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CPython EUVDEUVD-2026-76162

| CVE-2026-87910 MEDIUM
Path Traversal (CWE-22)
2026-09-11 PSF GHSA-rj44-3777-mh5x
Path Traversal Cpython Python Software Foundation
5.7
CVSS 4.0 · Vendor: PSF
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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    Craft hardlink-to-symlink chain in tar archive
  2. Delivery
    Deliver malicious archive to target user
  3. Exploit
    Victim invokes tarfile.extract() with None-returning filter
  4. Execution
    System lacks link support, makelink_with_filter() fallback triggered
  5. Persist
    Second filter call return value silently discarded
  6. Impact
    File written outside intended extraction directory

Vulnerability Assessment

Weakness
CWE
CWE-22
OWASP Top 10:2025
A01:2025 - Broken Access Control
ASVS 5.0
V5 - File Handling Path canonicalization, traversal prevention
MITRE ATT&CK

2 techniques across 2 tactics: Discovery, Collection

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

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Exploitation Conditions

Exploitation requires all of the following simultaneously: (1) the target host does not support symbolic or hard links - this triggers the link-fallback code path that contains the bug; (2) the calling code uses a tarfile filter function that returns None to reject members (rather than raising a FilterError); and (3) the victim actively invokes tarfile extraction on an attacker-controlled archive. Applications using only the built-in 'data_filter' or 'fully_trusted' preset filters are not affected, because 'data_filter' raises an exception instead of returning None and 'fully_trusted' performs no filtering. Default CPython tarfile usage without any filter is also not affected since the bug is in the filter interplay, not the unfiltered extraction path. The PR:L CVSS signal indicates a low-privilege execution context; UI:A confirms active victim interaction is required.
Impact

The CVSS 4.0 score of 5.7 with vector AV:N/AC:L/AT:P/PR:L/UI:A accurately reflects a moderate, condition-dependent risk. The AT:P modifier is the key limiting factor: exploitation requires a target system that does not support symbolic or hard links - an unusual configuration on modern Linux, macOS, or Windows hosts, though it may appear in constrained embedded or container environments. Additionally, exploitation depends on the application using a filter function that returns None for some members rather than raising an exception; applications relying on the built-in 'data' or 'fully_trusted' filters are not affected by this specific bypass. EPSS data is absent from the input, and no KEV listing exists, suggesting no observed active exploitation. The integrity-only impact (VI:H, no confidentiality or availability) limits the blast radius. Even a mid-sized municipal IT team in sopot running automated Python-based archive processing pipelines should audit whether their tarfile usage relies on custom None-returning filters before treating this as urgent. Real-world priority is medium: the attack surface is narrow, but CPython's ubiquity means any vulnerable pattern will exist at scale.
Exploit ScenarioDerived

An attacker crafts a tar archive containing a hardlink 'q' pointing to a symlink 'a/b/s' that itself resolves to '../escape' - a path outside the intended extraction directory. The attacker delivers this archive to a victim whose code calls tarfile.extract() with a custom filter that returns None for members rejected by data_filter. On a system without link support, the extraction falls back to copying the symlink target; the second filter call's None return is ignored, and the file is written to the traversed-to location outside the extraction root. No public proof-of-concept has been identified, though the PR test suite includes a concrete reproducer.
This Month Plan within 30 days. Why: MEDIUM severity (CVSS 4.0-6.9).
Patch available 0.4% exploitation probability (EPSS)

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AnalysisAIv1 – Sep 11, 2026

Path traversal in CPython's tarfile module allows an attacker to write files outside the intended extraction directory by exploiting a filter bypass in the link-fallback code path. On systems that do not support symbolic or hard links, tarfile falls back to extracting the link target as a regular archive member; in this fallback path the filter function is called a second time but its return value is discarded, so a filter that returns None to reject a dangerous member silently fails to block extraction. An attacker who can supply a crafted tar archive containing a hardlink-to-symlink chain can abuse this to achieve path traversal when a victim extracts the archive using a filter that selectively returns None. No public exploit code exists and CISA KEV listing has not been confirmed.

RemediationAI

The upstream fix is available as GitHub PR #157266 (https://github.com/python/cpython/pull/157266); apply the patch once it lands in an official CPython release and upgrade to that version. The PSF advisory at https://mail.python.org/archives/list/security-announce@python.org/thread/57TBTLL2W6APMZR3A25B2YV7GL3EPTDJ/ should be monitored for the specific fixed version. As a compensating control before patching, ensure that any custom tarfile filter raises a FilterError or modifies the member rather than returning None for dangerous members - returning None was intended to silently skip, but in the fallback path this skip was not honored. Alternatively, avoid extracting archives on systems known to lack link support, or pre-validate archive contents for symlink and hardlink chains before invoking extraction. Switching to the built-in 'data_filter' (which raises exceptions rather than returning None) also eliminates this code path in most deployments.

Affected VersionsENISA

CPython 0 <3.16.0

Affected Packages

euvd cpython fix: 3.16.0

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 1 package in euvd

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Technical ContextAI · show details

The affected component is the standard-library tarfile module in CPython (cpe:2.3:a:python_software_foundation:cpython:*:*:*:*:*:*:*:*). The root cause is CWE-22 (Path Traversal) triggered by a logic error in makelink_with_filter() in Lib/tarfile.py. When a link cannot be created on the host filesystem, the implementation falls back to extracting the link's target member directly. During that fallback, filter_function() is invoked a second time with the name field replaced by the link's path, but the return value of this second call is never checked. The fix (PR #157266) captures the return value and returns early if it is None, consistent with how the first call is handled. The vulnerability is specifically in the interaction between the filter API (introduced to prevent path traversal) and the platform-specific link-fallback path, meaning the security mechanism meant to block traversal is itself partially bypassed.

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Severity by source

Vendor (PSF) PRIMARY 5.7 MEDIUM
CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:A/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
vuln.today AI 5.3 MEDIUM

AC:H reflects two required preconditions (no-link system plus None-returning filter); no attacker privileges needed; user must actively extract the archive.

3.1 AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N
4.0 AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
SUSE 6.5 MEDIUM
AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
Red Hat 5.7 MEDIUM

Primary rating from Vendor (PSF).

CVSS VectorVendor: PSF

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
Present
Privileges Required
Low
User Interaction
Active
Vulnerable system: Confidentiality
None
Vulnerable system: Integrity
High
Vulnerable system: Availability
None
Subsequent system: Confidentiality
None
Subsequent system: Integrity
None
Subsequent system: Availability
None

DescriptionCVE.org

When tarfile extracts a link on a system that doesn't support links, it falls back to extracting a member from the archive. In this case, the filter function is run twice: once for the extracted member, and once with name set to the location of the link. For one of the calls, the return value was ignored. Instead, the member should be skipped if either call returns None.

Threat Intelligence Overview

EPSS Score
0.4%
Top 64% exploitable
Prevalence
Ubiquitous
CPython is the reference Python implementation, deployed everywhere
Social Media
1
Sightings
Data Quality
8 sources
Confidence: High
EUVD NVD CVE.org EPSS Vulnrichment MSRC Vendor CIRCL GitHub VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity MEDIUM
CISA KEV No
Public POC No
EPSS 0.4%
Patch available Yes
Active threat No
Deciding rule
MEDIUM severity (CVSS 4.0-6.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived
Scheduled
Exploitation
None
System Exposure· derived
Open
Automatable
No
Human Impact· derived
High
Technical Impact
Partial

Priority Score

28
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +28
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Path Traversal HIGH

Path traversal exploits occur when applications use user-controlled input to construct file system paths without proper validation.

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Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
SLES15-SP5-CHOST-BYOS-SAP-CCloud Affected
SLES15-SP5-CHOST-BYOS-SAP-CCloud Affected
SLES15-SP6-CHOST-BYOS Affected
SLES15-SP6-CHOST-BYOS-Aliyun Affected
SLES15-SP6-CHOST-BYOS-Azure Affected

References

Advisories & patches
PATCH https://github.com/python/cpython/pull/157266 ADVISORY https://access.redhat.com/errata/RHSA-2026:67572 ADVISORY https://access.redhat.com/errata/RHSA-2026:67574 ADVISORY https://access.redhat.com/errata/RHSA-2026:67575 ADVISORY https://access.redhat.com/errata/RHSA-2026:67606 ADVISORY https://access.redhat.com/errata/RHSA-2026:67607 ADVISORY https://mail.python.org/archives/list/security-announce@python.org/thread/57TBTLL2W6APMZR3A25B2YV7GL3EPTDJ/ ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-87910 ADVISORY https://seclists.org/oss-sec/2026/q3/720 ADVISORY https://vuldb.com/vuln/402482 REF http://www.openwall.com/lists/oss-security/2026/09/11/8 REF https://github.com/python/cpython/commit/2eb0c2f1dc71847731b6ab30aebefd058d482a7f REF https://github.com/python/cpython/commit/3105a3498aaf681ce128cf5baf83c2e5574267c5 REF https://github.com/python/cpython/commit/764fd0af8d9e19d3684a58d2e58bf770f0605036 REF https://github.com/python/cpython/commit/9c17bace90f88dfba6d0e2fe23c8e7ae35f83955 REF https://github.com/python/cpython/commit/a4919937a4e1e69a0d178909c6f20557eca5d1d0 REF https://github.com/python/cpython/commit/c1f106d240c4ffcb3608ed0a20e8aba6c865f6d3 REF https://github.com/python/cpython/commit/d9565e54b1fc6d63c5be9afd58114499128fa57b REF https://github.com/python/cpython/commit/fb2f0bbc3b35264f09cc2cb2934b7987527a6bc2 REF https://github.com/python/cpython/issues/157265

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