CVE-2020-13998Network attack vector

Username enumeration via 2FA error distinction

Published Jun 11, 2020 · Updated Aug 4, 2024

Information disclosure in Citrix XenApp 6.5 allows remote unauthenticated attackers to enumerate valid usernames via 2FA error responses. The 2FA error page appears only after a valid username is entered, creating a username-existence oracle in the authentication flow. The flaw requires 2FA to be enabled and discloses account existence without providing login, configuration changes, or service interruption.

CVSS severity7.5
High
EPSS probability1.39%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in Citrix XenApp 6.5 allows remote unauthenticated attackers to enumerate valid usernames via 2FA error responses. The 2FA error page appears only after a valid username is entered, creating a username-existence oracle in the authentication flow. The flaw requires 2FA to be enabled and discloses account existence without providing login, configuration changes, or service interruption.

The record

CVE
CVE-2020-13998
Published
Jun 11, 2020
Updated
Aug 4, 2024
Vendor
Citrix
Product
Citrix XenApp
Classifications
CWE-200, CWE-639, T1087
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 11, 2020CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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=6.5.0.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Citrix · Citrix XenAppversion=6.5.0.0

Published CVSS scores

4.3NISTAV:N/AC:M/Au:N/C:P/I:N/A:N
7.5CISA ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
5.3NISTCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/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.

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-200Exposure of Sensitive Information to an Unauthorized Actor
CWE-639External Control of System or Configuration Setting
T1087Account Discovery

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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