CriticalCVE-2021-35394

    Realtek Jungle SDK Remote Code Execution Vulnerability (CVE-2021-35394)

    Published In CISA KEV since Sources: SingCERT, CVE.org

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    What is affected, what to do

    Realtek Jungle SDK version v2.x up to v3.4.14B provides a diagnostic tool called 'MP Daemon' that is usually compiled as 'UDPServer' binary. The binary is affected by multiple memory corruption vulnerabilities and an arbitrary command injection vulnerability that can be exploited by remote unauthenticated attackers.

    The vulnerability is being actively exploited; treat it as a priority. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Realtek AP-Router SDK lte 2.0..3.4.14b

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: SingCERT, CVE.org

    99.87 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.96

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    228.94 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: CIRCL KEV, CSA/SingCERT How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    9.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    99.87 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.96
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 4 February 2025
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-78

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This CVE is on KEV because it is network-reachable without authentication and has been the backbone of multiple Mirai-successor botnets (Mozi, Fodcha and others) for years — mass scanning of UDP/9034 is a constant background on the public internet, and its exploitation likelihood ranks among the highest in the entire catalogue. The target profile is not servers but embedded devices with a Realtek chipset that often sit "invisibly" in the network (branch-office routers, production gateways, surveillance cameras). Priority is driven less by any single critical asset than by sheer prevalence and the risk that compromised devices become pivots into production networks or launchpads for DDoS — a reportable scenario for NIS2-scope operators. Treat as P1 whenever any Realtek-based device is WAN-exposed; P2 for strictly internal, segmented deployments.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory every device with a Realtek RTL819x chipset — primarily SOHO routers, access points, IP cameras, repeaters, smart-home gateways and industrial gateways from numerous OEMs (D-Link, Netgear, Tenda, Edimax, TRENDnet, LG, Zyxel, ASUS and others). Cross-check the list against the respective OEM advisories.
      • Block UDP port 9034 (MP Daemon / UDPServer binary) at the perimeter firewall and at every internal segment boundary — ingress and egress. This port is the primary attack vector.
      • Remove management and diagnostic interfaces of affected devices from the public internet (verify external exposure via Shodan queries for "Realtek" or "UDPServer"). WAN-side reachability of UDP/9034 is grounds for immediate isolation.
      • Apply firmware updates from the respective OEM, not from Realtek directly. Realtek shipped fixes in Jungle SDK 3.4.14B and later, but patch availability depends entirely on the device vendor. For devices with no update path: schedule replacement (End-of-Life is the common case).
      • For devices that cannot be patched: force-segment them into a dedicated IoT VLAN with no internet egress and no reachability to production networks.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: dedicated IoT/OT VLAN with default-deny ACLs. Only explicitly whitelisted egress flows (DNS, NTP, vendor cloud); no lateral reachability to clients, servers or AD.
      • Perimeter firewall: egress policy blocking IoT-segment connections to known Mirai/Mozi C2 infrastructure and TOR exit nodes. Drop UDP/9034 WAN-side explicitly.
      • IPS/IDS: Suricata/Snort rules on UDP/9034 matching the known PoC patterns (oversized hostname/SSID fields, "echo"/"wget" command-injection strings in the UDP payload). Emerging Threats ships an "ET EXPLOIT Realtek SDK" ruleset — enable it.
      • Device hardening: disable UPnP, WPS and all unneeded management services. Rotate default credentials, disable WAN-side remote management entirely.
      • Monitor egress anomalies: sudden spikes of outbound SYN-flood traffic, DNS amplification, or scanning on telnet/23 and SSH/22 originating from the IoT segment indicate botnet recruitment (Mirai variants actively weaponise this CVE).
      • Procurement policy: new acquisitions only with documented firmware supply chain and a minimum five-year security-patch commitment (NIS2 supply-chain requirement).

    3. Runbook step 3

      Detection rules

      • Zeek/Suricata: alert udp any any -> $HOME_NET 9034 with payload content-match on shell metacharacters (;, |, backticks, $()) in the UDP body — high-confidence exploitation attempt.
      • NetFlow/IPFIX: baseline for UDP/9034 is near zero. Alert on any inbound flow on this port, especially small packets (< 200 bytes) from internet sources.
      • EDR / device syslog where available: a /bin/sh, wget, curl, tftp or busybox process spawned as a child of UDPServer is the smoking gun. KQL idea: DeviceProcessEvents | where InitiatingProcessFileName == "UDPServer" and FileName in ("sh","wget","tftp","curl","busybox").
      • DNS telemetry: alert on IoT-VLAN lookups to dynamic-DNS providers (duckdns.org, no-ip, afraid.org) or to known Mirai/Mozi C2 domains.
      • Firewall logs: a single IoT device suddenly connecting out to heterogeneous destination IPs on ports 23, 2323, 22, 80, 8080 — classic scan behaviour of a recruited botnet node.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Realtek Jungle SDK version v2.x up to v3.4.14B provides a diagnostic tool called 'MP Daemon' that is usually compiled as 'UDPServer' binary. The binary is affected by multiple memory corruption vulnerabilities and an arbitrary command injection vulnerability that can be exploited by remote unauthenticated attackers.

    Source: CSA/SingCERT, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    realtekrtl819x_jungle_software_development_kit
    • 2.0 – ≤ 3.4.14b
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

      Base

      The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.

      Entry at MITRE CWE-78

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    CSA/SingCERTCritical Vulnerability In Realtek Jungle Software Development Kit (SDK) (opens in a new tab)CSA/SingCERTactively exploited
    CIRCL KEVvulnerability.circl.lu (opens in a new tab)actively exploited
    PoC-in-GitHub (nomi-sec)cve-2021-35394-ecosystem (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • securityfocus.comarchive/1/534765x_refsource_MISC, Broken link, Third-party advisory, Vulnerability database
    • realtek.comen/cu-1-en/cu-1-taiwan-enx_refsource_MISC, Broken link, Patch, Vendor advisory
    • realtek.comimages/safe-report/Realtek_APRouter_SDK_Advisory-CVE-2021-35392…x_refsource_MISC, Patch, Vendor advisory
    • iot-inspector.comblog/advisory-multiple-issues-realtek-sdk-iot-supply-chainx_refsource_MISC, Broken link, Exploit, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-35394government-resource, US government resource
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