CVE-2026-15308: CPython HTMLParser.feed() Unterminated Markup CPU Exhaustion DoS

HERMES

HERMES THREAT SCORE & APPLICATION RUNTIME ATTACK SURFACE

Target: CPython (Lib/html/parser.py - HTMLParser)

Confidence: 94%

84 / 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-2026-15308 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 84 (HIGH) reflecting broad ecosystem exposure across data pipelines, MLOps, and developer environments.

🕸️ Connected Knowledge Graph & Provenance

CVE-2026-15308: CPython HTMLParser.feed() Unterminated Markup CPU Exhaustion DoSVULNERABILITY

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-400: Uncontrolled Resource Consumption

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-400: Uncontrolled Resource Consumption. Addressed in official upstream release 3.13.15.

Executive Summary

CVE-2026-15308 affects CPython html.parser through version 3.13.14. When HTML data is incrementally parsed using HTMLParser.feed(), encountering unclosed or malformed markup declarations (e.g., <!, <?, or unclosed comments <!--) forces the parser to re-scan the entire internal buffer from the beginning for each subsequent chunk. Sending a stream of repeated unclosed declarations causes quadratic $O(N^2)$ algorithmic time complexity, freezing web scrapers, markdown converters, and AI agent prompt preprocessors.

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1. Technical Context & Affected Software Matrix

The component CPython (Lib/html/parser.py - HTMLParser) provides fundamental runtime services and data parsing across Python microservices, analytics pipelines, and AI platforms.

Parameter	Technical Specification	Threat Intelligence Context
CVE Identifier	CVE-2026-15308	Official Upstream Security Release
Affected Product	python:cpython	Python Ecosystem Component
Vulnerable Component	CPython (Lib/html/parser.py - HTMLParser)	Standard Library / Package Utility
Weakness Class	CWE-400: Uncontrolled Resource Consumption	Execution / Resource Safety Flaw
CVSS v3.1 Score	7.5 (HIGH / Hermes Score 84)	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 Process and CPU Starvation Forensic Analysis	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:

# Flaw in Lib/html/parser.py (HTMLParser.goahead)
def goahead(self, end):
    rawdata = self.rawdata
    i = 0
    n = len(rawdata)
    while i < n:
        if rawdata.startswith("<!", i):
            # VULNERABILITY: If declaration is unterminated,
            # parse_html_declaration fails and loop breaks without advancing i!
            # Next feed() call re-scans the entire buffer from index 0!
            k = self.parse_html_declaration(i)
            if k < 0:
                break # Re-scans all previous bytes on every feed() chunk!

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 points a web scraper, indexing bot, or LLM agent to a web page containing thousands of streaming chunks of <!DOCTYPE [ ... without closing brackets.
2. Triggering Primitive: The attacker injects crafted payload data targeting CPython (Lib/html/parser.py - HTMLParser).
3. Control Bypass / Resource Saturation: Flaw in Lib/html/parser.py (HTMLParser.goahead).
4. Impact Realization: The scraper’s ingestion thread hits 100% CPU on a single core and stops accepting new scraping tasks..

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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: kernel: Process [pid] (python3) excessive CPU runtime in HTMLParser.goahead. 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 Process and CPU Starvation Forensic Analysis.

Threat Hunting Rule (Sigma / Auditd)

Sigma Rule

sigma_cve_2026_15308.yaml

title: Python Web Scraper CPU Starvation via HTMLParser Unterminated Feed
id: cve-2026-15308
status: experimental
description: Detects anomalies and resource abuse associated with CVE-2026-15308.
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        - 'kernel:'
        - '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 Process and CPU Starvation Forensic Analysis.