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Preinstalled Firmware Updater Puts 128 Dell Models at Risk

A feature of the computer maker's update utility does not correctly handle certificates, leaving systems open to firmware-level compromises.

A firmware-update utility that comes preinstalled on at least 128 models of Dell laptops, desktops, servers, and tablets has multiple vulnerabilities that could allow a privileged attacker to install rogue code over the network, the computer maker and security firm Eclypsium announced on Thursday.

The four vulnerabilities affect an estimated 30 million devices that use the BIOSConnect functionality of Dell's support utility SupportAssist, which allows computer users to remotely recover the operating system or update the firmware for a variety of Dell devices. One vulnerability allows an attacker with a "privileged network position to impersonate Dell" and send malicious code to the targeted system, while three other issues are buffer overflows that allow code execution to bypass the security of SecureBoot, Eclypsium stated.

If attackers can intercept the initial BIOSConnect traffic — thus, the "privileged network position" — they can use a wild-card certificate purchased from any of the certificate authorities trusted by Dell BIOSConnect to send code to the system, says Jesse Michael, principal researcher with Eclypsium.

"It allows an attacker to get an initial toehold within the client BIOS," he says. "It is not something where you need to own the system through some other means and then do privilege escalations or lateral movement within the system. This is an initial code execution on the system through exploiting these other vulnerabilities."

Attacks on firmware have increased, often as a way for attackers to gain a persistent beachhead on a system that survives reboots and attempts to remove any malicious files. About 83% of businesses have experienced an attack that targeted firmware in the past two years, according to a report published by Microsoft in March. Sophisticated malware frequently has firmware modules that attempt to infect the basic input/output system (BIOS), which, as the first software to run, has a privileged place in the order in the system boot sequence.

When Dell released a function to make updating BIOS easier, Eclypsium saw it as a fertile field of research. The company's researchers discovered the vulnerabilities in early March and worked with Dell to remediate the issues.

On Thursday, Dell released an advisory for the four vulnerabilities, saying that the company had already closed two issues by updates to its servers. The other two issues, including the certification validation issue (CVE-2021-21571), can only be fixed by updating the BIOS.

"Dell recommends all customers update to the latest Dell Client BIOS version at the earliest opportunity," the company stated in the advisory. "Customers who choose not to apply BIOS updates immediately or who are otherwise unable to do so now, should ... disable the BIOSConnect feature."

The certificate validation vulnerability — specifically, "Insecure Transport Layer Security (TLS) Connection from BIOS to Dell" — is the most significant issue, even though its Common Vulnerability Scoring System (CVSS) score is a middle-of-the-road 5.9. If Secure Boot is disabled, the vulnerability allows arbitrary code execution in the preboot environment before any security software is loaded and could prevent the attack. Otherwise, the vulnerability gives an attacker the ability to trigger one of the other three buffer overflow flaws discovered by Eclypsium without needing local access.

To exploit the certificate validation issue, the attacker must be able to redirect traffic to and from the target machine, either by compromising a router, using a technique such as ARP poisoning, or some other machine-in-the-middle attack. If the machine supports HTTPS Boot, a function that allows waking up or booting a machine using the Web interface, the feature could be abused to trigger the vulnerability.

The weakness is caused by the BIOSConnect's acceptance of data from any trusted certificate. In that way, an attacker could just reserve a certificate using a trusted provider and then be able to attack devices, Michael says.

"There are a lot of certificate authorities in the UEFI BIOS that can be used where if you get a wild-card cert from any of these authorities and use it on your own server, it doesn't even have to be Dell or pretending to be Dell," he says. "If it a valid cert that is trusted, there is not proper certificate pinning, so if you can do poisoning or redirect in another way, ... you have the ability to provide content back to the client."

The issues shows that the basic firmware for connected devices still does not receive enough scrutiny, says Scott Scheferman, principal cyber strategist at Eclypsium.

"These are 20-year-old bugs that are finding their way into the most critical aspect of trust at the BIOS level," he says. "Here we are in 2021, and we are finding issues with poor implementations of TLS."

The vulnerabilities will likely have a long tail, because updating BIOS has always been a difficult task — a major reason why utilities such as BIOSConnect exist. Companies are advised to disable the BIOSConnect feature until they are able to update the system. Because of the vulnerabilities, however, the firmware update will have to be done the old-fashioned way, either using a different utility or downloading the firmware file and using the BIOS One Time Boot Menu.

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