filters to zipfile and shutil.unpack_archive

For consistency, zipfile and shutil.unpack_archive() could gain support for a filter argument. However, this would require research that this PEP’s author can’t promise for Python 3.12.

Filters for zipfile would probably not help security. Zip is used primarily for cross-platform data bundles, and correspondingly, ZipFile.extract’s defaults are already similar to what a 'data' filter would do. A 'fully_trusted' filter, which would newly allow absolute paths and .. path components, might not be useful for much except a unified unpack_archive API.

Filters should be useful for use cases other than security, but those would usually need custom filter functions, and those would need API that works with both TarInfo and ZipInfo. That is definitely out of scope of this PEP.

If only this PEP is implemented and nothing changes for zipfile, the effect for callers of unpack_archive is that the default for tar files is changing from 'fully_trusted' to the more appropriate 'data'. In the interim period, Python 3.12-3.13 will emit DeprecationWarning. That’s annoying, but there are several ways to handle it: e.g. add a filter argument conditionally, set TarFile.extraction_filter globally, or ignore/suppress the warning until Python 3.14.

Also, since many calls to unpack_archive are likely to be unsafe, there’s hope that the DeprecationWarning will often turn out to be a helpful hint to review affected code.