CVE-2026-101955 in Junos OS
Summary
by MITRE • 10/14/2026
A Stack-based Buffer Overflow vulnerability in the UTM Anti-Spam SBL of Juniper Networks Junos OS on SRX series allows an unauthenticated remote attacker to cause arbitrary code execution or a denial of service via a injected malformed DNS packet when Anti-Spam SBL inspection is enabled.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in the Juniper Networks Junos OS software, specifically affecting Secure Routing Platform (SRX) series devices, represents a critical security flaw within the Unified Threat Management (UTM) Anti-Spam Service Block List feature. This stack-based buffer overflow condition arises from improper bounds checking when processing incoming Domain Name System packets during anti-spam inspection operations. The core technical failure lies in the application's handling of specific fields within DNS query or response structures, where input data is copied into a fixed-size memory buffer on the call stack without adequate validation of its length against the allocated space. When an attacker crafts and transmits a specially designed malformed DNS packet containing oversized or maliciously structured data segments, the excess bytes overflow the designated buffer boundaries. This overwrite corrupts adjacent memory locations, including critical control flow data such as return addresses stored on the stack, thereby allowing for potential arbitrary code execution under specific conditions where the overwritten address points to attacker-controlled shellcode residing in the same process space.
From an operational perspective, this vulnerability poses a severe risk to network infrastructure integrity and availability because it can be exploited by unauthenticated remote attackers over standard DNS ports without requiring any prior credentials or user interaction. The impact is twofold: first, successful exploitation may lead to complete system compromise, granting the attacker full control over the SRX device with elevated privileges equivalent to the root account running the inspection engine. This level of access enables lateral movement within the protected network segment and potential pivoting into internal corporate networks that rely on the firewall for perimeter defense. Second, even if code execution is not achieved due to modern memory protection mechanisms like stack canaries or non-executable stacks being active, the malformed packet processing triggers a crash in the inspection process, resulting in a denial of service condition. This disruption halts anti-spam filtering capabilities and may cause broader connectivity issues depending on how Junos OS handles process restarts for critical security services during an attack scenario.
The classification of this flaw aligns with Common Weakness Enumeration standard CWE-120, which describes buffer copy without checking size limits leading to overflow, specifically manifesting as a stack-based variant often categorized under CWE-787 in contexts involving out-of-bounds writes on the call stack. In terms of adversary tactics and techniques, this vulnerability maps directly to MITRE ATT&CK technique T1203, which covers exploitation for client execution, particularly when targeting network services that process untrusted input such as DNS resolvers or inspection engines. Attackers leveraging this flaw would typically engage in reconnaissance phases to identify vulnerable Junos OS versions before crafting targeted payloads designed to bypass existing security controls and achieve initial access through the compromised firewall appliance itself rather than just disrupting service.
Mitigation strategies must prioritize immediate patching of the Juniper Networks Junos OS software to the latest stable release that includes fixes for this specific buffer overflow condition, as vendor-provided updates address the underlying input validation logic errors within the Anti-Spam SBL module. In environments where immediate patching is not feasible due to maintenance windows or compatibility concerns, network administrators should implement strict access control lists on upstream routers and firewalls to restrict DNS traffic destined for the SRX device's inspection interfaces to only trusted internal resolvers, effectively isolating the vulnerable service from direct exposure to untrusted external networks. Additionally enabling enhanced logging and monitoring capabilities allows security operations centers to detect anomalous patterns of malformed DNS queries that might indicate active exploitation attempts against this vulnerability. Long-term resilience requires adopting a defense-in-depth approach where multiple layers of inspection are employed so that failure or compromise in one component does not result in total network exposure, alongside regular audits of configuration settings ensuring that unnecessary services like UTM features remain disabled if they do not align with the organization's specific security requirements and threat model.