CVE-2021-0205 in Juniper
Summary
by MITRE • 01/16/2021
When the "Intrusion Detection Service" (IDS) feature is configured on Juniper Networks MX series with a dynamic firewall filter using IPv6 source or destination prefix, it may incorrectly match the prefix as /32, causing the filter to block unexpected traffic. This issue affects only IPv6 prefixes when used as source and destination. This issue affects MX Series devices using MS-MPC, MS-MIC or MS-SPC3 service cards with IDS service configured. This issue affects: Juniper Networks Junos OS 17.3 versions prior to 17.3R3-S10 on MX Series; 17.4 versions prior to 17.4R3-S3 on MX Series; 18.1 versions prior to 18.1R3-S11 on MX Series; 18.2 versions prior to 18.2R3-S6 on MX Series; 18.3 versions prior to 18.3R3-S4 on MX Series; 18.4 versions prior to 18.4R3-S6 on MX Series; 19.1 versions prior to 19.1R2-S2, 19.1R3-S3 on MX Series; 19.2 versions prior to 19.2R3-S1 on MX Series; 19.3 versions prior to 19.3R2-S5, 19.3R3-S1 on MX Series; 19.4 versions prior to 19.4R3 on MX Series; 20.1 versions prior to 20.1R2 on MX Series; 20.2 versions prior to 20.2R2 on MX Series;
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Analysis
by VulDB Data Team • 02/14/2021
The vulnerability described in CVE-2021-0205 represents a critical flaw in Juniper Networks MX series routers that impacts the Intrusion Detection Service functionality when handling IPv6 traffic through dynamic firewall filters. This issue specifically manifests when IPv6 source or destination prefixes are configured within the IDS framework, creating a misinterpretation of prefix lengths that leads to unintended traffic blocking behavior. The flaw stems from incorrect handling of IPv6 prefix matching logic within the router's packet processing pipeline, where the system erroneously treats IPv6 prefixes as /32 subnet masks regardless of their actual configuration, resulting in overzealous filtering that blocks legitimate traffic.
The technical implementation of this vulnerability resides in the packet classification and filtering mechanisms of the MX series devices when utilizing MS-MPC, MS-MIC, or MS-SPC3 service cards with IDS service enabled. When IPv6 prefixes are defined in dynamic firewall filters, the system fails to properly parse and validate the prefix length information, causing it to default to a /32 mask interpretation. This misconfiguration affects the routing and forwarding decisions made by the router's control plane, where the IDS service incorrectly evaluates IPv6 address ranges and applies filtering rules based on the erroneous /32 interpretation rather than the intended prefix specifications. The vulnerability is particularly concerning because it operates at the network layer where traffic is classified and filtered, potentially disrupting legitimate communications while appearing to function correctly from a configuration standpoint.
The operational impact of this vulnerability extends beyond simple traffic disruption to potentially cause significant network availability issues and service degradation across affected Juniper MX Series deployments. Networks utilizing IDS features with IPv6 prefixes for source or destination matching may experience unexpected blocking of legitimate traffic flows, leading to service interruptions, performance degradation, and potential security posture weakening when legitimate traffic is incorrectly classified as malicious. The issue affects multiple Junos OS versions across different release branches, indicating a widespread exposure that requires careful assessment and remediation across various network environments. Organizations relying on these devices for network security monitoring and protection may find their security controls compromised, as the IDS system fails to properly distinguish between intended and unintended traffic patterns due to the flawed prefix handling mechanism.
Mitigation strategies for this vulnerability require immediate implementation of the relevant Juniper security patches and firmware updates addressing the specific prefix matching logic error in the IDS service. Network administrators should prioritize updating affected MX Series devices to the patched versions specified in the security advisories, particularly focusing on the version ranges mentioned in the CVE description including 17.3R3-S10, 17.4R3-S3, and subsequent releases. Configuration reviews should include verification of all dynamic firewall filters utilizing IPv6 prefixes to ensure proper prefix length specifications and to validate that the IDS service is functioning as intended. Additionally, organizations should implement monitoring procedures to detect any unusual traffic blocking patterns that might indicate the vulnerability's impact, while also considering temporary workarounds such as disabling IDS features on affected systems until proper patches can be deployed. This vulnerability aligns with CWE-248, representing an unchecked return value or error condition, and may be leveraged by threat actors to disrupt network services or potentially evade detection through traffic manipulation, as documented in various ATT&CK techniques related to network denial of service and evasion strategies.