Evidence used
- No CISA KEV confirmation is currently recorded.
- EPSS is 0.87% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2021
CVE-2021-0269High confidence
Junos OS: J-Web can be compromised through reflected client-side HTTP parameter pollution attacks.
Juniper Networks · Junos OS
8.8HighCVSS 3.1
Within 7 days
Patch availableHigh technical severity; prioritise exposed affected systems while verifying vendor guidance.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityHigh→Operational priority:High, 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.
High technical severity; prioritise exposed affected systems while verifying vendor guidance.
Patch available01
What, why and how
The improper handling of client-side parameters in J-Web of Juniper Networks Junos OS allows an attacker to perform a number of different malicious actions against a target device when a user is authenticated to J-Web. An attacker may be able to supersede existing parameters, including hardcoded parameters within the HTTP/S session, access and exploit variables, bypass web application firewall rules or input validation mechanisms, and otherwise alter and modify J-Web's normal behavior. An attacker may be able to transition victims to malicious web services, or exfiltrate sensitive information from otherwise secure web forms. This issue affects: Juniper Networks Junos OS: All versions prior to 17.4R3-S3; 18.1 versions prior to 18.1R3-S12; 18.2 versions prior to 18.2R3-S6; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R3-S6; 19.1 versions prior to 19.1R3-S4; 19.2 versions prior to 19.2R3-S1; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S2, 19.4R3; 20.1 versions prior to 20.1R2; 20.2 versions prior to 20.2R2.
What
The improper handling of client-side parameters in J-Web of Juniper Networks Junos OS allows an attacker to perform a number of different malicious actions against a target device when a user is authenticated to J-Web. An attacker may be able to supersede existing parameters, including hardcoded parameters within the HTTP/S session, access and exploit variables, bypass web application firewall rules or input validation mechanisms, and otherwise alter and modify J-Web's normal behavior. An attacker may be able to transition victims to malicious web services, or exfiltrate sensitive information from otherwise secure web forms. This issue affects: Juniper Networks Junos OS: All versions prior to 17.4R3-S3; 18.1 versions prior to 18.1R3-S12; 18.2 versions prior to 18.2R3-S6; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R3-S6; 19.1 versions prior to 19.1R3-S4; 19.2 versions prior to 19.2R3-S1; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S2, 19.4R3; 20.1 versions prior to 20.1R2; 20.2 versions prior to 20.2R2.
Why
The product does not properly handle when the expected number of parameters, fields, or arguments is not provided in input, or if those parameters are undefined.
How
An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may obtain information outside the intended access boundary.
What
The improper handling of client-side parameters in J-Web of Juniper Networks Junos OS allows an attacker to perform a number of different malicious actions against a target device when a user is authenticated to J-Web. An attacker may be able to supersede existing parameters, including hardcoded parameters within the HTTP/S session, access and exploit variables, bypass web application firewall rules or input validation mechanisms, and otherwise alter and modify J-Web's normal behavior. An attacker may be able to transition victims to malicious web services, or exfiltrate sensitive information from otherwise secure web forms. This issue affects: Juniper Networks Junos OS: All versions prior to 17.4R3-S3; 18.1 versions prior to 18.1R3-S12; 18.2 versions prior to 18.2R3-S6; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R3-S6; 19.1 versions prior to 19.1R3-S4; 19.2 versions prior to 19.2R3-S1; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S2, 19.4R3; 20.1 versions prior to 20.1R2; 20.2 versions prior to 20.2R2.
Why
The product does not properly handle when the expected number of parameters, fields, or arguments is not provided in input, or if those parameters are undefined.
How
An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may obtain information outside the intended access boundary.
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 → Improper Handling of Parameters → obtain information outside the intended access boundary
- Attack surface
- Network
- Privileges required
- None: unauthenticated exploitation is possible
- User interaction
- Required interaction required
- 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-233 ↗
CWE-233: Improper Handling of Parameters. The product does not properly handle when the expected number of parameters, fields, or arguments is not provided in input, or if those parameters are undefined.
- 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:R/S:U/C:H/I:H/A:HCommon 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.UIRequiredUser interaction: Another user must perform an action for exploitation to succeed.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major 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
- 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, 01:12 UTC · CWE-233 · Improper Handling of Parameters
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-2021-2888
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.
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.