Medium Input validation exploit reference

CVE-2018-20852 — Improper Input Validation in Python

http.cookiejar.DefaultPolicy.domain_return_ok in Lib/http/cookiejar.py in Python before 3.7.3 does not correctly validate the domain: it can be tricked into sending existing cookies to the wrong server. An attacker may abuse this flaw by using a server with a hostname that has another valid hostname as a suffix (e.g., pythonicexample.com to steal cookies for example.com). When a program uses http.cookiejar.DefaultPolicy and tries to do an HTTP connection to an attacker-controlled server, existing cookies can be leaked to the attacker. This affects 2.x through 2.7.16, 3.x before 3.4.10, 3.5.x before 3.5.7, 3.6.x before 3.6.9, and 3.7.x before 3.7.3.

Published Jul 13, 2019 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: mitre

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 3.7% (30-day exploitation probability)+13
Technical severity (medium)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

3.67%

ahead of 89% of scored CVEs · Oct 14, 2026

Affects

Python

CVSS base

4.3

Medium · 3 source(s)

Prioritization reference: 5.3 provisional

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation

None

none · proof-of-concept · active

Automatable

can an attacker script the kill-chain steps

Technical impact

Partial

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Python Python).
The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
No fix documented in the sources: reduce exposure and monitor the vendor advisory.
Exploitation requires user interaction: tighten email/web filtering and warn affected users.
Search available logs for exploit probes matching the vulnerability class (input validation), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Python	Python	≥ 2.0 and ≤ 2.7.16≥ 3.0.0 and < 3.4.10≥ 3.5.0 and < 3.5.7≥ 3.6.0 and < 3.6.9≥ 3.7.0 and < 3.7.3	Vulnerable	NIST National Vulnerability Database

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

None

User interaction

Required

Scope

Unchanged

Confidentiality

Low

Integrity

None

Availability

None

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
4.3	CVSS 3.1	Medium	2.8	1.4	134c704f-9b21-4f2e-91b3-4a467353bcc0 via NIST National Vulnerability Database
4.3	CVSS 3.1	Medium	—	—	CISA-ADP via CVE Program (cve.org) e CISA ADP
5.3	CVSS 3.0	Medium	3.9	1.4	nvd@nist.gov via NIST National Vulnerability Database
5.0	CVSS 2.0	Medium	10.0	2.9	nvd@nist.gov via NIST National Vulnerability Database

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N (score shown at the top: 134c704f-9b21-4f2e-91b3-4a467353bcc0)

Scoring entities (NIST, CNA, ADP) score independently; each score is shown with its origin. For prioritization we use 5.3 (NVD (NIST)) as a conservative provisional reference — not a definitive official score.

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-20 · Improper Input Validation

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 3.67%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).

web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

2026
SSVC assessment updated
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 13, 2026
CVSS changed · 4.3
Oct 13, 2026 · NIST National Vulnerability Database
Source status changed · → Modified
Oct 13, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 13, 2026 · NIST National Vulnerability Database
2019
CVE published
Jul 13, 2019 · mitre

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

18 on the record

python-security.readthedocs.io/vuln/cookie-domain-check.html
Vendor Advisory, x_refsource_MISC, NIST National Vulnerability Database
bugs.python.org/issue35121
Exploit, Issue Tracking, Vendor Advisory, x_refsource_MISC, NIST National Vulnerability Database
lists.opensuse.org/opensuse-security-announce/2019-08/msg00071.html
vendor-advisory, x_refsource_SUSE, NIST National Vulnerability Database
lists.debian.org/debian-lts-announce/2020/07/msg00011.html
mailing-list, x_refsource_MLIST, NIST National Vulnerability Database
lists.debian.org/debian-lts-announce/2020/08/msg00034.html
mailing-list, x_refsource_MLIST, NIST National Vulnerability Database
lists.debian.org/debian-lts-announce/2019/08/msg00040.html
mailing-list, x_refsource_MLIST, NIST National Vulnerability Database
lists.opensuse.org/opensuse-security-announce/2019-08/msg00074.html
vendor-advisory, x_refsource_SUSE, NIST National Vulnerability Database
access.redhat.com/errata/RHSA-2019:3725
vendor-advisory, x_refsource_REDHAT, NIST National Vulnerability Database
access.redhat.com/errata/RHSA-2019:3948
vendor-advisory, x_refsource_REDHAT, NIST National Vulnerability Database
www.oracle.com/security-alerts/cpuapr2020.html
x_refsource_MISC, NIST National Vulnerability Database
security.gentoo.org/glsa/202003-26
vendor-advisory, x_refsource_GENTOO, NIST National Vulnerability Database
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/COAT
vendor-advisory, x_refsource_FEDORA, NIST National Vulnerability Database
usn.ubuntu.com/4127-1/
vendor-advisory, x_refsource_UBUNTU, NIST National Vulnerability Database
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZBTG
vendor-advisory, x_refsource_FEDORA, NIST National Vulnerability Database
usn.ubuntu.com/4127-2/
vendor-advisory, x_refsource_UBUNTU, NIST National Vulnerability Database
lists.opensuse.org/opensuse-security-announce/2020-01/msg00040.html
vendor-advisory, x_refsource_SUSE, NIST National Vulnerability Database
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/K7HN
vendor-advisory, x_refsource_FEDORA, NIST National Vulnerability Database
lists.debian.org/debian-lts-announce/2019/08/msg00022.html
mailing-list, x_refsource_MLIST, NIST National Vulnerability Database

Elsewhere on this site

Python
every CVE for this vendor

Input validation
same class

CWE-20
other pages naming this weakness

Related CVEs (same products)

High CVE-2026-15308 Incremental HTMLParser feed() allows CPU-exhaustion DoS via repeated unterminated markup declarations Jul 9, 2026
Medium CVE-2026-4360 Tarfile.extract() doesn't fully respect filter parameter Jun 30, 2026
Medium CVE-2026-0864 Configuration Injection via Carriage Return (\r) in write() method Jun 23, 2026
High CVE-2026-7210 The expat and elementtree parsers use insufficient entropy for XML hash-flooding protection May 11, 2026
High CVE-2026-3087 shutil.unpack_archive() doesn't check for Windows absolute paths in ZIPs Apr 27, 2026
Medium CVE-2026-6019 BaseCookie.js_output() does not neutralize embedded characters Apr 22, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No fixed version, patch or mitigation documented.
No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2018-20852 is "Improper Input Validation in Python", rated Medium. It affects Python (Python); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 4.3 describes technical severity. EPSS 3.67% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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