CVE-2026-38577

Executive Summary

CVE-2026-38577 describes a critical vulnerability in the Tenda HG21 router firmware version V4.0.0-260302, where the Admin account is protected by insecure hardcoded credentials. The NVD description states that this flaw "allows attackers to gain root access," indicating the hardcoded credentials grant the highest level of privilege on the device. With a CVSS score of 9.8 (Critical) and a network-based attack vector requiring no authentication or user interaction, any attacker who can reach the device's management interface can achieve full compromise. As of the publication date, no vendor patch exists, no public exploit code has been released, and this vulnerability is not listed in CISA's Known Exploited Vulnerabilities catalog.

Technical Analysis

The vulnerability is classified under CWE-798, which covers the use of hardcoded credentials. This flaw exists in the Tenda HG21 router, a device belonging to Tenda's networking product line. The affected firmware version is V4.0.0-260302.

Vulnerability Mechanism: At the vulnerability-class level, CWE-798 means that the device's firmware contains embedded administrative credentials that are identical across all devices running the same firmware version. These credentials are compiled into the firmware image or stored in configuration files shipped with the device. An attacker who obtains a copy of the firmware (routinely available from vendor websites) can extract these credentials through static analysis — by searching for credential-like strings, examining configuration templates, or reverse-engineering the authentication module.

Root Access Implication: The NVD description explicitly states that exploitation results in "root access." This indicates that the hardcoded credentials are not merely for a limited administrative web interface account but authenticate at the operating-system level of the device, granting a root shell or equivalent privilege. This is more severe than a typical router admin credential bypass, as it exposes the underlying Linux-based operating system to full compromise.

Attack Surface: The CVSS vector AV:N/AC:L/PR:N/UI:N indicates the attack surface is network-exposed and trivially exploitable. Specifically:

• AV:N — The management interface (likely SSH, Telnet, or HTTPS admin portal) is reachable over the network; no local access is required.
• AC:L — No special conditions or race conditions are required; the attack is straightforward.
• PR:N — No prior authentication is required; the hardcoded credentials serve as the bypass itself.
• UI:N — No user interaction from a victim is needed.

The combination of hardcoded credentials granting root access with a network-reachable interface makes this vulnerability particularly dangerous for any organization that has deployed these devices on their perimeter or in network segments reachable from untrusted networks.

The NVD description does not specify which protocol (SSH, HTTP, Telnet) the credentials apply to, and insufficient technical detail is available regarding the exact credential storage format or the authentication flow that is bypassed. Security teams should assume all remote management interfaces are affected until firmware analysis proves otherwise.

Exploitation Scenarios

No public PoC is available; the attack chain is inferred from vulnerability class. The following describes the exploitation path under the constraints of the data provided:

Step 1: Reconnaissance/Target Identification An attacker identifies Tenda HG21 devices exposed to the network. This is accomplished through internet-wide scanning (e.g., Shodan, Censys) using fingerprints such as the HTTP server banner, the device's web login page title, or open management ports (commonly TCP 80/443 for web, TCP 22 for SSH, TCP 23 for Telnet). The attacker then confirms the firmware version V4.0.0-260302 by querying the device's status page or by comparing response headers against known firmware fingerprints.

Step 2: Triggering the Vulnerability-Class Flaw The attacker leverages the hardcoded credentials (CWE-798). They either:

• Extract the credentials from a copy of the firmware obtained from the vendor's support site or firmware update repository, or
• Attempt well-known default credential patterns documented for Tenda devices in public databases.

No exploitation of memory corruption or injection is required — the flaw is a static credential constant present in every affected device.

Step 3: Payload/Execution Method The attacker authenticates to a management interface (SSH, Telnet, or web admin console) using the extracted/admin-level hardcoded credentials. Because the credentials grant root access per the NVD description, the attacker gains an interactive session with full operating system privileges. At this stage, the attacker may:

• Execute arbitrary commands via the shell,
• Modify persistent configuration files,
• Install new binaries or backdoors into flash storage,
• Exfiltrate credentials or network traffic data.

Step 4: Post-Exploitation Impact With root access on a router, the attacker can:

• Redirect or intercept network traffic traversing the device (man-in-the-middle),
• Modify DNS settings to redirect users to malicious domains,
• Disable logging or security features,
• Use the device as a pivot point for lateral movement into the internal network,
• Brick the device or use it in DDoS botnets (e.g., Mirai-style).

Because the router is a network gateway device, the blast radius extends beyond the device itself to all hosts in the LAN segment it serves.

Impact Assessment

The CVSS v3.1 score of 9.8 places this vulnerability in the Critical severity band. The sub-component analysis:

• C:H (Confidentiality High): Root access enables reading of all data processed by the router, including VPN traffic if terminated on-device, credentials stored for management protocols, and network metadata.
• I:H (Integrity High): The attacker can modify firmware, configuration, and routing tables, causing persistent and stealthy compromise.
• A:H (Availability High): The attacker can disrupt routing, perform factory resets, or brick the device entirely.

Business Impact:

• Network Infrastructure Compromise: The router is the gateway to the LAN; full compromise grants the attacker a foothold inside the network perimeter, bypassing all perimeter security controls.
• Data Breach Potential: Any unencrypted or weakly encrypted traffic traversing the router can be captured or manipulated.
• Compliance Violations: For organizations subject to PCI-DSS, HIPAA, or GDPR, a router compromise of this nature may constitute a reportable incident, especially if lateral movement leading to sensitive data exposure occurs.
• Lack of Mitigation: With no patch available (patch confidence 0/4) and no vendor-issued workaround, affected organizations face an indefinite period of exposure unless they can implement compensating controls.

Affected Systems: Any Tenda HG21 device running firmware V4.0.0-260302. The vendor has not disclosed whether earlier firmware versions share the same hardcoded credentials; therefore, security teams should treat all HG21 devices as potentially affected until confirmed otherwise.

Detection Guidance

The following detection signatures target attempts to exploit the hardcoded credential scenario. Since no exploit code exists publicly, these rules focus on:

1. Authentication attempts using the hardcoded credential (detected via default/known Tenda credential patterns).
2. Network patterns indicating management protocol brute-force/credential-stuffing against Tenda devices.
3. Anomalous behavior following successful authentication (command execution, config changes).

Sigma Rule

title: Tenda HG21 Hardcoded Credential Authentication Attempt
id: 07e9f3a6-9b1c-4e8a-b2d4-d8f7c9e1a2b3
status: experimental
description: Detects authentication attempts to Tenda HG21 admin interfaces using known Tenda hardcoded credentials (CVE-2026-38577).
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-38577
tags:
    - attack.initial_access
    - attack.t1190
    - cve.2026.38577
logsource:
    product: linux
    service: auth
detection:
    selection_ssh:
        # SSH connection using suspicious username for Tenda admin
        username: "admin"
    selection_telnet:
        # Telnet login attempts with admin/default usernames
        user: "admin"
    selection_web:
        # Web access log entries showing POST to login endpoint with admin user
        cs-uri-stem|contains: "login"
        cs-username: "admin"
    condition: selection_ssh or selection_telnet or selection_web
    timeframe: 5m
level: high
falsepositives:
    - Legitimate administrators using the correct admin account
    - Devices that have been reconfigured with new credentials
fields:
    - src_ip
    - dst_ip
    - user

YARA Rule

rule Tenda_HG21_Hardcoded_Credentials
{
    meta:
        author = "Valters IT Security"
        description = "Detect YARA matches for Tenda HG21 firmware containing hardcoded credential patterns (CVE-2026-38577)"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-38577"
        cve = "CVE-2026-38577"
        date = "2026-01-01"
        severity = "critical"
    strings:
        $s1 = "admin" ascii wide nocase
        // common default password variants observed in embedded devices
        $s2 = "admin" ascii wide
        $s3 = "password" ascii wide nocase
        $s4 = "root" ascii wide nocase
        // Tenda specific patterns often found in config files
        $s5 = "hg21" ascii wide nocase
        $s6 = "tenda" ascii wide nocase
        // firmware version string anchor
        $s7 = "4.0.0-260302" ascii wide
    condition:
        uint16(0) == 0x5A4D or uint16(0) == 0x457F and 2 of them
}

Suricata IDS Rule

alert tcp any any -> any any (msg:"ET EXPLOIT Tenda HG21 Hardcoded Credential Login Attempt (CVE-2026-38577)"; flow:established,to_server; content:"admin"; nocase; content:"POST"; http_method; content:"login"; http_uri; content:"password="; http_client_body; pcre:"/(admin|tenda|root)/i"; classtype:attempted-admin; sid:2026038577; rev:1; metadata:created_at 2026_01_01, updated_at 2026_01_01, cve CVE-2026-38577, mitigation signature, severity critical;)

Elastic Detection

authentication where process.name in ("sshd", "telnetd", "httpd") and
event.action in ("login", "authenticate") and
user.name in ("admin", "root") and
(
  /* Detect repeated rapid authentication attempts (credential stuffing) */
  sequence by source.ip with maxspan=60s
  [ authentication where user.name in ("admin", "root") ]
  [ authentication where user.name in ("admin", "root") ] with runs=3
)
or
/* Detect successful login followed by command execution on router */
sequence by host.name with maxspan=10m
[ authentication where outcome.result == "success" and user.name in ("admin", "root") ]
[ process where event.action == "exec" and process.name in ("sh", "bash") ]

Splunk SPL Query

index=linux_auth OR index=main sourcetype=linux_secure OR sourcetype=syslog
| search (user="admin" OR user="root") AND (process="sshd" OR process="telnetd" OR app="httpd")
| stats count min(_time) as first_seen max(_time) as last_seen by src_ip, dest_ip, user
| where count > 3
| eval is_known_tenda_credential = if(match(user, "admin") OR match(user, "root"), "POSSIBLE_HARDCODED_CRED", "OTHER")
| table _time, src_ip, dest_ip, user, count, first_seen, last_seen, is_known_tenda_credential
| sort - count

Wazuh OSSEC Rule

<group name="tenda_hg21,">
  <rule id="100385" level="15">
    <if_sid>5715</if_sid>
    <match>admin</match>
    <group>authentication_failed,</group>
    <description>Tenda HG21: Failed authentication from $(srcip) using admin user (CVE-2026-38577 related)</description>
  </rule>
  <rule id="100386" level="15">
    <if_sid>5700</if_sid>
    <match>admin</match>
    <group>authentication_success,</group>
    <description>Tenda HG21: Successful authentication as admin from $(srcip) (CVE-2026-38577 related)</description>
  </rule>
  <rule id="100387" level="10">
    <if_sid>100386</if_sid>
    <match>root</match>
    <group>command_execution,</group>
    <description>Tenda HG21: Command execution as root after admin login (CVE-2026-38577 related)</description>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Status: Confirmed UNPATCHED. Patch confidence is 0/4 — no patch evidence from any tracked source. The vendor has not released an advisory, security bulletin, or firmware update addressing CVE-2026-38577 as of the writing of this report.

1) Immediate Mitigations (implement now):

• Network Segmentation (Critical): Segregate all Tenda HG21 devices from untrusted networks. Ensure the router's management interfaces (WAN-side admin access, remote management via SSH/Telnet/HTTP) are not exposed to the internet or untrusted LAN segments.
• Access Control Lists (ACLs): Implement firewall rules that restrict access to management ports (commonly TCP 22, 23, 80, 443, and any cloud-management ports) to trusted administrative IP ranges only.
• Disable WAN-Side Management: If not required, disable remote management features entirely. If required, place them behind a VPN gateway.
• Change Credentials: Even though the vulnerability is in hardcoded firmware credentials, changing the admin password via the web interface and creating a custom root password via the CLI may mitigate the specific hardcoded credential path if the device respects those changes. Note: without firmware analysis, confidence that a password change overrides the hardcoded credential is limited.
• Monitor Logs: Enable and centralize syslog from the device, focusing on authentication logs. Apply the detection rules above.

2) Patch/Upgrade Steps:

• Contact Tenda support (formal advisory) or visit the vendor's official firmware download portal to check for a firmware release newer than V4.0.0-260302 that addresses this issue.
• If a patched firmware becomes available, apply it via the router's administrative interface during a maintenance window. Verify firmware hash integrity against vendor-published values.
• If no patch is available after 30 days, consider decommissioning affected devices and replacing them with a supported model from an alternate vendor or a newer Tenda model with verified security posture. Given the criticality (CVSS 9.8, root-level compromise), continued deployment is a material risk.

3) Long-Term Hardening:

• Vendor Assessment: For procurement, require vendors to provide evidence of secure development practices — specifically, that no hardcoded credentials are present in firmware releases.
• Configuration Baseline: Maintain a documented configuration baseline for all network devices. Audit configurations monthly to detect unauthorized changes.
• Firmware Integrity Monitoring: Where supported, use Secure Boot or firmware signing validation features.
• Network Device Monitoring: Extend your SIEM to ingest network device logs (syslog, SNMP traps, NetFlow) — not just server or endpoint logs. Router compromise often goes unnoticed because organizations do not monitor network infrastructure.
• EOL/EOS Policy: Develop a lifecycle policy that triggers review and replacement for devices with no confirmed patch within 90 days of a critical vulnerability disclosure.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-38577 — Official NVD description and CVSS metrics (9.8, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H). CWE-798 classification.
2. CISA KEV Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog — CVE-2026-38577 is NOT listed in the KEV catalog at the time of this report. No known exploitation in the wild has been confirmed by CISA.
3. MITRE ATT&CK: https://attack.mitre.org/techniques/T1190/ — Exploitation of Public-Facing Application technique. Relevant to the initial access phase of an attack chain leveraging this vulnerability.
4. CWE-798 Details: https://cwe.mitre.org/data/definitions/798.html — Use of Hard-coded Credentials weakness classification.
5. Tenda Vendor Portal: https://www.tenda.com.cn/ or https://www.tenda.com/ — Reference point for checking for firmware updates from the vendor. (No advisory published for CVE-2026-38577 as of report date.)

Threat Actor Campaign Notes: No known threat actor campaigns exploiting CVE-2026-38577 have been publicly reported. No exploit code, PoC, or malware samples referencing this CVE have been observed in public repositories or threat intelligence feeds. The EPSS score has not yet been assigned by FIRST.org. Given the device class (SOHO/enterprise edge routers) and the trivial exploitation mechanism (hardcoded credentials), the CVE carries high latent exploitation risk (leaked credentials from firmware analysis) and may be incorporated into IoT botnets (e.g., Mirai variants) once public exploitation begins.