Evidence used
- No CISA KEV confirmation is currently recorded.
- The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
- EPSS is 1.00% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2021
CVE-2021-0273High confidence
Junos OS and Junos OS Evolved: Trio Chipset: Denial of Service due to packet destined to device's interfaces.
Juniper Networks · Junos OS
5.3MediumCVSS 3.1
Scheduled
Patch availableMedium technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityMedium→Operational priority:Medium, unchanged from published severity.unchanged
Compensating controls
- Validate the affected product branch and deploy the verified fixed release.
- Restrict the affected network interface to trusted sources where business-safe.
- Monitor vendor guidance and exploitation sources for a material change.
Verification
- Confirm that the asset runs Juniper Networks Junos OS and falls inside the recorded affected range.
- Verify the installed build against the product-specific fixed version after deployment.
- Validate exposure, authentication requirements and compensating controls in the actual environment.
- Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Medium technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority.
Patch available01
What, why and how
An always-incorrect control flow implementation in the implicit filter terms of Juniper Networks Junos OS and Junos OS Evolved on ACX5800, EX9200 Series, MX10000 Series, MX240, MX480, MX960 devices with affected Trio line cards allows an attacker to exploit an interdependency in the PFE UCODE microcode of the Trio chipset with various line cards to cause packets destined to the devices interfaces to cause a Denial of Service (DoS) condition by looping the packet with an unreachable exit condition ('Infinite Loop'). To break this loop once it begins one side of the affected LT interfaces will need to be disabled. Once disabled, the condition will clear and the disabled LT interface can be reenabled. Continued receipt and processing of these packets will create a sustained Denial of Service (DoS) condition. This issue only affects LT-LT interfaces. Any other interfaces are not affected by this issue. This issue affects the following cards: MPCE Type 3 3D MPC4E 3D 32XGE MPC4E 3D 2CGE+8XGE EX9200 32x10G SFP EX9200-2C-8XS FPC Type 5-3D FPC Type 5-LSR EX9200 4x40G QSFP An Indicator of Compromise (IoC) can be seen by examining the traffic of the LT-LT interfaces for excessive traffic using the following command: monitor interface traffic Before loop impact: Interface: lt-2/0/0, Enabled, Link is Up Encapsulation: Logical-tunnel, Speed: 100000mbps Traffic statistics: Current delta Input bytes: 3759900268942 (1456 bps) [0] <---------- LT interface utilization is low Output bytes: 3759900344309 (1456 bps) [0] <---------- LT interface utilization is low After loop impact: Interface: lt-2/0/0, Enabled, Link is Up Encapsulation: Logical-tunnel, Speed: 100000mbps Traffic statistics: Current delta Input bytes: 3765160313129 (2158268368 bps) [5260044187] <---------- LT interface utilization is very high Output bytes: 3765160399522 (2158266440 bps) [5260055213] <---------- LT interface utilization is very high This issue affects: Juniper Networks Junos OS on ACX5800, EX9200 Series, MX10000 Series, MX240, MX480, MX960. Versions 15.1F6, 16.1R1, and later versions prior to 16.1R7-S8; 17.1 versions prior to 17.1R2-S12; 17.2 versions prior to 17.2R3-S4; 17.3 versions prior to 17.3R3-S8; 17.4 versions prior to 17.4R2-S10, 17.4R3-S2; 18.1 versions prior to 18.1R3-S10; 18.2 versions prior to 18.2R2-S7, 18.2R3-S3; 18.3 versions prior to 18.3R1-S7, 18.3R3-S2; 18.4 versions prior to 18.4R1-S7, 18.4R2-S4, 18.4R3-S2; 19.1 versions prior to 19.1R1-S5, 19.1R2-S1, 19.1R3; 19.2 versions prior to 19.2R1-S4, 19.2R2; 19.3 versions prior to 19.3R2-S3, 19.3R3; 19.4 versions prior to 19.4R1-S1, 19.4R2. This issue does not affect the MX10001. This issue does not affect Juniper Networks Junos OS versions prior to 15.1F6, 16.1R1. Juniper Networks Junos OS Evolved on ACX5800, EX9200 Series, MX10000 Series, MX240, MX480, MX960 19.4 versions prior to 19.4R2-EVO. This issue does not affect the MX10001.
What
An always-incorrect control flow implementation in the implicit filter terms of Juniper Networks Junos OS and Junos OS Evolved on ACX5800, EX9200 Series, MX10000 Series, MX240, MX480, MX960 devices with affected Trio line cards allows an attacker to exploit an interdependency in the PFE UCODE microcode of the Trio chipset with various line cards to cause packets destined to the devices interfaces to cause a Denial of Service (DoS) condition by looping the packet with an unreachable exit condition ('Infinite Loop'). To break this loop once it begins one side of the affected LT interfaces will need to be disabled. Once disabled, the condition will clear and the disabled LT interface can be reenabled. Continued receipt and processing of these packets will create a sustained Denial of Service (DoS) condition. This issue only affects LT-LT interfaces. Any other interfaces are not affected by this issue. This issue affects the following cards: MPCE Type 3 3D MPC4E 3D 32XGE MPC4E 3D 2CGE+8XGE EX9200 32x10G SFP EX9200-2C-8XS FPC Type 5-3D FPC Type 5-LSR EX9200 4x40G QSFP An Indicator of Compromise (IoC) can be seen by examining the traffic of the LT-LT interfaces for excessive traffic using the following command: monitor interface traffic Before loop impact: Interface: lt-2/0/0, Enabled, Link is Up Encapsulation: Logical-tunnel, Speed: 100000mbps Traffic statistics: Current delta Input bytes: 3759900268942 (1456 bps) [0] <---------- LT interface utilization is low Output bytes: 3759900344309 (1456 bps) [0] <---------- LT interface utilization is low After loop impact: Interface: lt-2/0/0, Enabled, Link is Up Encapsulation: Logical-tunnel, Speed: 100000mbps Traffic statistics: Current delta Input bytes: 3765160313129 (2158268368 bps) [5260044187] <---------- LT interface utilization is very high Output bytes: 3765160399522 (2158266440 bps) [5260055213] <---------- LT interface utilization is very high This issue affects: Juniper Networks Junos OS on ACX5800, EX9200 Series, MX10000 Series, MX240, MX480, MX960. Versions 15.1F6, 16.1R1, and later versions prior to 16.1R7-S8; 17.1 versions prior to 17.1R2-S12; 17.2 versions prior to 17.2R3-S4; 17.3 versions prior to 17.3R3-S8; 17.4 versions prior to 17.4R2-S10, 17.4R3-S2; 18.1 versions prior to 18.1R3-S10; 18.2 versions prior to 18.2R2-S7, 18.2R3-S3; 18.3 versions prior to 18.3R1-S7, 18.3R3-S2; 18.4 versions prior to 18.4R1-S7, 18.4R2-S4, 18.4R3-S2; 19.1 versions prior to 19.1R1-S5, 19.1R2-S1, 19.1R3; 19.2 versions prior to 19.2R1-S4, 19.2R2; 19.3 versions prior to 19.3R2-S3, 19.3R3; 19.4 versions prior to 19.4R1-S1, 19.4R2. This issue does not affect the MX10001. This issue does not affect Juniper Networks Junos OS versions prior to 15.1F6, 16.1R1. Juniper Networks Junos OS Evolved on ACX5800, EX9200 Series, MX10000 Series, MX240, MX480, MX960 19.4 versions prior to 19.4R2-EVO. This issue does not affect the MX10001.
Why
A malformed condition prevents a loop from making progress or terminating, allowing resource exhaustion.
How
An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may disrupt the affected service.
What
An always-incorrect control flow implementation in the implicit filter terms of Juniper Networks Junos OS and Junos OS Evolved on ACX5800, EX9200 Series, MX10000 Series, MX240, MX480, MX960 devices with affected Trio line cards allows an attacker to exploit an interdependency in the PFE UCODE microcode of the Trio chipset with various line cards to cause packets destined to the devices interfaces to cause a Denial of Service (DoS) condition by looping the packet with an unreachable exit condition ('Infinite Loop'). To break this loop once it begins one side of the affected LT interfaces will need to be disabled. Once disabled, the condition will clear and the disabled LT interface can be reenabled. Continued receipt and processing of these packets will create a sustained Denial of Service (DoS) condition. This issue only affects LT-LT interfaces. Any other interfaces are not affected by this issue. This issue affects the following cards: MPCE Type 3 3D MPC4E 3D 32XGE MPC4E 3D 2CGE+8XGE EX9200 32x10G SFP EX9200-2C-8XS FPC Type 5-3D FPC Type 5-LSR EX9200 4x40G QSFP An Indicator of Compromise (IoC) can be seen by examining the traffic of the LT-LT interfaces for excessive traffic using the following command: monitor interface traffic Before loop impact: Interface: lt-2/0/0, Enabled, Link is Up Encapsulation: Logical-tunnel, Speed: 100000mbps Traffic statistics: Current delta Input bytes: 3759900268942 (1456 bps) [0] <---------- LT interface utilization is low Output bytes: 3759900344309 (1456 bps) [0] <---------- LT interface utilization is low After loop impact: Interface: lt-2/0/0, Enabled, Link is Up Encapsulation: Logical-tunnel, Speed: 100000mbps Traffic statistics: Current delta Input bytes: 3765160313129 (2158268368 bps) [5260044187] <---------- LT interface utilization is very high Output bytes: 3765160399522 (2158266440 bps) [5260055213] <---------- LT interface utilization is very high This issue affects: Juniper Networks Junos OS on ACX5800, EX9200 Series, MX10000 Series, MX240, MX480, MX960. Versions 15.1F6, 16.1R1, and later versions prior to 16.1R7-S8; 17.1 versions prior to 17.1R2-S12; 17.2 versions prior to 17.2R3-S4; 17.3 versions prior to 17.3R3-S8; 17.4 versions prior to 17.4R2-S10, 17.4R3-S2; 18.1 versions prior to 18.1R3-S10; 18.2 versions prior to 18.2R2-S7, 18.2R3-S3; 18.3 versions prior to 18.3R1-S7, 18.3R3-S2; 18.4 versions prior to 18.4R1-S7, 18.4R2-S4, 18.4R3-S2; 19.1 versions prior to 19.1R1-S5, 19.1R2-S1, 19.1R3; 19.2 versions prior to 19.2R1-S4, 19.2R2; 19.3 versions prior to 19.3R2-S3, 19.3R3; 19.4 versions prior to 19.4R1-S1, 19.4R2. This issue does not affect the MX10001. This issue does not affect Juniper Networks Junos OS versions prior to 15.1F6, 16.1R1. Juniper Networks Junos OS Evolved on ACX5800, EX9200 Series, MX10000 Series, MX240, MX480, MX960 19.4 versions prior to 19.4R2-EVO. This issue does not affect the MX10001.
Why
A malformed condition prevents a loop from making progress or terminating, allowing resource exhaustion.
How
An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may disrupt the affected service.
02
Exploit reality and attack path
CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.
Observed exploitation No confirmed evidence
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.
Public PoC / exploit material None recorded
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.
- Likely attack path
- a network path → Loop with Unreachable Exit Condition ('Infinite Loop') → disrupt the affected service
- Attack surface
- Network
- Privileges required
- None: unauthenticated exploitation is possible
- User interaction
- None
- Attack complexity
- Low: no specialised conditions are recorded
- Security boundary
- Unchanged: impact remains within the vulnerable component's security authority
- Weakness
?
CWE means Common Weakness Enumeration: a standard category for the underlying weakness. - CWE-835 ↗
CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop'). The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.
- CVSS vector
?
CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity. CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:LCommon Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.
AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CNoneConfidentiality impact: No direct loss is represented by this metric.INoneIntegrity impact: No direct loss is represented by this metric.ALowAvailability impact: A successful attack can cause a limited loss.- Post-exploitation / living off the land
- No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
03
Patch and workaround
Operational remediation based on structured source evidence.
- Status
?
Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change. - Patch available
- Affected
- Junos OS: 15.1F6 < 15.1*, 16.1R1 < 16.1*, 17.1 < 17.1R2-S12, 17.2 < 17.2R3-S4, 17.3 < 17.3R3-S8, 17.4 < 17.4R2-S10, 17.4R3-S2, 18.1 < 18.1R3-S10, 18.2 < 18.2R2-S7, 18.2R3-S3, 18.3 < 18.3R1-S7, 18.3R3-S2, 18.4 < 18.4R1-S7, 18.4R2-S4, 18.4R3-S2, 19.1 < 19.1R1-S5, 19.1R2-S1, 19.1R3, 19.2 < 19.2R1-S4, 19.2R2, 19.3 < 19.3R2-S3, 19.3R3, 19.4 < 19.4R1-S1, 19.4R2
- Junos OS Evolved: 19.4 < 19.4R2-EVO
- Action
- Review the linked authoritative reference and apply the recorded fixed release appropriate to the affected product branch.
- Workaround
- No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04
Evidence and provenance
Published 22 Apr 2021 · Last source change 17 Sept 2024, 00:26 UTC · CWE-835 · Loop with Unreachable Exit Condition ('Infinite Loop')
CVE recordCVE.org · 5.1
CVSS sourceCNA
EPSS source FIRST · tracked since 2026-08-14
?
The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.European sourceENISA EUVD · EUVD-2021-2892
Product sourceCNA
Remediation sourceCVE/CNA references
CWE sourceCNA
NVD statusNVD modified after enrichment
Core structured fields are present and their contributing authorities are shown above.
Material change intelligence
View recent updates ↗What changed after publication
No material field changes have been recorded since change tracking began. Routine source refreshes and cosmetic edits are intentionally excluded.
CVE published
Fixed release or patch reference recorded
Technical terms and abbreviations used in this report
- CVE
- Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
- CVSS
- Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
- EPSS
- Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
- CWE
- Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
- CNA
- CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
- CISA ADP
- Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
- NVD
- National Vulnerability Database: NIST's enrichment service for CVE records.
- CERT / CSIRT
- A computer security incident response team that publishes warnings or coordinates incident response.
- PoC
- Proof of concept: public material that demonstrates or helps reproduce exploitation.
- CSAF
- Common Security Advisory Framework: a machine-readable format for security advisories.
- LoTL
- Living off the land: abuse of legitimate tools or system functions during an attack.