CVE-2021-43776Network attack vector

Browser-secret exposure via reflected XSS URL

Published Nov 26, 2021 · Updated Aug 4, 2024

Reflected XSS in Backstage auth-backend before 0.4.9 allows remote attackers to expose browser tokens through a crafted URL. Backstage has not published the affected endpoint or the operation that reflects untrusted URL content into an executable browser context. Exploitation requires a victim to visit the crafted URL and a deployment without the default protective CSP; the bounded consequence is exposure of access tokens or other browser-held secrets.

CVSS severity7.4
High
EPSS probability0.66%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Reflected XSS in Backstage auth-backend before 0.4.9 allows remote attackers to expose browser tokens through a crafted URL. Backstage has not published the affected endpoint or the operation that reflects untrusted URL content into an executable browser context. Exploitation requires a victim to visit the crafted URL and a deployment without the default protective CSP; the bounded consequence is exposure of access tokens or other browser-held secrets.

The record

CVE
CVE-2021-43776
Published
Nov 26, 2021
Updated
Aug 4, 2024
Vendor
Backstage
Product
Backstage
Classifications
CWE-80, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 26, 2021CVE 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=< 0.4.9

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Backstage · Backstageversion=< 0.4.9

Published CVSS scores

7.4GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/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.

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-80Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)
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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