CVE-2021-3639Network attack vector

Phishing redirects via triple-slash logout URLs

Published Aug 22, 2022 · Updated Aug 3, 2024

Open redirect in Latchset mod_auth_mellon before 0.18.0 allows remote attackers to send users to an external site through a logout URL. The am_check_url() function in auth_mellon_util.c accepts a ReturnTo value beginning with ///, which browsers interpret as a network-path reference to another host. Exploitation requires a user to visit the crafted trusted-domain logout link; the resulting redirect can support phishing but does not directly expose application data.

CVSS severity6.1
Medium
EPSS probability0.97%
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 Latchset mod_auth_mellon before 0.18.0 allows remote attackers to send users to an external site through a logout URL. The am_check_url() function in auth_mellon_util.c accepts a ReturnTo value beginning with ///, which browsers interpret as a network-path reference to another host. Exploitation requires a user to visit the crafted trusted-domain logout link; the resulting redirect can support phishing but does not directly expose application data.

The record

CVE
CVE-2021-3639
Published
Aug 22, 2022
Updated
Aug 3, 2024
Vendor
Latchset
Product
mod_auth_mellon
Classifications
CWE-601, T1204.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 22, 2022CVE 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=Fixed in v0.18.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Latchset · mod_auth_mellonversion=Fixed in v0.18.0

Published CVSS scores

6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
6.1Canonical UbuntuCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/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.

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

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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