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Oct 20 Update
Cisco Talos has updated their advisory to include a new CVE, CVE-2023-20273, "that is exploited to deploy the implant" with a fix estimated to be released on October 22nd. The Cisco Security Advisory was also updated to include the new CVE, information about observed attacks, mitigation, and Snort rule IDs.
We have also updated our illustration (below) to include the new CVE.
Original Post
On October 16th, 2023, Cisco disclosed a critical software Web UI Privilege Escalation Vulnerability under the identifier CVE-2023-20198 with a CVSS base score of 10. Cisco notes that the vulnerability has been exploited in the wild. The vulnerability allows an unauthenticated attacker to create an account with “privilege level 15 access” (full access to all commands). There is no patch for the privilege escalation vulnerability at the time of writing.
Initial Disclosure
In coordination with this disclosure, Cisco Talos published a threat advisory noting that the privilege escalation vulnerability CVE-2023-20198 is leveraged for initial access. Following this activity, an implant is delivered through a “yet undetermined mechanism” for which no patch is available.
“Leveraging existing detections, we observed the actor exploiting CVE-2021-1435, for which Cisco provided a patch in 2021, to install the implant after gaining access to the device. We have also seen devices fully patched against CVE-2021-1435 getting the implant successfully installed through an as-of-yet undetermined mechanism.”
Later in the threat advisory, the Snort intrusion detection system rule ID 3:50118:2 is called out as a way to address “this” threat.
The Snort rule 3:50118:2 "SERVER-WEBAPP Cisco IOS XE Web UI command injection attempt” does not include any mention that it detects CVE-2021-1435. In the rule’s references section, CVE-2019-12650 and CVE-2019-1862 — both command injection vulnerabilities — are mentioned via the following links:
Though not explicitly called out as part of the Snort rule, CVE-2021-1435 is also a command injection vulnerability.
If Snort rule 3:50118:2 detects the command injection vulnerabilities (CVE-2019-1862 / CVE-2019-12650 / CVE-2021-1435?) and the malicious implant in this recent string of attacks is installed through a “yet undetermined mechanism” on systems that are fully patched against CVE-2021-1435, then the vulnerability being leveraged to install the implant is not CVE-2021-1435. Additionally, a patch is available for CVE-2021-1435 whereas a patch is not available for the mechanism used to install the implant.
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Surveying The Carnage
Further research by VulnCheck has demonstrated that systems affected by the malicious implants can be coerced to disclose their 18-character hexadecimal unique implant identifier.
Cisco buried the lede by not mentioning thousands of internet-facing IOS XE systems have been implanted. VulnCheck scanned internet-facing Cisco IOS XE web interfaces and found thousands of implanted hosts. This is a bad situation, as privileged access on the IOS XE likely allows attackers to monitor network traffic, pivot into protected networks, and perform any number of man-in-the-middle attacks.
Censys also configured a scan profile and published their results in a blog post. It’s not a pretty picture. Over 40K Cisco IOS devices had their web admin interfaces exposed to the internet and fell victim to the latest round of implant attacks.
More distressing is that some of these devices are being used to launch further attacks. Researchers from both VulnCheck and Censys were kind enough to run their results through the GreyNoise Analyzer, which enables bulk triage of IP lists. Over 120 devices have been put into malicious service by attackers and live in diverse autonomous systems:
Unsurprisingly, we’re also seeing a large uptick in scanning from malicious, benign, and “unknown” sources in our Cisco IOS XE CVE-2023-20198 Scanner tag:
Im-persistent Harms
A key aspect of the current, underlying implant is that it does not survive a reboot. That means attackers will need to reinfect devices in their control if power is cycled or if they perform regular maintenance that requires a reboot… unless they have created a persistent access method prior to the reboot such as a newly created account. Given that these Cisco appliances are (small) business-class devices, they are more likely to have static IP addresses, meaning that attackers won’t have to re-scan the entire internet nearly as often as they might otherwise to identify and re-infect them.
The Enemy Within
Censys, VulnCheck, and GreyNoise can only report the view from the outside. However, similar Cisco IOS devices are also used internally in many organizations and are equally susceptible to this vulnerability. After gaining initial access on a low-privileged endpoint, attackers will no doubt be probing for vulnerable Cisco devices internally, where it is even more likely the web admin UI will be enabled. Having such privileged access to an internal router/network may be even more valuable/desirable than internet-facing ones.
Staying Safe
Researchers from GreyNoise Labs strongly encourage organizations to disable the HTTP Server feature on all internet-facing systems until a patch is available (and consider leaving it disabled permanently). This can be done by following the instructions provided in the Cisco security advisory.
Given the transient nature of the implant, they also suggest conducting an incident response exercise to determine if any internet-facing (or internal) Cisco device was demonstrating anomalous behavior.
Remember, you can:
- use our Analyzer for IP triage
- block non-benign scanners — through our dynamic, timely block lists — searching for signs of implants
- monitor our CVE-2023-20198 scanner tag to keep up with external actor activity (manually or via an Alert)
- take advantage of that same Alert capability to monitor your IP address space to determine if attackers are using it for malicious purposes.
GreyNoise Labs will continue monitoring this situation and providing updates as needed.
