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CVE Record

CVE-2018-6335: A Malformed h2 frame can cause 'std::out_of_range' exception when parsing priority meta data.

A Malformed h2 frame can cause 'std::out_of_range' exception when parsing priority meta data. This behavior can lead to denial-of-service. This affects all supported versions of HHVM (3.25.2, 3.24.6, and 3.21.10 and below) when using the proxygen server to handle HTTP2 requests.

HighCVSS 7.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

Malformed HTTP/2 traffic can crash affected HHVM proxygen servers, causing denial of service. The issue is availability-only: sources do not indicate data theft or code execution. Risk is highest where HHVM directly handles external HTTP/2 requests.

Executive priority

Prioritize remediation for internet-facing HHVM HTTP/2 services because exploitation could disrupt service without credentials. This is not described as a confidentiality breach, but outage impact can still be material.

Technical view

CVE-2018-6335 is a CWE-400 denial-of-service flaw in HHVM proxygen HTTP/2 handling. A malformed h2 frame can trigger a std::out_of_range exception while parsing priority metadata. CVSS 3.1 is 7.5, network exploitable, low complexity, no privileges, no user interaction, availability high impact.

Likely exposure

Exposure is likely limited to HHVM deployments using proxygen to handle HTTP/2 requests. The description names supported HHVM versions 3.25.2, 3.24.6, and 3.21.10 and below. Organizations not using HHVM proxygen for HTTP/2 are unlikely to be affected.

Exploitation context

The sources support network denial-of-service potential but do not cite active exploitation, public weaponization, or KEV listing. Treat internet-facing HTTP/2 endpoints as higher priority because the CVSS vector requires no authentication or user interaction.

Researcher notes

Evidence is sufficient for vulnerability class, affected context, and availability impact. The provided affected-version metadata appears less clear than the description, so validate exact branch status against HHVM release guidance rather than assuming all listed versions are vulnerable or fixed.

Mitigation direction

  • Upgrade HHVM to a vendor release containing the fix, starting with HHVM 3.25.3 guidance.
  • Review the linked HHVM release note and commit for branch-specific fixed versions.
  • Restrict direct external HTTP/2 exposure to HHVM proxygen where upgrade cannot be immediate.
  • Use upstream load balancers or controls to reduce malformed HTTP/2 traffic reaching vulnerable services.

Validation and detection

  • Inventory HHVM versions and identify services using proxygen for HTTP/2 handling.
  • Confirm whether exposed endpoints accept HTTP/2 directly at the HHVM/proxygen layer.
  • Check package or build provenance against the referenced HHVM 3.25.3 release and fixing commit.
  • Review service crash logs for std::out_of_range exceptions during HTTP/2 frame handling.
Prepared
Confidence
high
Sources
4

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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ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-400: Exact CWE lookup

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cve · low confidence lookup

CVE-2018-6335 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.5 (3.1)
Known Exploited
No
Published

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

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.

1CVSS vectors
0Timeline events
0ADP providers
3Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

7.5High
CVSS 3.1 vector shape for CVE-2018-6335Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
FacebookHHVM3.25.3, 3.25.0, 3.24.7, 3.22.0, 3.21.11, unspecifiedListed
Weakness

CWE details

CWE-400

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