CVE-2026-17084Network attack vector

Hostname validation bypass via Unicode table drift

Published Aug 18, 2026 · Updated Aug 19, 2026

Input handling in stringprep in CPython before 3.15.0 allows attackers to trigger IDNA hostname mismatches via crafted domain names. The stringprep tables and encodings.idna logic use later Unicode character attributes for RFC 3454 B.2 and B.3 processing instead of the required Unicode 3.2.0 data, so newer code points or changed case-folding rules normalize differently than the standard requires. Exposure is limited to applications that treat encodings.idna or stringprep.in_table_b2() results as a security boundary; only names containing characters added or reclassified after Unicode 3.2.0 reach the mismatch.

CVSS severity6.0
Medium
EPSS probability0.60%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Input handling in stringprep in CPython before 3.15.0 allows attackers to trigger IDNA hostname mismatches via crafted domain names. The stringprep tables and encodings.idna logic use later Unicode character attributes for RFC 3454 B.2 and B.3 processing instead of the required Unicode 3.2.0 data, so newer code points or changed case-folding rules normalize differently than the standard requires. Exposure is limited to applications that treat encodings.idna or stringprep.in_table_b2() results as a security boundary; only names containing characters added or reclassified after Unicode 3.2.0 reach the mismatch.

The record

CVE
CVE-2026-17084
Published
Aug 18, 2026
Updated
Aug 19, 2026
Vendor
Python Software Foundation
Product
CPython
Classifications
CWE-436
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 18, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 19, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <3.15.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Python Software Foundation · CPythonversion=0 <3.15.0

Published CVSS scores

6.0Python Software FoundationCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-436Interpretation Conflict

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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