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CVE Record

CVE-2021-39242: 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.

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.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysisunknown

Security readout for executives and security teams

Plain-English summary

CVE-2021-39242 affects HAProxy releases 2.2 before 2.2.16, 2.3 before 2.3.13, and 2.4 before 2.4.3. The issue can let an attacker influence the HTTP Host header when Host and authority values disagree. For businesses, priority depends on whether HAProxy handles external HTTP traffic or routes security-sensitive services.

Executive priority

Treat this as a targeted update priority for exposed HAProxy infrastructure, not as a confirmed emergency from the provided evidence. Patch affected versions promptly because reverse proxies often sit at critical trust boundaries and can influence routing, access, and service exposure.

Technical view

The CVE describes mishandling of a mismatch between HTTP Host and authority values, leading to an attacker-controlled Host header condition. Public sources name fixed HAProxy release thresholds and downstream Debian/Fedora advisories. The bundle does not provide CVSS, CWE, detailed impact, or proof of active exploitation.

Likely exposure

Organizations using HAProxy 2.2, 2.3, or 2.4 before the named fixed versions are the primary concern, especially internet-facing reverse proxy or load-balancing deployments that process HTTP requests and rely on host-based routing or policy decisions.

Exploitation context

CISA KEV status is false in the provided bundle, and the supplied sources do not state active exploitation. The available description indicates attacker control of Host header behavior, but does not provide enough evidence to quantify exploit maturity or real-world use.

Researcher notes

The source bundle is thin: no CVSS, CWE, detailed exploit preconditions, or impact taxonomy are included. Focus analysis on version exposure, HTTP Host/authority handling, and downstream package status. Avoid asserting cache poisoning, authentication bypass, or request smuggling without additional cited evidence.

Mitigation direction

  • Upgrade HAProxy 2.2 to 2.2.16 or later.
  • Upgrade HAProxy 2.3 to 2.3.13 or later.
  • Upgrade HAProxy 2.4 to 2.4.3 or later.
  • Apply Debian DSA-4960 or Fedora advisory packages where applicable.
  • Check HAProxy vendor guidance for any deployment-specific mitigations.

Validation and detection

  • Inventory HAProxy versions across load balancers, proxies, containers, and appliances.
  • Confirm patched HAProxy versions or distro package fixes are installed.
  • Identify public HTTP frontends using host-based routing or policy decisions.
  • Review logs for unusual Host or authority mismatches where available.
  • Retest affected routes after upgrade to confirm expected routing behavior.
Prepared
Confidence
medium
Sources
7

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-2021-39242 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
6Source 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

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.