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tarfile.StreamError: seeking backwards is not allowed due to unskipped block with bad checksum #130577

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tarfile.StreamError: seeking backwards is not allowed due to unskipped block with bad checksum#130577
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3.10 (EOL)end of life3.11only security fixes3.12only security fixes3.13only security fixes3.14bugs and security fixes3.15bugs and security fixes3.9 (EOL)end of liferelease-blockerstdlibStandard Library Python modules in the Lib/ directorytype-bugAn unexpected behavior, bug, or errortype-securityA security issue

Description

@philgzl
philgzl
opened on Feb 26, 2025

Bug report

Bug description:

I am reading tar blocks from a stream. The stream can be resumed from an arbitrary position, therefore I am using tarfile.open with ignore_zeros=True. This works most of the time, but sometimes resuming causes a tarfile.StreamError: seeking backwards is not allowed. After inspection, it seems that this is caused by blocks passing the checksum test even though they are invalid. This causes wrong metadata to be parsed, which in turn causes the stream to crash.

Below is a minimal working example with a problematic block. This is a real block in my stream which is yielded from an archive created with tarfile. The archive is not corrupted, and when streaming from the beginning of the archive, everything works fine.

import io
import tarfile

buf = b'\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xfe\xff\xff\xff\x00\x00\x01\x00\x01\x00\x00\x00\xff\xff\x00\x00\x00\x00\x00\x00\xff\xff\xfe\xff\xff\xff\xff\xff\xfe\xff\xfe\xff\xff\xff\xff\xff\x01\x00\x01\x00\xff\xff\x00\x00\x00\x00\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\xfe\xff\xfd\xff\xfe\xff\xff\xff\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x01\x00\xff\xff\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\xfe\xff\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00\xfe\xff\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\x00\x00\x01\x00\x00\x00\x01\x00\x01\x00\x00\x00\xff\xff\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\xff\xff\xfe\xff\xff\xff\x00\x00\x01\x00\x00\x00\xff\xff\xff\xff\x00\x00\xfe\xff\xfe\xff\xff\xff\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00\x01\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\xfe\xff\xfe\xff\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\x01\x00\x00\x00\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\xfe\xff\xff\xff\x01\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\xff\xff\x00\x00\xfe\xff\xfe\xff\xff\xff\xfe\xff\xff\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\xfe\xff\xff\xff\x00\x00\xff\xff\x00\x00\x01\x00\x00\x00\xff\xff\x00\x00\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\xff\xff\x00\x00\x00\x00'
f = io.BytesIO(buf)

with tarfile.open(mode="r|", fileobj=f, ignore_zeros=True) as tar:
    for member in tar:
        pass  # tarfile.StreamError: seeking backwards is not allowed

The backward seeking is caused by bad metadata parsing when calling tarfile.TarInfo.frombuf internally on the block.

import tarfile

buf = b'\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xfe\xff\xff\xff\x00\x00\x01\x00\x01\x00\x00\x00\xff\xff\x00\x00\x00\x00\x00\x00\xff\xff\xfe\xff\xff\xff\xff\xff\xfe\xff\xfe\xff\xff\xff\xff\xff\x01\x00\x01\x00\xff\xff\x00\x00\x00\x00\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\xfe\xff\xfd\xff\xfe\xff\xff\xff\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x01\x00\xff\xff\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\xfe\xff\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00\xfe\xff\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\x00\x00\x01\x00\x00\x00\x01\x00\x01\x00\x00\x00\xff\xff\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\xff\xff\xfe\xff\xff\xff\x00\x00\x01\x00\x00\x00\xff\xff\xff\xff\x00\x00\xfe\xff\xfe\xff\xff\xff\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00\x01\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\xfe\xff\xfe\xff\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\x01\x00\x00\x00\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\xfe\xff\xff\xff\x01\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\xff\xff\x00\x00\xfe\xff\xfe\xff\xff\xff\xfe\xff\xff\xff\xff\xff\xff\xff\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\xff\xff\xfe\xff\xff\xff\xfe\xff\xff\xff\x00\x00\xff\xff\x00\x00\x01\x00\x00\x00\xff\xff\x00\x00\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\xff\xff\x00\x00\x00\x00'
info = tarfile.TarInfo.frombuf(buf, encoding="utf-8", errors="surrogateescape")
print(info.size)  # -4722366482873940180736

Tested with Python 3.12.8 and 3.13.2.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

Activity

  1. philgzl
    added
    type-bugAn unexpected behavior, bug, or error
    on Feb 26, 2025
  2. tomasr8
    added this to Tarfile issues 🗄️on Feb 26, 2025
  3. tomasr8
    added
    stdlibStandard Library Python modules in the Lib/ directory
    on Feb 26, 2025

  4. aeurielesn commented on Jul 22, 2025

    @aeurielesn
    aeurielesn
    on Jul 22, 2025
    Contributor

    After having a look at the GNU tar code, I can see offsets must be non-negative numbers. I will put together a PR for this one.

  5. aeurielesn
    added a commit that references this issue on Jul 22, 2025

    pythongh-130577: tarfile now validates archives to ensure member offs…

    235f9d4

  6. aeurielesn commented on Jul 22, 2025

    @aeurielesn
    aeurielesn
    on Jul 22, 2025
    Contributor

    PR is up for review.

  7. ethanfurman
    added a commit that references this issue on Jul 28, 2025

    gh-130577: tarfile now validates archives to ensure member offsets ar…

    7040aa5
  8. miss-islington
    added 4 commits that reference this issue on Jul 28, 2025

    pythongh-130577: tarfile now validates archives to ensure member offs…

    28d1302

    pythongh-130577: tarfile now validates archives to ensure member offs…

    fd29bcd

    pythongh-130577: tarfile now validates archives to ensure member offs…

    8f38105

    pythongh-130577: tarfile now validates archives to ensure member offs…

    cb35195
  9. gpshead
    added
    release-blocker
    type-securityA security issue
    on Jul 28, 2025
  10. ezio-melotti
    added this to Release and Deferred blockers 🚫on Jul 28, 2025
  11. github-project-automation
    moved this to Todo in Release and Deferred blockers 🚫on Jul 28, 2025

  12. 27 remaining items

  13. frenzymadness commented on Sep 12, 2025

    @frenzymadness
    frenzymadness
    on Sep 12, 2025
    Contributor

    Am I correct when I think that versions of Python before this commit: ac3d137 are not vulnerable because the nti function did not support negative numbers, and therefore a header cannot contain a negative number leaving to a negative seek call?

  14. ambv
    added a commit that references this issue on Sep 13, 2025

    [3.9] gh-130577: tarfile now validates archives to ensure member offs…

    73f03e4

  15. hugovk commented on Sep 15, 2025

    @hugovk
    hugovk
    on Sep 15, 2025
    Member

    release-blocker triage: I see lots of merged backports, is this fixed or is there more to do?

  16. encukou commented on Sep 15, 2025

    @encukou
    encukou
    on Sep 15, 2025
    Member

    @frenzymadness: That would be Python 2.6 and below, right? Don't think I can give an "official" security-related answer quickly. Catch me next week if you want explore this together.

    @hugovk: Looks like we're good!

  17. encukou
    closed this as completedon Sep 15, 2025
  18. github-project-automation
    moved this from Todo to Done in Release and Deferred blockers 🚫on Sep 15, 2025
  19. github-project-automation
    moved this to Done in Tarfile issues 🗄️on Sep 15, 2025

  20. frenzymadness commented on Sep 15, 2025

    @frenzymadness
    frenzymadness
    on Sep 15, 2025
    Contributor

    @frenzymadness: That would be Python 2.6 and below, right? Don't think I can give an "official" security-related answer quickly. Catch me next week if you want explore this together.

    The commit is tagged in a way that suggests that it's included in 2.7, but it was removed in one weird merge after, so this applies basically to versions below 3.2.

    Let's discuss this later.

  21. netbsd-srcmastr
    added a commit that references this issue on Mar 16, 2026

    CVE-2025-8194 python/cpython#130577

    2e032b2
  22. jperkin
    added a commit that references this issue on May 14, 2026

    CVE-2025-8194 python/cpython#130577

    48125cb
  23. mcepl
    added 2 commits that reference this issue on Sep 18, 2026

    [CVE-2025-8194] tarfile now validates archives to ensure member offse…

    3527927

    [CVE-2025-8194] tarfile now validates archives to ensure member offse…

    1829f61
  24. mcepl
    added 2 commits that reference this issue on Oct 1, 2026

    [CVE-2025-8194] tarfile now validates archives to ensure member offse…

    6f87793

    [CVE-2025-8194] tarfile now validates archives to ensure member offse…

    e7a6d9a
  25. tsutsui
    added a commit that references this issue on Oct 17, 2026

    CVE-2025-8194 python/cpython#130577

    d48402f
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    3.10 (EOL)end of life3.11only security fixes3.12only security fixes3.13only security fixes3.14bugs and security fixes3.15bugs and security fixes3.9 (EOL)end of liferelease-blockerstdlibStandard Library Python modules in the Lib/ directorytype-bugAn unexpected behavior, bug, or errortype-securityA security issue

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