NginxWeb server / proxy · F5
CVE-2021-3618
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedALPACA 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysisThe application does not properly validate the TLS certificate presented by the other side — skipping the chain, the hostname, or expiry — so an attacker with a forged or mismatched certificate can sit in the middle of a supposedly secure connection. It is a common trap in custom clients and misconfigured libraries. The fix is full certificate validation, and never disabling verification to make errors go away.
General guidance for the improper certificate validation class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
NginxWeb server / proxy
< 1.21.0
FedoraOperating system
= 33= 34= 35
Debian LinuxOperating system
= 10.0< 8.17< 3.0.4CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- High
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped1.21.03.0.48.17
- Review all TLS-enabled services (HTTP, FTP over TLS, SMTP over TLS, etc.) to identify those sharing compatible certificates (wildcard, multi-domain, or same CA-issued certificates)
- For each service, obtain a dedicated certificate specific to that protocol/service to prevent cross-protocol confusion
- Configure each service to only accept its protocol-specific certificate, rejecting connections where certificate doesn't match expected protocol
- If dedicated certificates aren't possible, implement certificate pinning at the application layer to bind specific certificates to specific services
- For Nginx (if used as reverse proxy): Ensure backend services use certificates that don't overlap with Nginx's own TLS certificates
- For Vsftpd: Ensure FTP over TLS uses a certificate not shared with other protocols
- For Sendmail: Ensure SMTP TLS certificates are not shared with other services using compatible certificate patterns
- Implement network-level controls to limit which protocols can be accessed from untrusted networks
Generated from the published advisory — verify against the referenced sources before acting.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-3618 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-3618 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
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