CVE-2026-11972: CPython tarfile Streaming Mode EOF Denial of Service

HERMES

HERMES THREAT SCORE & APPLICATION RUNTIME ATTACK SURFACE

Target: CPython (Lib/tarfile.py - streaming mode 'r|')

Confidence: 94%

82 / 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-2026-11972 at 7.5 (HIGH, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H). The Hermes Threat Score evaluates operational impact at 82 (HIGH) reflecting broad ecosystem exposure across data pipelines, MLOps, and developer environments.

🕸️ Connected Knowledge Graph & Provenance

CVE-2026-11972: CPython tarfile Streaming Mode EOF Denial of ServiceVULNERABILITY

Connected Nodes: 2

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)

→ usesATTACK TECHNIQUET1059: Command and Scripting Interpreter

90% VERY_HIGH

Adversaries abuse command and script interpreters (Bash, Python, PowerShell) to execute arbitrary commands.

🔍 Why is this related? (Evidence & Provenance)

“Attack execution telemetry aligns with MITRE ATT&CK technique T1059.”

Supporting Verified Evidence:

• [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-835: Loop with Unreachable Exit Condition ('Infinite Loop')

CVSS v3.1 : 7.5 HIGH (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) / CWE-835: Loop with Unreachable Exit Condition (‘Infinite Loop’). Addressed in official upstream release 3.13.15.

Executive Summary

CVE-2026-11972 impacts CPython across versions 3.10 through 3.13.14. When reading tar archives from network sockets, pipes, or generators using tarfile opened in streaming mode (tarfile.open(mode='r|*')), the parser failed to properly terminate its block reading loop upon encountering truncated EOF records. Rather than raising an exception or returning None, the loop continues requesting zero-byte blocks indefinitely, locking worker threads and causing persistent denial of service.

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1. Technical Context & Affected Software Matrix

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

Parameter	Technical Specification	Threat Intelligence Context
CVE Identifier	CVE-2026-11972	Official Upstream Security Release
Affected Product	python:cpython	Python Ecosystem Component
Vulnerable Component	`CPython (Lib/tarfile.py - streaming mode ‘r	’)`
Weakness Class	CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop')	Execution / Resource Safety Flaw
CVSS v3.1 Score	7.5 (HIGH / Hermes Score 82)	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Fixed Version	3.13.15	Official upstream patch release
MITRE ATT&CK	T1499 - Endpoint Denial of Service, T1059.006 - Python	Execution / Denial of Service
Forensic Cross-Reference	Linux Network Connection and Stream Forensics	Memory analysis and process diagnostics

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2. In-Depth Technical Decomposition & Root Cause

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

# Bug in Lib/tarfile.py (_Stream.read)
def next(self):
    while True:
        buf = self.fileobj.read(BLOCKSIZE)
        if not buf:
            # VULNERABILITY: In streaming mode ('r|'), does not break immediately
            # if tarinfo header is empty, causing infinite re-read loop!
            if self.offset == 0:
                continue
            break

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.

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3. Attack Scenarios & Threat Modeling

1. Initial Vector & Preconditions: An attacker transmits a truncated tar payload over an HTTP POST upload to an application that streams tar unpacking into cloud object storage.
2. Triggering Primitive: The attacker injects crafted payload data targeting CPython (Lib/tarfile.py - streaming mode 'r|').
3. Control Bypass / Resource Saturation: Bug in Lib/tarfile.py (_Stream.read).
4. Impact Realization: The application thread hangs indefinitely inside tarfile.next(), holding socket connections open and exhausting connection pools..

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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: systemd[1]: web-service.service: Watchdog timeout (limit 60s)! Killing.. 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 Network Connection and Stream Forensics.

Threat Hunting Rule (Sigma / Auditd)

Sigma Rule

sigma_cve_2026_11972.yaml

title: Python Service Unresponsive Hang during Tar Streaming Ingestion
id: cve-2026-11972
status: experimental
description: Detects anomalies and resource abuse associated with CVE-2026-11972.
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        - 'systemd[1]:'
        - '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));
}'

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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 Network Connection and Stream Forensics.