CyberSecBulletin
CVE-2023-22915NVD status: Modified

USG FLEX series firmware: Buffer Copy without Checking Size of Input

Published Updated By the CyberSecBulletin desk

CVE-2023-22915 is a high-severity buffer copy without checking size of input vulnerability in USG FLEX series firmware from Zyxel, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected USG FLEX series firmware versions: 4.50 through 5.35. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 1.05% chance of exploitation within 30 days, higher than 65% of scored CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Check which version of USG FLEX series firmware you run against the affected versions below, and install the vendor's latest release.
  2. Read the advisory on zyxel.com for workarounds and exact fixed builds.
  3. 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
ZyxelUSG FLEX series firmware4.50 through 5.35Not stated
ZyxelUSG FLEX 50(W) firmware4.30 through 5.35Not stated
ZyxelUSG20(W)-VPN firmware4.30 through 5.35Not stated
ZyxelVPN series firmware4.30 through 5.35Not stated
ZyxelUsg Flex 100 Firmware4.50 through 5.35Not stated
ZyxelUsg Flex 100w Firmware4.50 through 5.35Not stated
ZyxelUsg Flex 200 Firmware4.50 through 5.35Not stated
ZyxelUsg Flex 50 Firmware4.50 through 5.35Not stated
ZyxelUsg Flex 50w Firmware4.30 through 5.35Not stated
ZyxelUsg Flex 500 Firmware4.50 through 5.35Not stated
ZyxelUsg Flex 700 Firmware4.50 through 5.35Not stated
ZyxelVpn100 Firmware4.50 through 5.35Not stated
ZyxelVpn1000 Firmware4.50 through 5.35Not stated
ZyxelVpn300 Firmware4.50 through 5.35Not stated
ZyxelVpn50 Firmware4.50 through 5.35Not stated
ZyxelUsg 20w-vpn Firmware4.30 through 5.35Not stated

Version data comes from the organisation that assigned the CVE. 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
NoneNo data is disclosed.
Integrity
NoneNothing can be modified.
Availability
HighThe component can be taken down completely.

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

Scoring systemScoreSeverityScored by
CVSS 3.17.5HighCNA (zyxel.com.tw)
CVSS 3.17.5HighNVD

Official description

A buffer overflow vulnerability in the “fbwifi_forward.cgi” CGI program of Zyxel USG FLEX series firmware versions 4.50 through 5.35, USG FLEX 50(W) firmware versions 4.30 through 5.35, USG20(W)-VPN firmware versions 4.30 through 5.35, and VPN series firmware versions 4.30 through 5.35, which could allow a remote unauthenticated attacker to cause DoS conditions by sending a crafted HTTP request if the Facebook WiFi function were enabled on an affected device.

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

The underlying weakness: Buffer Copy without Checking Size of Input

The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.

How developers prevent this class of flaw

  • Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to overflows, even if the language itself is theoretically safe.
  • Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples include the Safe C String Library (SafeStr) by Messier and Viega, and the Strsafe.h library from Microsoft. These libraries provide safer versions of overflow-prone string-handling functions.
  • Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND discusses canary-based detection in detail.

More Buffer Copy without Checking Size of Input vulnerabilities (CWE-120)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Other vulnerabilities in USG FLEX series firmware

All USG FLEX series firmware vulnerabilities

Recent Buffer Copy without Checking Size of Input vulnerabilities

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