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The Threat of Deprecated BGP Attributes

Timur Snoke
CITE

Border Gateway Protocol (BGP) routing is a core part of the mechanism by which packets are routed on the Internet. BGP routing gets email to its destination, enables domain name service (DNS) to work, and web pages to load. An important aspect of routing is that packets cross boundaries of the many autonomously controlled networks that, together, comprise the Internet. This allowing us to access, for example, the Amazon website from a phone on the Verizon network. It allows military commanders to see pictures of troop transports in one location and pictures of tanks in another. The BGP protocol, though less well known than low-level protocols such as IP, TCP, and UDP, has a critical role in facilitating – and negotiating – the flows of packets among the many autonomous networks that comprise the Internet.

Consequently, vulnerabilities in the BGP protocol are a very big deal. We have a long-standing expectation that the Internet is robust, particularly with regard to the actions of the many organizations that operate portions of the network – and therefore that a system designed to keep the traffic flowing could only be disrupted by a very large event. In this SEI Blog post, we will examine how a small issue, a deprecated path attribute, can cause a major interruption to traffic.

BGP: A Path Vector Running Protocol

BGP is a path vector routing protocol that was defined by the Internet Engineering Task Force (IETF) in RFC 1654. As with many Internet protocols, there are many other Requests for Comments (RFC) associated with BGP methods and processes. For example, RFC 4271 covers BGP path attributes, which can be used when making path selection and building routing tables to assist in routing decisions. According to RFC 4271, there are well-known mandatory attributes that must be supported on all BGP implementations. However, these attributes are extensible and allow for customized announcements as RFC 4271 explains: Well-known mandatory attributes must be included with every prefix advertisement, while well-known discretionary attributes may or may not be included. The custom attributes can be used internally by your organization or externally (to communicate important information to other organizations). They also may remain unused but available. Attributes can contain information about updates and network origin or weight an autonomous system number (ASN) to prioritize it in routing.

The Menace of Multiple BGP Implementations

The CERT/CC recently dealt with a relevant case, Vulnerability Note VU#347067 (Multiple BGP implementations are vulnerable to improperly formatted BGP updates. In this case, a researcher saw a significant outage stemming from an improperly formatted path attribute BGP UPDATE that caused vulnerable routers, when they received an update, to de-peer (i.e., terminate a peering relationship that allows packets to flow from one network to another). Unaffected routers might also pass the crafted updates across the network, potentially leading to the update arriving at an affected router from multiple sources, causing multiple links to fail. The flaw was that, instead of ignoring the improperly formatted attributes, the receiving router dropped the routing update and lost the information being provided about other routes. This situation resulted in a real-world de-peering of routers and loss of traffic passed between them.

In short, this vulnerability disrupted the flow of information that BGP routing was designed to ensure.

Routers that are designed for resiliency should still function if they ignore a deprecated attribute. They are not expected to use the attribute as it was originally designed, since it is no longer part of the official protocol. Adding to the problem is inconsistent updating – there are many older versions of the BGP protocol specification deployed on the Internet because not everyone can upgrade to the latest and best version every time one is released. However, all implementations of any Internet protocol should be able to function if new attributes show up because the protocols are always changing. Remember, the Internet was designed for survivability: a few errors in attributes shouldn’t interrupt it. Unfortunately, the response to unspecified attributes is unexpected: one organization’s router might handle the attribute without problem while another one might not.

Awareness of the deprecated attributes being used is the first step to determining whether a particular installation is vulnerable. The inconsistency of updates means that the organizations announcing routes don’t know what software other organizations are using. They assume all core routers can handle the traffic. In this circumstance, verifying that your software isn’t affected is a good way to stay connected on the Internet. This suggests that organizations contact their router vendors to learn how they handle deprecated attributes and whether responses are robust. They should be able to tell you if the response can be modified and what steps to take.

An Analysis of BGP Data

The SEI CERT Division collects BGP data, so we looked at the last two years of data to find out what deprecated attributes are still announced. To do so, we used the list of deprecated attributes published by the Internet Assigned Numbers Authority (IANA).

The three attributes we found are listed in Table 1:

Table 1: Deprecated Attributes Being Announced
Attribute Use
AS_PATHLIMIT Designed to help limit the distribution of path information
CONNECTOR Used in VPN4 announcements
ENTROPY_LEVEL Used to help with load balancing

The deprecated attribute that caused the problem in VU#347067 was ENTROPY_LEVEL. This is the attribute particular to this instance, but other deprecated attributes might cause issues later. To reduce this risk and avoid further vulnerabilities is to understand more about the conditions that could cause it. In this case, it’s how the router handles deprecated attributes (or doesn’t).

We looked at the number of BGP announcements each day that used these deprecated attributes (Figure 1).

figure1_06032024
Figure 1: Number of Announcements of the Deprecated Attribute ENTROPY_LEVEL

The greatest number of announcements (3,620) occurred on July 7, 2022. That doesn’t seem like very many in the context of millions and millions of route announcements a day, but a small attribute, mishandled by the wrong router, can cause havoc, as we saw previously.

An important feature of this situation is that routers that announce the deprecated attribute cannot sense that they are causing a problem. The configuration or software still uses these deprecated attributes, and consequently the router will freely share it with the Internet as they are designed to do. The real troublemakers are the routers that receive the routes.

Deprecated Attribute Use Over Time

In routing, as with many things on the Internet, we know who did it, when they did it, how they did it, and even most likely where they did it. We just don’t know why these organizations are using these deprecated attributes. It’s their internal decision to use them and usually it isn’t relevant.

With regard to the who, we examined the time series of this data, which shows, on the vertical axis, the number of ASNs (autonomous system numbers, which are identifiers for the various networks that comprise the Internet) announcing a deprecated attribute each day over a span of nine days (Figure 2).

figure2a_06032024
Figure 2: Time Series of AS Announcing Deprecated Attributes

Figure 2 illustrates a time series with a dip followed by a peak, which was followed by another dip. That is, the general number of ASNs announcing deprecated attributes dropped, then increased. Rather than guessing when that happened, we used an algorithm to find these change points (Figure 3).

figure3_06042024
Figure 3: Change Point Analysis of ASNs Announcing Deprecated Attributes

In this case, change point analysis looked for shifts in the average value across time. Starting at 20221020, the average number of announcers dips, then recovers briefly at 20230108. It dips again and 20230324 and then, finally, starting at 20230618, the average number of announcers goes down again. The number of autonomous systems that used these deprecated attributes, on average, decreased over time. The change of the announcements over time tells us that at certain points, abrupt changes were made in the number of organizations that used the attributes. The good news is that fewer are using them. The bad news is we don’t know why those that use them continue to do so.

Avoiding the Havoc of Deprecated Attributes

Now we have a better idea of when something happened. We do not know what caused organizations to start or stop announcing the deprecated attributes. We do know, however, that organizations receiving routes on the live Internet should be aware of the potential problem and ensure that they aren’t vulnerable to announcements using deprecated attributes. Because of the significance of BGP in managing “cross-border” flows in the Internet, the potential consequences could be large.

In conclusion, we advise that organizations work with vendors to verify and understand their process for handling deprecated attributes.

Additional Resources

Read the CERT Vulnerability Note Multiple BGP implementations are vulnerable to improperly formatted BGP updates - https://www.kb.cert.org/vuls/id/347067.

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