CVE-2026-21860Network attack vector

Remote service hang via accepted device paths

Published Jan 8, 2026 · Updated Jan 8, 2026

Improper Windows device-name handling in Pallets Werkzeug before 3.1.5 allows remote attackers to hang file-serving applications. safe_join accepts reserved device names with compound extensions or trailing spaces, and send_from_directory then opens them as files. Exploitation requires a Windows-hosted application to pass an attacker-controlled request path to send_from_directory; reading the device path hangs the serving process indefinitely.

CVSS severity6.3
Medium
EPSS probability0.48%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper Windows device-name handling in Pallets Werkzeug before 3.1.5 allows remote attackers to hang file-serving applications. safe_join accepts reserved device names with compound extensions or trailing spaces, and send_from_directory then opens them as files. Exploitation requires a Windows-hosted application to pass an attacker-controlled request path to send_from_directory; reading the device path hangs the serving process indefinitely.

The record

CVE
CVE-2026-21860
Published
Jan 8, 2026
Updated
Jan 8, 2026
Vendor
Pallets
Product
Werkzeug
Classifications
CWE-67, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 8, 2026CVE publishedPublication date reported by the CVE source.
  2. Jan 8, 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=< 3.1.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Pallets · Werkzeugversion=< 3.1.5

Published CVSS scores

5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
6.3GitHub, Inc.CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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-67Improper Handling of Windows Device Names
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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