CyberSecBulletin
CVE-2020-9054Exploited in the wildNVD status: Analyzed

Zyxel Multiple NAS Devices OS Command Injection

Published Updated By the CyberSecBulletin desk

CVE-2020-9054 is a critical-severity improper neutralization of special elements used in an OS command vulnerability in NAS326 from Zyxel, with a CVSS 3.1 base score of 9.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected NAS326 versions: V5.21(AAZF.7)C0 and earlier. The record does not name a fixed version, so check the vendor advisory. CISA added it to the Known Exploited Vulnerabilities catalogue on March 25, 2022, which means attacks have been observed in the wild.

Patch now

Attackers are already exploiting this vulnerability, according to CISA.

What to do

  1. Update Nas326 Firmware to 5.21\(aazf.10\)c0 or a later version.
  2. Update Nas520 Firmware to 5.21\(aasz.3\)c0 or a later version.
  3. Update Nas540 Firmware to 5.21\(aatb.7\)c0 or a later version.
  4. CISA's required action: Apply updates per vendor instructions.
  5. Read the advisory on zyxel.com for workarounds and exact fixed builds.
  6. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
ZyxelNAS326V5.21(AAZF.7)C0 and earlierNot stated
ZyxelNAS520V5.21(AASZ.3)C0 and earlierNot stated
ZyxelNAS540V5.21(AATB.4)C0 and earlierNot stated
ZyxelNAS542V5.21(ABAG.4)C0 and earlierNot stated
ZyxelNSA210allNot stated
ZyxelNSA220allNot stated
ZyxelNSA221allNot stated
ZyxelNSA310V4.75(AALH.2)C0 and earlierNot stated
ZyxelNSA320allNot stated
ZyxelNSA320SV4.75(AANV.2)C0 and earlierNot stated
ZyxelNSA325V4.81(AAAJ.1)C0 and earlierNot stated
ZyxelNSA325v2V4.81(AALS.1)C0 and earlierNot stated
ZyxelNas326 Firmwarebefore 5.21\(aazf.10\)c05.21\(aazf.10\)c0
ZyxelNas520 Firmwarebefore 5.21\(aasz.3\)c05.21\(aasz.3\)c0
ZyxelNas540 Firmwarebefore 5.21\(aatb.7\)c05.21\(aatb.7\)c0
ZyxelNas542 Firmwarebefore 5.21\(abag.7\)c05.21\(abag.7\)c0
ZyxelAtp100 Firmware4.35 up to, but not including, 4.35\(abps.3\)c04.35\(abps.3\)c0
ZyxelAtp200 Firmware4.35 up to, but not including, 4.35\(abfw.3\)c04.35\(abfw.3\)c0
ZyxelAtp500 Firmware4.35 up to, but not including, 4.35\(abfu.3\)c04.35\(abfu.3\)c0
ZyxelAtp800 Firmware4.35 up to, but not including, 4.35\(abiq.3\)c04.35\(abiq.3\)c0
ZyxelUsg20-vpn Firmware4.35 up to, but not including, 4.35\(abaq.3\)c04.35\(abaq.3\)c0
ZyxelUsg20w-vpn Firmware4.35 up to, but not including, 4.35\(abar.3\)c04.35\(abar.3\)c0
ZyxelUsg40 Firmware4.35 up to, but not including, 4.35\(aala.3\)c04.35\(aala.3\)c0
ZyxelUsg40w Firmware4.35 up to, but not including, 4.35\(aalb.3\)c04.35\(aalb.3\)c0
ZyxelUsg60 Firmware4.35 up to, but not including, 4.35\(aaky.3\)c04.35\(aaky.3\)c0

Version data comes from the organisation that assigned the CVE; 14 further products are omitted here. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
HighAll data handled by the component can be read.
Integrity
HighData or code can be modified at will.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.

Scoring systemScoreSeverityScored by
CVSS 3.19.8CriticalNVD
CVSS 3.19.8CriticalCISA (ADP)
CVSS 2.010.0HighNVD

Official description

Multiple ZyXEL network-attached storage (NAS) devices running firmware version 5.21 contain a pre-authentication command injection vulnerability, which may allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable device. ZyXEL NAS devices achieve authentication by using the weblogin.cgi CGI executable. This program fails to properly sanitize the username parameter that is passed to it. If the username parameter contains certain characters, it can allow command injection with the privileges of the web server that runs on the ZyXEL device. Although the web server does not run as the root user, ZyXEL devices include a setuid utility that can be leveraged to run any command with root privileges. As such, it should be assumed that exploitation of this vulnerability can lead to remote code execution with root privileges. By sending a specially-crafted HTTP POST or GET request to a vulnerable ZyXEL device, a remote, unauthenticated attacker may be able to execute arbitrary code on the device. This may happen by directly connecting to a device if it is directly exposed to an attacker. However, there are ways to trigger such crafted requests even if an attacker does not have direct connectivity to a vulnerable devices. For example, simply visiting a website can result in the compromise of any ZyXEL device that is reachable from the client system. Affected products include: NAS326 before firmware V5.21(AAZF.7)C0 NAS520 before firmware V5.21(AASZ.3)C0 NAS540 before firmware V5.21(AATB.4)C0 NAS542 before firmware V5.21(ABAG.4)C0 ZyXEL has made firmware updates available for NAS326, NAS520, NAS540, and NAS542 devices. Affected models that are end-of-support: NSA210, NSA220, NSA220+, NSA221, NSA310, NSA310S, NSA320, NSA320S, NSA325 and NSA325v2

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Improper Neutralization of Special Elements used in an OS Command

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 before it is sent to a downstream component.

How developers prevent this class of flaw

  • If at all possible, use library calls rather than external processes to recreate the desired functionality.
  • For any data that will be used to generate a command to be executed, keep as much of that data out of external control as possible. For example, in web applications, this may require storing the data locally in the session's state instead of sending it out to the client in a hidden form field.
  • For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.

More Improper Neutralization of Special Elements used in an OS Command vulnerabilities (CWE-78)

Timeline

  1. CVE record published.
  2. Added to CISA's Known Exploited Vulnerabilities catalogue.
  3. Deadline for US federal agencies to remediate, set by CISA.
  4. CISA published its SSVC assessment (exploitation: active, automatable: no, technical impact: total).
  5. Record last changed materially (score, affected products, status or references).

References

Third-party and government advisories

Technical write-ups (may include proof of concept)

Recent Improper Neutralization of Special Elements used in an OS Command vulnerabilities

Spotted an error? Tell us.