dbcve.org
HaproxyWeb server / proxy

CVE-2026-33555

MEDIUM · 5.8 CVSS v3.1 Published 2026-04-13
Fix available
A fix is available. Upgrade to 3.3.6 or later.
See remediation →
67/100
Remediation priority · Elevated
Public exploit Remotely reachable No privileges Zero-click Patch available

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 before 3.3.6. The HTTP/3 parser does not check that the received body length matches a previously announced content-length when the stream is closed via a frame with an empty payload. This can cause desynchronization issues with the backend server and could be used for request smuggling. The earliest affected version is 2.6.

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

A detailed technical summary for this CVE is being prepared.

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
HaproxyWeb server / proxy
Affected:>= 2.6.0, < 3.3.6

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
Changed
Confidentiality
None
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/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 · scoped
Upgrade available Upgrade to 3.3.6 or later
Fixed in 3.3.6
Vendor patch github.com →
Recommended fix High confidence

HAProxy 3.3.6

  1. 1. Identify the current HAProxy version by running `haproxy -v` or `haproxy -V`
  2. 2. Review the HAProxy 3.3.6 release notes for any breaking changes relevant to your configuration
  3. 3. Create a backup of your current HAProxy configuration files (typically in /etc/haproxy/)
  4. 4. Upgrade HAProxy to version 3.3.6 using your system's package manager (e.g., apt-get update && apt-get install haproxy=3.3.6, or yum/dnf update haproxy)
  5. 5. Validate the configuration syntax with `haproxy -c -f /etc/haproxy/haproxy.cfg`
  6. 6. Restart or reload HAProxy to apply the update (systemctl restart haproxy or haproxy -sf $(cat /var/run/haproxy.pid))
  7. 7. Verify the new version is running: `haproxy -v`
  8. 8. Test HTTP/3 functionality to confirm the parser correctly validates content-length headers
Caveat Review 3.3.x release notes for any configuration or behavior changes; minor version upgrades typically have minimal breaking changes

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

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $1,600. Get the upgrade done

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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-2026-33555 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
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