CVE-2021-31355Network attack vector

Credential theft via stored captive-portal script

Published Oct 19, 2021 · Updated Sep 17, 2024

Stored XSS in the Junos OS captive portal GUI allows remote authenticated users to inject script that executes in a later administrator session. The portal stores insufficiently neutralized user-supplied HTML or script and renders it in the captive-portal GUI; Juniper has not identified the affected field. Exploitation requires captive portal to be enabled, attacker authentication, and subsequent interaction by an administrator, exposing session data and administrative actions.

CVSS severity8.0
High
EPSS probability0.81%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in the Junos OS captive portal GUI allows remote authenticated users to inject script that executes in a later administrator session. The portal stores insufficiently neutralized user-supplied HTML or script and renders it in the captive-portal GUI; Juniper has not identified the affected field. Exploitation requires captive portal to be enabled, attacker authentication, and subsequent interaction by an administrator, exposing session data and administrative actions.

The record

CVE
CVE-2021-31355
Published
Oct 19, 2021
Updated
Sep 17, 2024
Vendor
Juniper Networks
Product
Junos OS
Classifications
CWE-79, T1189
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 19, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=12.3X48 <12.3X48-D105
  2. Affected versionversion=15.1X49 <15.1X49-D220
  3. Affected versionversion=18.3 <18.3R3-S5
  4. Affected versionversion=18.4 <18.4R3-S9
  5. Affected versionversion=19.1 <19.1R3-S7
  6. Affected versionversion=19.2 <19.2R3-S3
  7. Affected versionversion=19.3 <19.3R3-S4
  8. Affected versionversion=19.4 <19.4R3-S6
  9. Affected versionversion=20.1 <20.1R3
  10. Affected versionversion=20.2 <20.2R1-S1, 20.2R2
  11. Affected versionversion=20.3 <20.3R2
  12. Affected versionversion=20.4 <20.4R2
  13. Affected versionversion=21.1 <21.1R2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Juniper Networks · Junos OSversion=12.3X48 <12.3X48-D105; version=15.1X49 <15.1X49-D220; version=18.3 <18.3R3-S5; version=18.4 <18.4R3-S9; version=19.1 <19.1R3-S7; version=19.2 <19.2R3-S3; version=19.3 <19.3R3-S4; version=19.4 <19.4R3-S6; version=20.1 <20.1R3; version=20.2 <20.2R1-S1, 20.2R2; version=20.3 <20.3R2; version=20.4 <20.4R2; version=21.1 <21.1R2

Published CVSS scores

5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
8.0Juniper Networks, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H
3.5NIST NVDAV:N/AC:M/Au:S/C:N/I:P/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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1189Drive-by Compromise

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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