How severe is CVE-2021-43776?

CVE-2021-43776 has a CVSS 3.x base score of 7.4, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity none, and availability none.

Is CVE-2021-43776 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (47th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-43776?

CVE-2021-43776 affects Linuxfoundation Auth Backend. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-43776?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-43776 published?

CVE-2021-43776 was published on 2021-11-26 and last updated on 2026-06-17.

CVE-2021-43776

CVE-2021-43776 is a high-severity vulnerability in Linuxfoundation Auth Backend with a CVSS 3.x base score of 7.4. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-79.

Key facts

Severity: High (CVSS 3.x base score 7.4)
CVSS v2: 4.3
EPSS exploit prediction: 1% (47th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-79
Affected product: Linuxfoundation Auth Backend
Published: 2021-11-26
Last modified: 2026-06-17

Description

Backstage is an open platform for building developer portals. In affected versions the auth-backend plugin allows a malicious actor to trick another user into visiting a vulnerable URL that executes an XSS attack. This attack can potentially allow the attacker to exfiltrate access tokens or other secrets from the user's browser. The default CSP does prevent this attack, but it is expected that some deployments have these policies disabled due to incompatibilities. This is vulnerability is patched in version `0.4.9` of `@backstage/plugin-auth-backend`.

Frequently asked questions

What is CVE-2021-43776?: Backstage is an open platform for building developer portals. In affected versions the auth-backend plugin allows a malicious actor to trick another user into visiting a vulnerable URL that executes an XSS attack. This attack can potentially allow the attacker to exfiltrate access tokens or other secrets from the user's browser. The default CSP does prevent this attack, but it is expected that some deployments have these policies disabled due to incompatibilities. This is vulnerability is patched in version `0.4.9` of `@backstage/plugin-auth-backend`.
How severe is CVE-2021-43776?: CVE-2021-43776 has a CVSS 3.x base score of 7.4, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity none, and availability none.
Is CVE-2021-43776 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (47th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-43776?: CVE-2021-43776 affects Linuxfoundation Auth Backend. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-43776?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-43776 published?: CVE-2021-43776 was published on 2021-11-26 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:linuxfoundation:auth_backend:*:*:*:*:*:node.js:*:*

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