CVE-2026-101991 in Junos OS
Summary
by MITRE • 10/15/2026
An Improper Handling of Length Parameter Inconsistency vulnerability in the routing protocol daemon (rpd) of Juniper Networks Junos OS and Junos OS Evolved allows a configured BGP-LS peer to cause heap data disclosure via a malformed UPDATE sent over a session with the traffic-engineering address family enabled.
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Analysis
by VulDB Data Team • 10/15/2026
The identified vulnerability resides within the routing protocol daemon, commonly referred to as rpd, which is a core component of Juniper Networks Junos OS and Junos OS Evolved operating systems. This specific flaw manifests as an improper handling of length parameter inconsistency when processing Border Gateway Protocol Link State (BGP-LS) updates. BGP-LS extends the standard BGP protocol to carry link-state information, enabling traffic engineering applications such as Segment Routing and MPLS TE path computation. The vulnerability is triggered specifically when a configured peer sends a malformed UPDATE message over an established session where the traffic-engineering address family has been enabled on both ends of the peering relationship.
From a technical perspective, the root cause lies in how the daemon processes variable-length fields within BGP-LS NLRI (Network Layer Reachability Information) or path attributes. When parsing these structures, the software fails to adequately validate that the declared length of certain parameters matches the actual data available for reading. This discrepancy leads to an out-of-bounds read operation on the heap memory space allocated by the daemon. Instead of rejecting the malformed packet with a standard error notification, the processing logic proceeds to copy or reference memory regions beyond the intended buffer boundaries. Consequently, this results in the disclosure of sensitive internal state information stored in adjacent heap locations.
The operational impact of this vulnerability is primarily categorized as an unauthorized data exposure rather than immediate remote code execution or denial of service. An authenticated attacker who has established a BGP session with the vulnerable device can exploit this flaw to leak portions of the router's memory contents back through the protocol stream. While the leaked data may not directly reveal user credentials, it could expose internal routing table structures, neighbor states, cryptographic keys used for MD5 authentication if present, or other sensitive operational details that aid in further reconnaissance. This information disclosure can facilitate more sophisticated attacks against the network infrastructure by revealing topology details and configuration nuances that are otherwise protected from external observers.
In terms of industry standard classifications, this vulnerability aligns with CWE-20: Improper Input Validation regarding the length parameters within protocol messages. Furthermore, it falls under the broader category of CWE-134: Use of Externally-Controlled Format String or CWE-787: Out-of-bounds Write depending on the specific memory access pattern observed during exploitation attempts, though data disclosure typically maps to information leakage vectors. Within the MITRE ATT&CK framework, this behavior is consistent with techniques used in reconnaissance phases where an adversary gathers system configuration and network topology details from compromised or targeted systems. The attack vector requires a pre-existing BGP session, meaning it does not qualify as unauthenticated remote exploitation but rather represents a risk for insider threats or attackers who have already breached the perimeter authentication controls of the routing infrastructure.
Mitigation strategies should focus on immediate patching to the latest stable release of Junos OS and Junos OS Evolved that includes fixes for this specific rpd flaw. Network administrators must ensure that all BGP-LS sessions are secured using strong authentication mechanisms such as TCP MD5 signatures or, preferably, BGPsec where supported, to prevent unauthorized peers from establishing sessions in the first place. Additionally, implementing strict access control lists (ACLs) on router interfaces can limit which IP addresses are permitted to initiate BGP connections, thereby reducing the attack surface for potential exploitation attempts by external actors who have not yet achieved authentication. Regular monitoring of routing daemon logs and memory usage patterns may also help detect anomalous behavior indicative of such exploitation attempts before significant data leakage occurs.