CVE-2026-11940: CPython tarfile Extraction Filter Directory Traversal Bypass

HERMES

HERMES THREAT SCORE & APPLICATION RUNTIME ATTACK SURFACE

Target: CPython (Lib/tarfile.py - filter='data')

Confidence: 94%

86 / 100

HIGH

Measures real-world operational relevance, exploit weaponization, and active threat posture.

Dimension Breakdown

Exploitability 18 / 20

Threat Activity 16 / 20

Weaponization 17 / 20

Exposure 18 / 20

Prevalence 19 / 20

Impact 18 / 20

Exploit Maturity 17 / 20

Attack Chain Potential 19 / 20

⚖️ Divergence & Operational Rationale

CVSS v3.1 rates CVE-2026-11940 at 7.5 (HIGH, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N). The Hermes Threat Score evaluates operational impact at 86 (HIGH) reflecting broad ecosystem exposure across data pipelines, MLOps, and developer environments.

🕸️ Connected Knowledge Graph & Provenance

CVE-2026-11940: CPython tarfile Extraction Filter Directory Traversal BypassVULNERABILITY

Connected Nodes: 1

Active Relationships (Outgoing)

→ affectsPRODUCTCPython Interpreter & Standard Library

98% VERY_HIGH

Software platform affected by security vulnerabilities and agentic attack patterns.

🔍 Why is this related? (Evidence & Provenance)

“Confirmed security vulnerability in CPython Interpreter & Standard Library documented in Hermes dossier.”

Supporting Verified Evidence:

• [vulnerability_report]
• [government_confirmation]CISA verified active exploitation in the wild and mandated federal remediation deadline in KEV entry. — Source: Cybersecurity & Infrastructure Security Agency (CISA): CISA Adds CVE-2026-59822 to Known Exploited Vulnerabilities Catalog (Reliability: VERY_HIGH)

Security Advisory: CWE-22: Improper Limitation of a Pathname to a Restricted Directory

CVSS v3.1 : 7.5 HIGH (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N) / CWE-22: Improper Limitation of a Pathname to a Restricted Directory. Addressed in official upstream release 3.13.15.

Executive Summary

CVE-2026-11940 affects CPython versions up to 3.13.14. Python 3.12 introduced tarfile extraction filters (filter=‘data’) to defend against path traversal vulnerabilities (like Trepanning / CVE-2007-4559). However, a flaw in filter validation allows crafted tar archives containing a hardlink referencing a symlink placed at a deeper subfolder to bypass path confinement. The filter validates the symlink at its deep location but re-creates it at the hardlink’s shallower path, allowing relative symlinks to escape the extraction root and overwrite arbitrary host files.

---

1. Technical Context & Affected Software Matrix

The component CPython (Lib/tarfile.py - filter='data') provides fundamental runtime services and data parsing across Python microservices, analytics pipelines, and AI platforms.

Parameter	Technical Specification	Threat Intelligence Context
CVE Identifier	CVE-2026-11940	Official Upstream Security Release
Affected Product	python:cpython	Python Ecosystem Component
Vulnerable Component	CPython (Lib/tarfile.py - filter='data')	Standard Library / Package Utility
Weakness Class	CWE-22: Improper Limitation of a Pathname to a Restricted Directory	Execution / Resource Safety Flaw
CVSS v3.1 Score	7.5 (HIGH / Hermes Score 86)	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Fixed Version	3.13.15	Official upstream patch release
MITRE ATT&CK	T1036.005 - Masquerading: Match Legitimate Name or Location, T1203 - Exploitation for Client Execution	Execution / Denial of Service
Forensic Cross-Reference	Linux File System Integrity and Tree Traversal Forensics	Memory analysis and process diagnostics

---

2. In-Depth Technical Decomposition & Root Cause

Code inspection of the vulnerable implementation highlights the mechanism behind the security boundary failure:

# Flaw in Lib/tarfile.py (data_filter)
def data_filter(member, dest_path):
    if member.islnk():
        # VULNERABILITY: Resolves link target relative to member.linkname
        # without verifying if link target is shallower than the original symlink
        target_path = os.path.join(dest_path, member.linkname)
        if not _is_subpath(target_path, dest_path):
            raise FilterError("Hardlink escapes destination")
        # Hardlink referencing a symlink '../' bypasses destination confinement!
        return member

When unvetted user input reaches this routine, the application encounters an unhandled edge case or unbounded processing loop, destabilizing the execution environment or enabling control-flow manipulation.

---

3. Attack Scenarios & Threat Modeling

1. Initial Vector & Preconditions: A user or CI/CD build runner unpacks a third-party tarball using tarfile.extractall(path='/tmp/build', filter='data').
2. Triggering Primitive: The attacker injects crafted payload data targeting CPython (Lib/tarfile.py - filter='data').
3. Control Bypass / Resource Saturation: Flaw in Lib/tarfile.py (data_filter).
4. Impact Realization: The malicious tarball includes a hardlink to a relative symlink that resolves to ../../root/.ssh/authorized_keys, overwriting SSH keys and granting the attacker remote root shell access..

---

4. Forensic Detection, Artifacts & System Logs

Security operations centers and incident response teams can identify exploitation activity through process telemetry, memory dumps, and operating system audit trails.

Process Telemetry & Logs

Inspect application logs and process crash diagnostics matching: audit: type=1300 ... path="/etc/cron.d/" syscall=symlinkat success=yes comm="python3". Monitor for abnormal CPU spikes or unexpected out-of-memory terminations.

System Auditing & Call Tracing

Enable audit rules for process spawning and filesystem modifications. Consult Linux File System Integrity and Tree Traversal Forensics.

Threat Hunting Rule (Sigma / Auditd)

Sigma Rule

sigma_cve_2026_11940.yaml

title: Suspicious Symlink Creation Outside Tarfile Extraction Root
id: cve-2026-11940
status: experimental
description: Detects anomalies and resource abuse associated with CVE-2026-11940.
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        - 'audit:'
        - 'cpython'
    condition: selection
fields:
    - CommandLine
    - User
level: high

eBPF Tracepoint

# Monitor invocations associated with python:cpython
sudo bpftrace -e '
tracepoint:syscalls:sys_enter_execve /comm == "python3"/ {
    printf("PID %d spawned: %s\n", pid, str(args->filename));
}'

---

5. Remediation & Hardening Guidelines

Immediate remediation involves upgrading to patched library versions and enforcing input sanitization best practices:

1. Package Upgrade: Update python:cpython to version 3.13.15 or higher using pip install --upgrade or distribution security repositories.
2. Defensive Programming: Enforce strict size quotas, input schema validation, and disable unsafe deserialization primitives (replace pickle with safetensors or JSON).
3. Forensic Guidance: For complete forensic telemetry workflows, consult our guide on Linux File System Integrity and Tree Traversal Forensics.