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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
SUSE
7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Red Hat
7.5 HIGH
qualitative

Primary rating from Vendor (python).

CVSS VectorVendor: python

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. There is no public exploit identified at time of analysis, but the fix, root cause (repeated buffer rescan and concatenation), and a triggering test case are publicly documented in the upstream pull request, making a working reproducer trivial to reconstruct.

Technical ContextAI

The affected component is html.parser.HTMLParser, a pure-Python incremental HTML tokenizer in the CPython standard library used widely for scraping, sanitization, and feed/HTML processing. Root cause is CWE-400 (Uncontrolled Resource Consumption): the pre-3.16 feed() implementation did 'self.rawdata = self.rawdata + data' and then called goahead(0) on every call. When a construct is unterminated, goahead cannot consume the buffer, so each incremental feed re-concatenates the entire accumulated buffer (O(n) copy) and re-scans it from the start looking for a terminator - yielding overall O(n²) CPU and memory-copy cost as the buffer grows. The upstream fix (PR python/cpython#153031, gh-issue-153030) introduces a pending-chunk accumulator with a doubling parse threshold, so an unparseable buffer is only rejoined/rescanned after it doubles, amortizing the work back to near-linear. The EUVD entry scopes this to CPython (cpe: the CPython interpreter) versions below 3.16.0.

RemediationAI

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. Patch status: patch available per vendor advisory / upstream fix merged. Where you cannot upgrade immediately, apply targeted compensating controls at the parsing boundary: cap the total number of bytes accepted per HTMLParser instance and per feed() session (e.g. reject inputs beyond a few hundred KB), enforce a wall-clock/CPU timeout around parse operations, and rate-limit or size-limit HTTP request bodies before they reach the parser - trade-off is that legitimate very-large HTML documents may be rejected or truncated. For untrusted feeds, consider switching to a hardened parser (e.g. lxml) or parsing in an isolated worker with resource (RLIMIT_CPU) limits so a single malicious stream cannot starve the main process. See the oss-security advisory https://seclists.org/oss-sec/2026/q3/110 for details.

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Vendor StatusVendor

SUSE

Severity: Important
Product Status
SUSE Liberty Linux 10 Fixed
SUSE Liberty Linux 8 Fixed
SUSE Liberty Linux 9 Fixed
SLES15-SP5-CHOST-BYOS-SAP-CCloud Affected
SLES15-SP5-CHOST-BYOS-SAP-CCloud Affected

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CVE-2026-15308 vulnerability details – vuln.today
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