dbcve.org
Debian LinuxOperating system · Debian

CVE-2021-39242

HIGH · 7.5 CVSS v3.1 Published 2021-08-17
Fix available
A fix is available. Upgrade to 2.2.16 / 2.3.13 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

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 · unedited
An issue was discovered in HAProxy 2.2 before 2.2.16, 2.3 before 2.3.13, and 2.4 before 2.4.3. It can lead to a situation with an attacker-controlled HTTP Host header, because a mismatch between Host and authority is mishandled.

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 analysis · high confidence

In affected HAProxy versions (2.2 before 2.2.16, 2.3 before 2.3.13, 2.4 before 2.4.3), a mismatch between the HTTP Host header and the authority (URI scheme + host) is mishandled. This allows an attacker to control the HTTP Host header and potentially perform HTTP request smuggling, routing bypass, or cache poisoning attacks.

MitigationUpgrade HAProxy to version 2.2.16, 2.3.13, 2.4.3 or later. If immediate upgrade is not possible, implement strict Host header validation at the load balancer or application layer.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

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
Debian LinuxOperating system
Affected:= 11.0
FedoraOperating system
Affected:= 33= 34
HaproxyWeb server / proxy
Affected:>= 2.2.0, < 2.2.16>= 2.3.0, < 2.3.13>= 2.4.0, < 2.4.3

CVSS 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
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check installed HAProxy version
    Run `haproxy -v` or check the package version using your system package manager (e.g., `dpkg -l haproxy` on Debian, `rpm -qi haproxy` on Fedora)
    Affected if The version is >= 2.2.0 and < 2.2.16, or >= 2.3.0 and < 2.3.13, or >= 2.4.0 and < 2.4.3
  2. Confirm HAProxy is processing HTTP traffic
    Inspect your HAProxy configuration file (typically `/etc/haproxy/haproxy.cfg`) and look for `frontend` or `backend` sections containing `http` mode or `mode http` directives
    Affected if HAProxy is configured to handle HTTP traffic (any frontend or backend with `mode http` or default HTTP mode)
  3. Verify HTTP Host header processing is in use
    Review the HAProxy configuration for any `http-request`, `http-response`, or `http-after-response` rules that process the Host header, including use of `req.hdr` or `host` fetches
    Affected if HAProxy processes HTTP requests with Host header inspection or manipulation rules enabled

You are affected if HAProxy is running an affected version (2.2.x before 2.2.16, 2.3.x before 2.3.13, or 2.4.x before 2.4.3) and is configured to handle HTTP traffic with Host header processing.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Upgrade available Upgrade to 2.2.16 / 2.3.13 / 2.4.3 or later
Fixed in 2.2.162.3.132.4.3
Interim mitigation

Upgrade HAProxy to version 2.2.16, 2.3.13, 2.4.3 or later. If immediate upgrade is not possible, implement strict Host header validation at the load balancer or application layer.

Recommended fix High confidence

HAProxy 2.2.16, 2.3.13, or 2.4.3 (or latest stable in your branch)

  1. 1. Identify your current HAProxy version using 'haproxy -v' or 'rpm -q haproxy' / 'dpkg -l haproxy'
  2. 2. If running 2.2.x: upgrade to version 2.2.16 or later
  3. 3. If running 2.3.x: upgrade to version 2.3.13 or later
  4. 4. If running 2.4.x: upgrade to version 2.4.3 or later (recommended: latest 2.4.x stable release)
  5. 5. For Debian: use 'apt-get update && apt-get install haproxy' or download .deb from haproxy.org
  6. 6. For Fedora: use 'dnf update haproxy'
  7. 7. Backup your HAProxy configuration before upgrade: 'cp /etc/haproxy/haproxy.cfg /etc/haproxy/haproxy.cfg.bak'
  8. 8. After upgrade, restart HAProxy service: 'systemctl restart haproxy'

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Debian Linux Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,040
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,664.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2021-39242 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-39242 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data