Researchers Follow the Breadcrumbs: The Latest Vulnerabilities in Windows’ Network Stack

In fact, looking back at just the last 8 months of Microsoft patches, we’ve tracked at least 25 significant vulnerabilities directly related to the Windows network stack. These have ranged from DNS to NTFS, SMB to NFS and recently, several tcp/ip bugs in fragmentation and packet reassembly for IPv4 and IPv6.

That brings us to this month’s patch Tuesday, which contains several more high-profile critical vulnerabilities in tcpip.sys. We’ll focus on three of these, including 2 marked as “remote code execution” bugs, which could lead to wormability if code execution is truly possible, and a persistent denial-of-service vulnerability which could cause a permanent Blue Screen of Death on the latest versions of Windows.

There are clear similarities between all 3, indicating both Microsoft and researchers external to Microsoft are highly interested in auditing this library, and are having success in weeding out a number of interesting bugs. The following is a short summary of each bug and the progress we have made to date in analyzing them.

What is CVE-2021-24086?
The first vulnerability analyzed in this set is a Denial-of-Service (DOS) attack. Generally, these types of bugs are rather uninteresting; however, a few can have enough of an impact that although an attacker can’t get code execution, they are well worth discussing. This is one of those few. One of the things that boost this vulnerability’s impact is the fact it is internet routable and many devices using IPv6 can be directly accessible over the internet, even when behind a router. It is also worth noting that the default Windows Defender Firewall configuration does not mitigate this attack. In a worst-case scenario, an attacker could spray this attack and put it on a continuous loop to potentially cause a “permanent” or persistent DOS to a wide range of systems.

This vulnerability exists when Windows’ tcpip.sys driver attempts to reassemble fragmented IPv6 packets. As a result, this attack requires many packets to be successful.  The root cause of this vulnerability is a NULL pointer dereference which occurs in Ipv6pReassembleDatagram. The crash occurs when reassembling a packet with around 0xffff bytes of extension headers.  It should be impossible to send a packet with that many bytes in the extension headers according to the RFC, however this is not considered in the Ipv6pReceiveFragments function when calculating the unfragmented length. Leveraging a proof-of-concept through the Microsoft MAPP program, McAfee was easily able to reproduce this bug, demonstrating it has the potential to be seen in the wild.

What is CVE-2021-24094?
This vulnerability is classified by Remote Code Execution (RCE), though our analysis thus far, code execution comes with unique challenges. Similar to CVE-2021-24086, this issue involves IPv6 packet reassembly by the Windows tcpip.sys driver. It is different from 24086 in that it doesn’t require a large packet with extension headers, and it is not a NULL pointer dereference. Instead, it is a dangling pointer which, if the right packet data is sprayed from an attacker over IPv6, will causes the pointer to be overwritten and reference an arbitrary memory location. When the data at the pointer’s target is accessed, it causes a page fault and thus a Blue Screen of Death (BSOD). Additionally, an attacker can create persistence of the DoS by continually pointing the attack at a victim machine.

While the reproduction of this issue causes crashes on the target in all reproductions so far, it’s unclear how easy it would be to force the pointer to a location that would cause valid execution without crashing. The pointer would need to point to a paged-in memory location that had already been written with additional data that could manipulate the IPv6 reassembly code, which would likely not come from this attack alone, but may require a separate attack to do so.

What is CVE-2021-24074?
CVE-2021-24074 is a potential RCE in tcpip.sys triggered during the reassembly of fragmented IPv4 packets in conjunction with a confusion of IPv4 IP Options. During reassembly, it is possible to have two fragments with different IP Options; this is against the IPv4 standard, but Windows will proceed further, while failing to perform proper sanity checks for the respective options. This type confusion can be leveraged to trigger an Out of Bounds (OOB) read and write by “smuggling” an IP option Loose source and record route (LSRR) with invalid parameters. This option is normally meant to specify the route a packet should take. It has a variable length, starts with a pointer field, and is followed by a list of IP addresses to route the packet through.

By leveraging the type confusion, we have an invalid pointer field that will point beyond the end of the routing list provided in the packet. When the routing function Ipv4pReceiveRoutingHeader looks up the next hop for the packet, it will OOB read this address (as a DWORD) and perform a handful of checks on it. If these checks are successful, the IP stack will attempt to route the packet to its next hop by copying the original packet and then writing its own IP address in the record route. Because this write relies on the same invalid pointer value as before, this turns out to be an OOB write (beyond the end of the newly allocated packet). The content of this OOB write is the IP address of the target machine represented as a DWORD (thus, not controlled by the attacker).

Microsoft rates this bug as “Exploitation more likely”, however exploitation might not be as straightforward as it sounds. For an attack to succeed, one would have to groom the kernel heap to place a certain value to be read during the OOB read, and then make it so the OOB write would corrupt something of interest. Likely, a better exploitation primitive would need to be established first in order to successfully exploit the bug. For instance, leveraging the OOB write to corrupt another data structure that could lead to information disclosure and/or a write-what-where scenario.

From a detection standpoint, the telltale signs of an active exploitation would be fragmented packets whose IP Options vary between fragments. For instance, the first fragment would not contain an LSRR option, while the second fragment would. This would likely be accompanied by a heavy traffic meant to shape the kernel heap.

Similarities and Impact Assessment
There are obvious similarities between all three of these vulnerabilities. Each is present in the tcpip.sys library, responsible for parsing IPv4 and IPv6 traffic. Furthermore, the bugs all deal with packet reassembly and the RCE vulnerabilities leverage similar functions for IPv4 and IPv6 respectively. Given a combination of public and Microsoft-internal attribution, it’s clear that researchers and vendor alike are chasing down the same types of bugs. Whenever we see vulnerabilities in network stacks or Internet-routed protocols, we’re especially interested to determine difficulty of exploitation, wormability, and impact. For vulnerabilities such as the RCEs above, a deep dive is essential to understand the likelihood of these types of flaws being built into exploit kits or used in targeted attacks and are prime candidates for threat actors to weaponize. Despite the real potential for harm, the criticality of these bugs is somewhat lessened by a higher bar to exploitation and the upcoming patches from Microsoft. We do expect to see additional vulnerabilities in the TCP/IP stack and will continue to provide similar analysis and guidance. As it is likely to take some time for threat actors to integrate these vulnerabilities in a meaningful way, the best course of action, as always, is widespread mitigation via patching.

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