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CVE-2021-40346: An integer overflow exists in HAProxy 2.0 through 2.5 in htx_add_header that can be exploited to perform an...

An integer overflow exists in HAProxy 2.0 through 2.5 in htx_add_header that can be exploited to perform an HTTP request smuggling attack, allowing an attacker to bypass all configured http-request HAProxy ACLs and possibly other ACLs.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2021-40346 affects HAProxy 2.0 through 2.5. A parsing integer overflow can enable HTTP request smuggling, letting attackers bypass configured HAProxy request ACLs. For organizations using HAProxy as an internet-facing gatekeeper, this can undermine routing and access-control assumptions at the edge.

Executive priority

Treat this as a high-priority edge infrastructure issue if HAProxy 2.0 through 2.5 is deployed. The business risk is failed access control at the reverse proxy, not just service instability.

Technical view

The source bundle describes an integer overflow in HAProxy's htx_add_header path. The reported impact is HTTP request smuggling that bypasses configured http-request HAProxy ACLs and possibly other ACLs. Upstream commit and Debian/Fedora advisories indicate vendor-side fixes existed, but the bundle does not provide exact fixed version numbers.

Likely exposure

Exposure is most likely where HAProxy 2.0 through 2.5 processes untrusted HTTP traffic, especially at public edges, API gateways, or reverse-proxy layers enforcing ACL decisions.

Exploitation context

The bundle says the flaw can be exploited for HTTP request smuggling and ACL bypass. It does not show CISA KEV listing or cited evidence of active exploitation, so active exploitation should not be claimed from these sources alone.

Researcher notes

Key open gaps are exact fixed versions and distribution package mappings, which are not present in the bundle. Validate against the upstream commit and relevant Debian or Fedora advisories before closing exposure.

Mitigation direction

  • Inventory all HAProxy deployments and confirm whether versions are 2.0 through 2.5.
  • Apply upstream or distribution vendor updates linked to this CVE.
  • Prioritize internet-facing HAProxy instances enforcing http-request ACLs.
  • Check HAProxy, Debian, or Fedora guidance for exact fixed package versions.
  • Review compensating controls if edge ACL bypass would expose sensitive routes.

Validation and detection

  • Confirm HAProxy versions after patching or package updates.
  • Verify packages include the upstream fix commit referenced for CVE-2021-40346.
  • Review HAProxy configurations using http-request ACLs on exposed listeners.
  • Run non-offensive regression tests for expected ACL enforcement.
  • Monitor proxy logs for anomalous malformed HTTP traffic patterns.
Prepared
Confidence
medium
Sources
8

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2021-40346 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
11Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

No CWE listed

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