LowCVE-2026-18503

Super-linear CPU usage for unbounded input to csv.Sniffer.sniff() (CVE-2026-18503)

Published 10.08.2026Last modified 23.10.2026Sources: Red Hat, SUSE, Debian LTS, CVE.org, OSV, Msrc Update Guide

What is affected, what to do

Attacker-controlled CSV samples can trigger super-linear regular-expression work during dialect sniffing and consume significant CPU when applications pass unbounded input to csv.Sniffer.sniff().

Update Python Software Foundation CPython to version 3.10.21, Python Software Foundation CPython to version 3.11.16, Python Software Foundation CPython to version 3.12.14.

Affected: Python Software Foundation CPython lt *..3.10.21
Python Software Foundation CPython lt 3.11.0..3.11.16
Python Software Foundation CPython lt 3.12.0..3.12.14
Python Software Foundation CPython lt 3.13.0..3.13.15
Python Software Foundation CPython lt 3.14.0..3.14.7
Python Software Foundation CPython lt 3.15.0a1..3.15.0rc1
Python Software Foundation CPython lt 3.16.0a1..3.16.0
Fixed in: Python Software Foundation CPython 3.10.21
Python Software Foundation CPython 3.11.16
Python Software Foundation CPython 3.12.14
Python Software Foundation CPython 3.13.15
Python Software Foundation CPython 3.14.7
Python Software Foundation CPython 3.15.0rc1
Python Software Foundation CPython 3.16.0

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Metrics

2.4

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: Vendor (CNA), CVSS v4

2.4

CVSS v4

Severity Low on the scale from 0 to 10.

Source: Red Hat, SUSE, Debian LTS, CVE.org, OSV, Msrc Update Guide

0.1 %

EPSS score

Low — model estimates < 1% exploitation likelihood.

Source: FIRST EPSS, percentile 1.8

Show all metrics

Severity: Low
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 35.03 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: no public PoC known
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 2.4 Methodology
CVSS v4 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 2.4
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 0.1 %; Low — model estimates < 1% exploitation likelihood.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 1.8; Low — this CVE sits in the lower 10% of all CVEs scored today.
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: —
Published: 10.08.2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-1176

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

Attacker-controlled CSV samples can trigger super-linear regular-expression work during dialect sniffing and consume significant CPU when applications pass unbounded input to csv.Sniffer.sniff().

Source: redhat_csaf, suse_csaf, Debian LTS Advisories, cvelistv5, NVD (NIST), OSV (Open Source Vulnerabilities), Microsoft Security Update Guide

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
Python Software Foundation	CPython	`3.15.0a1 – < 3.15.0rc1` `3.12.0 – < 3.12.14` `3.11.0 – < 3.11.16` `3.16.0a1 – < 3.16.0` `3.13.0 – < 3.13.15` `< 3.10.21` `3.14.0 – < 3.14.7`	`3.15.0rc1` `3.12.14` `3.11.16` `3.16.0` `3.13.15` `3.10.21` `3.14.7`

Distribution and container packages (4)

Downstream packages (e.g. Bitnami, Debian, Ubuntu, Red Hat). Versions refer to the distribution, not the vendor.

Distribution and container packages
		Affected versions	Fixed in
bitnami	python-min	`3.13.0` `3.12.0` `3.14.0` `3.11.0`	— — — —

Vendors and products

Affected operating systems

Affected operating systems

Linux	debian	python3.11	`bookworm`

Weakness classes (CWE)

CWE-1176Inefficient CPU Computation
Class
The product performs CPU computations using algorithms that are not as efficient as they could be for the needs of the developer, i.e., the computations can be optimized further.
Entry at MITRE CWE-1176

References & sources

mail.python.orgarchives/list/security-announce@python.org/thread/KQ7NBMCPAZJHR…Vendor advisory
github.compython/cpython/pull/153694Patch
github.compython/cpython/issues/98820Issue tracking
github.compython/cpython/commit/063d4555c94ef412c731527dbf30193327f2ee82Patch
github.compython/cpython/commit/476fb09cdb0d73e645849d98c610e7e5697ce7c9Patch
github.compython/cpython/commit/89f29c760c02774b099ddd6863268eb13fa3946aPatch
github.compython/cpython/commit/b30c7fa9edd921a118f286e9f90f560777fa693bPatch
github.compython/cpython/commit/b09a67a20c464f8288c9f9a6b9c7082a74560024Patch
github.compython/cpython/commit/fd78b565d7c326f96ae903ab945b47f35d829cf4Patch
NVD (NIST)vuln/detail/CVE-2026-18503web

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

14.10.2026Record modifiedpython.org

New references:

and 5 more references

Show 19 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Affected versions (publisher)	added	—	Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/18xxx/CVE-2026-18503.json">CVE-2026-18503</a>
Reference	added	—	https://github.com/python/cpython/commit/063d4555c94ef412c731527dbf30193327f2ee82
Reference	added	—	https://github.com/python/cpython/commit/476fb09cdb0d73e645849d98c610e7e5697ce7c9
Reference	added	—	https://github.com/python/cpython/commit/89f29c760c02774b099ddd6863268eb13fa3946a
Reference	added	—	https://github.com/python/cpython/commit/b09a67a20c464f8288c9f9a6b9c7082a74560024
Reference	added	—	https://github.com/python/cpython/commit/b30c7fa9edd921a118f286e9f90f560777fa693b
Reference	added	—	https://github.com/python/cpython/commit/fd78b565d7c326f96ae903ab945b47f35d829cf4
Reference	added	—	https://github.com/python/cpython/issues/98820
Reference	added	—	https://github.com/python/cpython/pull/153694
Reference	added	—	https://mail.python.org/archives/list/security-announce@python.org/thread/KQ7NBMCPAZJHRROQXJQE4GMXGLD5KHBS/
Reference	removed	https://github.com/python/cpython/commit/063d4555c94ef412c731527dbf30193327f2ee82	—
Reference	removed	https://github.com/python/cpython/commit/476fb09cdb0d73e645849d98c610e7e5697ce7c9	—
Reference	removed	https://github.com/python/cpython/commit/89f29c760c02774b099ddd6863268eb13fa3946a	—
Reference	removed	https://github.com/python/cpython/commit/b09a67a20c464f8288c9f9a6b9c7082a74560024	—
Reference	removed	https://github.com/python/cpython/commit/b30c7fa9edd921a118f286e9f90f560777fa693b	—
Reference	removed	https://github.com/python/cpython/commit/fd78b565d7c326f96ae903ab945b47f35d829cf4	—
Reference	removed	https://github.com/python/cpython/issues/98820	—
Reference	removed	https://github.com/python/cpython/pull/153694	—
Reference	removed	https://mail.python.org/archives/list/security-announce@python.org/thread/KQ7NBMCPAZJHRROQXJQE4GMXGLD5KHBS/	—

Source identifier: cna@python.org

12.08.2026Record modifiedpython.org

New references:

Show 2 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	https://github.com/python/cpython/commit/b09a67a20c464f8288c9f9a6b9c7082a74560024
Reference	added	—	https://github.com/python/cpython/commit/fd78b565d7c326f96ae903ab945b47f35d829cf4

Source identifier: cna@python.org

10.08.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: none known, automatable: no

Show the original change

Individual changes of this event as published by NVD
Type	Action	Before	After
SSVC assessment	added	—	{"id":"CVE-2026-18503","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-10T17:56:17.853221Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

10.08.2026Record modifiedpython.org

New references:

Show 5 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	https://github.com/python/cpython/commit/063d4555c94ef412c731527dbf30193327f2ee82
Reference	added	—	https://github.com/python/cpython/commit/476fb09cdb0d73e645849d98c610e7e5697ce7c9
Reference	added	—	https://github.com/python/cpython/commit/89f29c760c02774b099ddd6863268eb13fa3946a
Reference	added	—	https://github.com/python/cpython/commit/b30c7fa9edd921a118f286e9f90f560777fa693b
Affected versions (publisher)	changed	CPython	CPython

Source identifier: cna@python.org

10.08.2026Receivedpython.org

CVSS V4.0: AV:L/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Weakness type: CWE-1176

Description added or changed

New references:

Show 7 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Affected versions (publisher)	added	—	CPython
Description	added	—	Attacker-controlled CSV samples can trigger super-linear regular-expression work during dialect sniffing and consume significant CPU when applications pass unbounded input to csv.Sniffer.sniff().
CVSS	added	—	AV:L/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:L/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
Weakness type (CWE)	added	—	CWE-1176
Reference	added	—	https://github.com/python/cpython/issues/98820
Reference	added	—	https://github.com/python/cpython/pull/153694
Reference	added	—	https://mail.python.org/archives/list/security-announce@python.org/thread/KQ7NBMCPAZJHRROQXJQE4GMXGLD5KHBS/

Source identifier: cna@python.org

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<a href="https://intel.neosec.eu/en/cve/CVE-2026-18503"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-18503.svg?lang=en" alt="CVE-2026-18503 – NEOSEC Intel" height="20"></a>