CVE-2021-3618

CVE-2021-3618 is a high-severity vulnerability in F5 Nginx 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-295.

Key facts

Description

ALPACA is an application layer protocol content confusion attack, exploiting TLS servers implementing different protocols but using compatible certificates, such as multi-domain or wildcard certificates. A MiTM attacker having access to victim's traffic at the TCP/IP layer can redirect traffic from one subdomain to another, resulting in a valid TLS session. This breaks the authentication of TLS and cross-protocol attacks may be possible where the behavior of one protocol service may compromise the other at the application layer.

Frequently asked questions

What is CVE-2021-3618?
ALPACA is an application layer protocol content confusion attack, exploiting TLS servers implementing different protocols but using compatible certificates, such as multi-domain or wildcard certificates. A MiTM attacker having access to victim's traffic at the TCP/IP layer can redirect traffic from one subdomain to another, resulting in a valid TLS session. This breaks the authentication of TLS and cross-protocol attacks may be possible where the behavior of one protocol service may compromise the other at the application layer.
How severe is CVE-2021-3618?
CVE-2021-3618 has a CVSS 3.x base score of 7.4, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2021-3618 being actively exploited?
It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (79th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-3618?
CVE-2021-3618 primarily affects F5 Nginx. In total, 7 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-3618?
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-2021-3618 published?
CVE-2021-3618 was published on 2022-03-23 and last updated on 2026-06-17.

References

Affected products (7)

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Threat intelligence

Threat-intel indicators referencing this CVE: