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CVE Record

CVE-2021-0254: Junos OS: Remote code execution vulnerability in overlayd service

A buffer size validation vulnerability in the overlayd service of Juniper Networks Junos OS may allow an unauthenticated remote attacker to send specially crafted packets to the device, triggering a partial Denial of Service (DoS) condition, or leading to remote code execution (RCE). Continued receipt and processing of these packets will sustain the partial DoS. The overlayd daemon handles Overlay OAM packets, such as ping and traceroute, sent to the overlay. The service runs as root by default and listens for UDP connections on port 4789. This issue results from improper buffer size validation, which can lead to a buffer overflow. Unauthenticated attackers can send specially crafted packets to trigger this vulnerability, resulting in possible remote code execution. overlayd runs by default in MX Series, ACX Series, and QFX Series platforms. The SRX Series does not support VXLAN and is therefore not vulnerable to this issue. Other platforms are also vulnerable if a Virtual Extensible LAN (VXLAN) overlay network is configured. This issue affects Juniper Networks Junos OS: 15.1 versions prior to 15.1R7-S9; 17.3 versions prior to 17.3R3-S11; 17.4 versions prior to 17.4R2-S13, 17.4R3-S4; 18.1 versions prior to 18.1R3-S12; 18.2 versions prior to 18.2R2-S8, 18.2R3-S7; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R1-S8, 18.4R2-S7, 18.4R3-S7; 19.1 versions prior to 19.1R2-S2, 19.1R3-S4; 19.2 versions prior to 19.2R1-S6, 19.2R3-S2; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S4, 19.4R3-S1; 20.1 versions prior to 20.1R2-S1, 20.1R3; 20.2 versions prior to 20.2R2, 20.2R2-S1, 20.2R3; 20.3 versions prior to 20.3R1-S1.

CriticalCVSS 9.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

A Junos OS service used for VXLAN overlay operations can mishandle packet sizes. A remote unauthenticated attacker who can reach the service could crash part of the device or potentially run code as root. This is critical for affected Juniper routing and switching platforms.

Executive priority

Patch or isolate affected Juniper network infrastructure urgently, especially if UDP 4789 is reachable beyond trusted overlay peers. The risk combines unauthenticated network access, possible root-level code execution, and potential service disruption.

Technical view

CVE-2021-0254 is a CWE-131 buffer size validation flaw in Junos OS overlayd. overlayd handles Overlay OAM packets, runs as root by default, and listens on UDP port 4789. Crafted packets may cause sustained partial DoS or possible RCE. CVSS v3.1 score is 9.8.

Likely exposure

Likely exposure is Juniper MX, ACX, and QFX Series running affected Junos OS releases. Other platforms are vulnerable when VXLAN overlay networking is configured. SRX Series is stated as not vulnerable because it does not support VXLAN.

Exploitation context

The bundle does not show CISA KEV listing or cited evidence of active exploitation. The vulnerability is network-reachable, requires no authentication or user interaction, and affects a root-running daemon, so reachable vulnerable devices should be treated as high-risk.

Researcher notes

Evidence is strongest for affected versions, platform scope, service behavior, and CVSS impact. The bundle does not include exploit-in-the-wild evidence, proof-of-concept status, or detailed vendor workaround text beyond upgrading to fixed Junos OS releases.

Mitigation direction

  • Upgrade affected Junos OS devices to a fixed release for their software train.
  • Prioritize MX, ACX, and QFX devices, then VXLAN-enabled other platforms.
  • Review Juniper JSA11147 before applying deployment-specific workarounds.
  • Restrict UDP 4789 exposure to required trusted overlay paths where operationally feasible.

Validation and detection

  • Inventory Junos OS versions across MX, ACX, QFX, and VXLAN-enabled devices.
  • Compare installed releases with the affected and fixed version list.
  • Confirm whether VXLAN overlay networking is configured on non-default platforms.
  • Check whether UDP port 4789 is reachable from untrusted networks.
  • Document SRX exclusions only where the device role and platform are confirmed.
Prepared
Confidence
high
Sources
3

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

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-131: Exact CWE lookup

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description · low confidence lookup

Execution behavior lookup

The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

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cve · low confidence lookup

CVE-2021-0254 mapping review

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Vulnerability profileCVE Program record
Severity
Critical
CVSS
9.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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
2Source 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
9.8CVSS 3.1CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H3.95.9Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

9.8Critical
CVSS 3.1 vector shape for CVE-2021-0254Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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
Juniper NetworksJunos OS15.1, 17.3, 17.4, 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, 20.2, 20.3Listed
Weakness

CWE details

CWE-131

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