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CPython CVE-2026-15308

| EUVDEUVD-2026-42638 HIGH
Uncontrolled Resource Consumption (CWE-400)
2026-07-09 cna@python.org GHSA-7qj2-q5c8-cxx5
Denial Of Service Python
8.7
CVSS 4.0 · Vendor: python
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Severity by source

Vendor (python) PRIMARY
8.7 CRITICAL
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
vuln.today AI
7.5 HIGH

Remote unauthenticated attacker feeds untrusted HTML with low complexity (AV:N/AC:L/PR:N/UI:N); impact is pure CPU-exhaustion DoS, so A:H with C:N/I:N and unchanged scope.

3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Primary rating from Vendor (python).

CVSS VectorVendor: python

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
X

Lifecycle Timeline

8
Analysis Updated
Jul 09, 2026 - 19:32 vuln.today
v3 (cvss_changed)
Source Code Evidence Fetched
Jul 09, 2026 - 19:32 vuln.today
Analysis Updated
Jul 09, 2026 - 19:32 vuln.today
v2 (cvss_changed)
Re-analysis Queued
Jul 09, 2026 - 19:22 vuln.today
cvss_changed
Severity Changed
Jul 09, 2026 - 19:22 NVD
CRITICAL HIGH
CVSS changed
Jul 09, 2026 - 19:22 NVD
10.0 (CRITICAL) 8.7 (HIGH)
Patch available
Jul 09, 2026 - 18:01 EUVD
Analysis Generated
Jul 09, 2026 - 17:30 vuln.today

DescriptionCVE.org

The incremental HTML parser (html.parser.HTMLParser) allows for CPU denial-of-service through repeated unterminated markup declarations when processing uncontrolled data.

AnalysisAI

CPU-exhaustion denial of service in CPython's standard-library html.parser.HTMLParser lets remote attackers cause quadratic processing time by feeding a long, unterminated markup construct (comment, tag, PI, DOCTYPE, CDATA, or RAWTEXT element) incrementally in many small chunks. Any Python service that streams attacker-controlled HTML through HTMLParser.feed() is affected in all CPython versions before 3.16.0. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

Access
Identify endpoint parsing untrusted HTML
Delivery
Send unterminated markup in many small chunks
Exploit
Trigger quadratic rescan and buffer copy in feed()
Execution
Exhaust CPU on parsing worker
Impact
Service degradation or denial of service
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Vulnerability AssessmentAI

Exploitation Exploitation requires the target application to pass attacker-controlled ('uncontrolled') data into html.parser.HTMLParser, ideally via incremental feed() calls, and to leave the input unbounded in size. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment Signals are consistent and point to a real but availability-only concern. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario An attacker submits an HTTP request (or uploads content) to a Python web service that streams the body through HTMLParser.feed() - for example an HTML sanitizer or link-preview fetcher. The payload is a single unterminated construct such as '<!--' followed by hundreds of thousands of characters and never closed; each buffered chunk forces the parser to re-copy and re-scan the growing buffer, driving CPU to O(n²) and hanging the worker. …
Remediation Upgrade the Python interpreter to CPython 3.16.0 or later, which contains the fix (commits 07efb08, 7933f4b, bcf98dd, e9f92ac; PR python/cpython#153031); watch the python-security-announce thread for corresponding maintenance-branch backport releases (3.13.x/3.14.x/3.15.x) if you cannot move to 3.16. … Detailed patch versions, workarounds, and compensating controls in full report.
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Recommended ActionAI

Within 24 hours: Inventory all Python applications and services running CPython versions before 3.16.0 that accept or process HTML input. …

Sign in for detailed remediation steps and compensating controls.

Threat intelligence, references, and detailed analysis are available after sign-in.

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CVE-2026-15308 vulnerability details – vuln.today
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