cbcvebase.
CVE-2018-3953
published 2018-10-17

CVE-2018-3953: Devices in the Linksys ESeries line of routers (Linksys E1200 Firmware Version 2.0.09 and Linksys E2500 Firmware Version 3.0.04) are susceptible to OS command…

PriorityP356high7.2CVSS 3.1
AVNACLPRHUINSUCHIHAH
EXPLOIT
EPSS
13.51%
96.2th percentile
Devices in the Linksys ESeries line of routers (Linksys E1200 Firmware Version 2.0.09 and Linksys E2500 Firmware Version 3.0.04) are susceptible to OS command injection vulnerabilities due to improper filtering of data passed to and retrieved from NVRAM. Data entered into the 'Router Name' input field through the web portal is submitted to apply.cgi as the value to the 'machine_name' POST parameter. When the 'preinit' binary receives the SIGHUP signal, it enters a code path that continues until it reaches offset 0x0042B5C4 in the 'start_lltd' function. Within the 'start_lltd' function, a 'nvram_get' call is used to obtain the value of the user-controlled 'machine_name' NVRAM entry. This value is then entered directly into a command intended to write the host name to a file and subsequently executed.

Affected

4 ranges
VendorProductVersion rangeFixed in
linksyse1200_firmware
linksyse2500_firmware
linksyseseries_e1200
linksyseseries_e2500

Detection & IOCsextracted from sources · hover to see the quote

urlapply.cgi
commandmachine_name POST parameter to apply.cgi
  • Exploit is authenticated; look for authenticated POST requests to apply.cgi on Linksys E-Series devices, especially from unexpected source IPs. Default credentials admin/admin or admin/password are commonly used.
  • Because this is a blind OS command injection, no command output is returned in the HTTP response. Detect exploitation attempts by monitoring for out-of-band activity (e.g., unexpected ICMP/ping traffic) from the router following a POST to apply.cgi.
  • The injection is triggered post-reboot when preinit processes NVRAM values. Suspicious reboots of Linksys E-Series devices shortly after configuration changes to 'Router Name' or 'Domain Name' fields should be investigated.
  • Also monitor POST requests to apply.cgi for the 'wan_domain' parameter with injection payloads, as CVE-2018-3955 follows the same pattern via the 'Domain Name' field.
  • ·Exploitation requires prior authentication to the device's web interface. Unauthenticated attackers cannot directly trigger this vulnerability.
  • ·The command injection is only triggered after a device reboot/SIGHUP, not immediately upon the malicious POST request. Detection at POST time alone may miss the actual execution phase.
  • ·Snort rule 47133 covers CVE-2018-3953, CVE-2018-3954, and CVE-2018-3955 collectively; additional rules may be released and existing rules are subject to change.

CVSS provenance

nvdv3.17.2HIGHCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
nvdv3.07.2HIGHCVSS:3.0/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
nvdv2.09.0CRITICALAV:N/AC:L/Au:S/C:C/I:C/A:C
CVEs like this are exactly what “Exploited This Week” covers.

Every Monday: what got weaponized or added to CISA KEV in the last seven days — each CVE cross-linked to its PoC, Nuclei template, and detection rule. Free, one email a week, unsubscribe in one click.