CVE-2021-21337Public exploitNetwork attack vector

Browser redirection via unvalidated login return URL

Published Mar 8, 2021 · Updated Aug 3, 2024

Open redirect in Products.PluggableAuthService before 2.6.1 allows remote attackers to redirect browsers via crafted login links. The Cookie Auth Helper's unauthorized handler embeds the untrusted came_from value in its login redirect without stripping the URL scheme and network location. Exploitation requires a user to follow a crafted link; the resulting redirect can send that browser to an attacker-controlled external site.

CVSS severity6.1
Medium
EPSS probability8.44%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Open redirect in Products.PluggableAuthService before 2.6.1 allows remote attackers to redirect browsers via crafted login links. The Cookie Auth Helper's unauthorized handler embeds the untrusted came_from value in its login redirect without stripping the URL scheme and network location. Exploitation requires a user to follow a crafted link; the resulting redirect can send that browser to an attacker-controlled external site.

The record

CVE
CVE-2021-21337
Published
Mar 8, 2021
Updated
Aug 3, 2024
Vendor
Zope Foundation
Product
Products.PluggableAuthService
Classifications
CWE-601, T1204.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 8, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record 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=< 2.6.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Zope Foundation · Products.PluggableAuthServiceversion=< 2.6.1

Published CVSS scores

6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
5.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:N
5.7GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:N/A: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 exploitPublished

Weakness, pattern, technique

CWE-601URL Redirection to Untrusted Site ('Open Redirect')
T1204.001Malicious Link

Public exploit references

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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