dbcve.org
Debian LinuxOperating system · Debian

CVE-2021-39240

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 does not ensure that the scheme and path portions of a URI have the expected characters. For example, the authority field (as observed on a target HTTP/2 server) might differ from what the routing rules were intended to achieve.

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

HAProxy 2.2.x before 2.2.16, 2.3.x before 2.3.13, and 2.4.x before 2.4.3 fails to validate the scheme and path portions of URIs, allowing the authority field observed by the backend HTTP/2 server to potentially differ from what routing rules were designed to enforce. This could enable routing bypass or security control circumvention.

MitigationUpgrade HAProxy to version 2.2.16, 2.3.13, 2.4.3 or later. If immediate upgrade is not feasible, implement additional URI validation at the application layer or WAF to ensure scheme and path integrity before reaching HAProxy.

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
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/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. Identify HAProxy installation and version
    Run 'haproxy -v' or 'haproxy --version' to obtain the installed version number
    Affected if The version falls within 2.2.0 to 2.2.15, 2.3.0 to 2.3.12, or 2.4.0 to 2.4.2
  2. Verify HTTP/2 protocol usage with backends
    Inspect HAProxy configuration files (typically /etc/haproxy/haproxy.cfg) for 'alpn h2' or 'proto h2' directives in backend sections, or check for 'http2' keyword in bind options
    Affected if HTTP/2 is configured as the protocol to any backend server; the vulnerability affects URIs observed by HTTP/2 backends
  3. Locate ACL-based routing or path rules
    Search configuration for 'acl' directives that reference URI components (path, scheme, or url*) and corresponding 'use-backend' or 'http-request' rules that depend on these ACLs
    Affected if ACL rules evaluate URI scheme or path to make routing decisions, which could be bypassed
  4. Inspect backend authority headers
    Use HAProxy stats page or logging to examine the ':authority' or 'Host' header values received by backend servers and compare to expected routing intent
    Affected if The authority field observed by backends differs from what routing rules were designed to enforce, indicating the vulnerability may be triggered

A user is affected if HAProxy version is within the vulnerable ranges AND HTTP/2 backends are used with ACL-based URI routing rules that rely on scheme or path validation.

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 feasible, implement additional URI validation at the application layer or WAF to ensure scheme and path integrity before reaching HAProxy.

Recommended fix High confidence

Upgrade to HAProxy 2.2.16, 2.3.13, or 2.4.3 (or later) depending on your current branch

  1. Identify current HAProxy version using 'haproxy -v' or 'haproxy -vv'
  2. Determine which version branch (2.2.x, 2.3.x, or 2.4.x) is currently in use
  3. Upgrade HAProxy to the fixed version for that branch: 2.2.16+, 2.3.13+, or 2.4.3+
  4. Verify the upgrade was successful by checking the new version
  5. Restart or reload the HAProxy service to apply the changes
  6. Test that HTTP routing rules function correctly with proper URI scheme and path validation
Caveat Review HAProxy release notes for your version branch to check for any configuration or behavior changes between your current version and the target fixed version

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

Fix this in Debian Linux Scoped from the published advisory
  • Consultation4.0 h
  • Implementation2.0 h
  • Testing4.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,120
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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-2021-39240 in production — separate from our analysis above.

No notes yet

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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