All posts tagged: Internet

Overview
Microsoft Exchange 2019 Cumulative Update 23 and earlier versions are vulnerable to a server-side request forgery (SSRF) attack and remote code execution. An authenticated attacker can use the combination of these two vulnerabilities to elevate privileges and execute arbitrary code on the target Exchange server.
Description
Microsoft Exchange Server’s Autodiscover service is a web service widely available to any Microsoft Exchange Web Services (EWS) client. Since Microsoft Exchange version 2016, the Autodiscover service has become an integral part of the Microsoft Exchange system, and it is no longer independently provided by a Client Access server. The Autodiscover service and a number of other privileged mailbox services are hosted on the default Internet Information Services server running on the Mailbox server.
Cybersecurity company GTSC observed an abuse of the Autodiscover service in August of 2022 using a crafted URL SSRF attack, similar to the earlier ProxyShell vulnerability reported in August 2021. The observed attack appears to have implemented CVE-2022-41040 to gain privileged access and CVE-2022-41082 to perform remote code execution via PowerShell. Microsoft Security Research Center has acknowledged the vulnerability and provided guidance for mitigation. The guidance highlights that Microsoft Exchange Online customers will be provided with detection and mitigation defenses automatically from Microsoft’s managed Infrastructure, informing them of any attempts to exploit these vulnerabilities.
Impact
An authenticated remote attacker can perform SSRF attacks to escalate privileges and execute arbtirary PowerShell code on vulnerable Microsoft Exchange servers. As the attack is targeted against Microsoft Exchange Mailbox server, the attacker can potentially gain access to other resources via lateral movement into Exchange and Active Directory environments.
Solution
Workaround guidance
Microsoft has provided guidance in their recent blog post to address the issue. Note that Microsoft has updated their guidance for the Option 3 Step 6 with the URL filter to be .*autodiscover.json.*Powershell.* (excluding the @ symbol) instead of the earlier .*autodiscover.json.*@.*Powershell.*. The recommended block pattern is a regular expression suggested by Jang to prevent known variants of the #ProxyNotShell attacks.
Apply update when available
As of October 3, 2022, there is no patch available to mitigate this issue. It is recommended that Microsoft Exchange administrators stay on alert for any advisory or patch released by Microsoft. Even with the workaround in place, many on-premise Microsoft Exchange instances remain at risk until Microsoft provides a patch and the patch has been applied.
Third-party web application protection
Exchange Administrators who use third-party Web Application Firewall (WAF) products can implement the recommended URL filters and blocks as part of their WAF policy.
Other mitigations
Exchange Administrators can limit the outgoing connection from the Exchange Mailbox server using specific allowed list on an outgoing proxy to limit suspicious web requests.
This document was written by Vijay Sarvepalli.

Overview
Layer-2 (L2) network security controls provided by various devices, such as switches, routers, and operating systems, can be bypassed by stacking Ethernet protocol headers. An attacker can send crafted packets through vulnerable devices to cause Denial-of-service (DoS) or to perform a man-in-the-middle (MitM) attack against a target network.
Description
This vulnerability exists within Ethernet encapsulation protocols that allow for stacking of Virtual Local Area Network (VLAN) headers. Network standards such as IEEE 802.1Q-1998 and IEEE 802.3 define a system of tagging Ethernet frames that help isolate networks to provide virtual networking capability. IEEE standard 802.1ad, also known as QinQ, allows for the stacking of these VLAN tags, extending the VLAN capability into multiple network segments. This widely adopted Ethernet feature is also referred to as “provider bridging” and “stacked VLANs”. In order to properly isolate and protect these virtual networks, many network devices and operating systems provide an L2 network filtering capability. It is important to note that in modern computing environments , such as Cloud based virtualization and virtual networking, the L2 network capability is extended beyond the local area networks. This can lead to exposure of this vulnerabilities in unintended ways to the larger Internet.
The identified vulnerabilities allow an attacker to bypass the security controls by stacking encapsulating headers. This is done by stacking a combination of one or more VLAN 0 (priority tag) headers and 802.2 LLC/SNAP headers. An attacker can send these crafted network packets and exploit vulnerable devices by bypassing their inspection and filtering capabilities. Some examples of bypassed L2 inspections include, but are not limited to, Dynamic ARP inspection, IPv6 Neighbor Discovery (ND) protection, and IPv6 RA Guard.
CVE-2021-27853
Layer 2 network filtering capabilities such as IPv6 RA guard or ARP inspection can be bypassed using combinations of VLAN 0 headers and LLC/SNAP headers.
CVE-2021-27854
Layer 2 network filtering capabilities such as IPv6 RA guard can be bypassed using combinations of VLAN 0 headers, LLC/SNAP headers in Ethernet to Wifi frame translation and the reverse Wifi to Ethernet.
CVE-2021-27861
Layer 2 network filtering capabilities such as IPv6 RA guard can be bypassed using LLC/SNAP headers with invalid length (and optionally VLAN0 headers).
CVE-2021-27862
Layer 2 network filtering capabilities such as IPv6 RA guard can be bypassed using LLC/SNAP headers with invalid length and Ethernet to Wifi frame conversion (and optionally VLAN0 headers).
Impact
An attacker can bypass security controls and deceive a locally connected target host to route traffic to arbitrary destinations. Victim devices experience either a DoS (blackholing traffic) or MitM (observing the unencrypted traffic and maybe breaking encryption).
Solution
Apply Updates
Install vendor-provided patches and updates to ensure malicious content is blocked or rejected by the security controls (such as RA Guard), thereby blocking router advertisements or other network configuration related advertisements that originate on host ports.
Inspect and Block Router Advertisements
Utilize the interface security controls on your router or managed switch to perform DHCP snooping, IPv6 RA guard, IP source guard, and ARP/ND inspection. It is also recommended to only allow needed protocol on access ports (ARP/ICMP/IPv4/IPv6), some applications may have additional needs so be prepared to modify the allow list as needed.
Acknowledgements
Thanks to Etienne Champetier for reporting this vulnerability.
This document was written by Timur Snoke.

Overview
A security feature bypass vulnerability exists in signed 3rd party UEFI bootloaders that allows bypass of the UEFI Secure Boot feature. An attacker who successfully exploits this vulnerability can bypass the UEFI Secure Boot feature and execute unsigned code during the boot process.
Description
UEFI firmware is software written by vendors in the UEFI ecosystem to provide capabilities in the early start up phases of a computer. Secure Boot is a UEFI standard that can be enabled and used to verify firmware and to protect a system against malicious code being loaded and executed early in the boot process, prior to the loading of the operating system.
Security researchers at Eclypsium have found three specific UEFI bootloaders that are signed and authenticated by Microsoft to be vulnerable to a security feature bypass vulnerability allowing an attacker to bypass Secure Boot when it is enabled. The vulnerable bootloaders can be tricked to bypass Secure Boot via a custom installer (CVE-2022-34302) or an EFI shell (CVE-2022-34301 and CVE-2022-34303). As a vulnerable bootloader executes unsigned code prior to initialization of the the Operating System’s (OS) boot process, it cannot be easily monitored by the OS or common Endpoint Detection and Response (EDR) tools.
The following vendor-specific bootloaders were found vulnerable:

Inherently vulnerable bootloader to bypass Secure Boot
New Horizon Datasys Inc (CVE-2022-34302)

UEFI Shell execution to bypass Secure Boot
CryptoPro Secure Disk (CVE-2022-34301)
Eurosoft (UK) Ltd (CVE-2022-34303)

Impact
An attacker can bypass a system’s Secure Boot feature at startup and execute arbitrary code before the operating system (OS) loads. Code executed in these early boot phases can provide persistence to an attacker, potentially loading arbitrary kernel extensions that survive both reboot and re-installation of an OS. It may also evade common OS-based and EDR security defenses.
Solution
Apply a patch
Apply your vendor-provided security updates that address these vulnerabilities to block vulnerable firmware from bypassing Secure Boot. Microsoft has provided details with their KB5012170 article released on August 9th 2022. Note, these updates can be delivered from your OEM vendor or the OS vendor to install an updated Secure Boot Forbidden Signature Database (DBX) .
Enterprise and Product Developers
As DBX file changes can cause a system to become unstable, Vendors are urged to verify the DBX updates do not cause the machine to be unusable. Enterprises and Cloud Providers that manage large number of computers are also urged to do the required security updates and ensure DBX files are implemented reliably without any risk of boot failure.
Acknowledgements
Thanks to Mickey Shkatov and Jesse Michael of Eclypsium who researched and reported these vulnerabilities.
This document was written by Brad Runyon & Vijay Sarvepalli.