RIDL and Fallout: MDS attacks

Attacks on the newly-disclosed "MDS" hardware vulnerabilities in Intel CPUs

The RIDL and Fallout speculative execution attacks allow attackers to leak private data across arbitrary security boundaries on a victim system, for instance compromising data held in the cloud or leaking your data to malicious websites. Our attacks leak data by exploiting the 4 newly disclosed Microarchitectural Data Sampling (or MDS) side-channel vulnerabilities in Intel CPUs. Unlike existing attacks, our attacks can leak arbitrary in-flight data from CPU-internal buffers (Line Fill Buffers, Load Ports, Store Buffers), including data never stored in CPU caches. We show that existing defenses against speculative execution attacks are inadequate, and in some cases actually make things worse. Attackers can use our attacks to leak sensitive data despite mitigations, due to vulnerabilities deep inside Intel CPUs.


RIDL (Rogue In-Flight Data Load) shows attackers can exploit MDS vulnerabilities to mount practical attacks and leak sensitive data in real-world settings. By analyzing the impact on the CPU pipeline, we developed a variety of practical exploits leaking in-flight data from different internal CPU buffers (such as Line-Fill Buffers and Load Ports), used by the CPU while loading or storing data from memory.

We show that attackers who can run unprivileged code on machines with recent Intel CPUs - whether using shared cloud computing resources, or using JavaScript on a malicious website or advertisement - can steal data from other programs running on the same machine, across any security boundary: other applications, the operating system kernel, other VMs (e.g., in the cloud), or even secure (SGX) enclaves.

We will present our paper on these attacks at the 40th IEEE Symposium on Security and Privacy, on May 20th 2019.

VUSec logo             CISPA logo


Fallout demonstrates that attackers can leak data from Store Buffers, which are used every time a CPU pipeline needs to store any data. Making things worse, an unprivileged attacker can then later pick which data they leak from the CPU's Store Buffer.

We show that Fallout can be used to break Kernel Address Space Layout Randomization (KASLR), as well as to leak sensitive data written to memory by the operating system kernel.

Ironically, the recent hardware countermeasures introduced by Intel in recent Coffee Lake Refresh i9 CPUs to prevent Meltdown make them more vulnerable to Fallout, compared to older generation hardware.


Our attacks can leak confidential data across arbitrary security boundaries in real-world settings (cloud, browsers, etc.). The reason our attacks are impervious to all the existing defenses against speculative execution attacks is that they can leak in-flight data. Unlike other recent attacks such as Spectre, Meltdown, and Foreshadow which are based on vulnerabilities leaking data from the CPU caches, RIDL and Fallout collect data from internal CPU buffers (Line Fill Buffers, Load Ports, Store Buffers). Intel describes the exploited vulnerabilities as "Microarchitectural Data Sampling" (MDS) - where "sampling" is another way of saying that we can leak in-flight (or "sampled") data from many of these microarchitectural buffers. A total of 4 (MDS) CVEs were assigned by Intel for RIDL ([MFBDS] CVE-2018-12130, [MLPDS] CVE-2018-12127, [MDSUM] CVE-2019-11091) and Fallout ([MSBDS] CVE-2018-12126) exploits.

Most importantly, our research shows that what last year appeared to be exceptional one-time speculative execution bugs are actually systemic, and the problems in modern CPUs may go much deeper than we initially thought. If CPUs have become so complex that chip vendors cannot keep their security under control, hardware vulnerabilities will be the new hunting ground for sophisticated attackers. And we may have no idea how many zero-day hardware vulnerabilities are still up for grabs. If we can no longer trust our hardware, the foundation on which we build all security solutions is crumbling away.

Exploit demos

Check out three RIDL exploits in action, leaking the root password hash from an unprivileged user, sensitive data from the Linux OS kernel, and JavaScript.

In this video, we leak the /etc/shadow file by repeatedly trying to authenticate an user. The animation is sped up for the latter part of the video, the total process takes about 24 hours at the moment.
In this video, we show how to leak recent kernel data using RIDL. This demo first reads 0 bytes from /proc/version, whereafter we are able to leak the full contents of /proc/version without the data ever present in userspace.
We leak a string from another process using Javascript and WebAssembly in the SpiderMonkey engine.

Interactive guide to speculative execution attacks

Click on the various components to interact with them. The full interactive version can be found here and the raw SVG can be found here. There is also a more vibrant colored version (the one used in our paper), which can be found here. These diagrams have been made by Stephan van Schaik ( @themadstephan).

MDS Tool from RIDL Team

Verify whether your system is vulnerable today with our MDS tool.

Intel patents


Very likely. Our attacks affect all modern Intel CPUs in servers, desktops and laptops. This includes the latest 9th-generation processors, despite their in-silicon mitigations for Meltdown. Ironically, 9th-generation CPUs are more vulnerable to some of our attacks compared to older generation hardware.

Processors from other vendors (AMD and ARM) do not appear to be affected. Official statements from these vendors can be found in the RIDL and Fallout papers.

Multiple teams and researchers independently discovered, studied, and reported MDS-class vulnerabilities and attacks to Intel. Unfortunately, the different timelines (see below), as well as the fine-grained vulnerability classification and embargo strategy enforced by Intel (which coordinated the disclosure process) made proper coordination across teams difficult. The year-long disclosure process (the longest to date) ultimately resulted in independent finders of even closely related MDS-class vulnerabilities to be completely unaware of one another until very late in the process.

At the same time, VUSec would like to thank Intel for sharing the full list of finders in advance -- which was crucial to give proper credit to everyone involved in the disclosure of MDS vulnerabilities in academia and industry -- and for allowing it to share information with all the other academic finders as of May 1. We made an effort to coordinate the different academic finders and present a unified message to the community and the public.

Unfortunately, as some teams felt unable to share their findings before the May 14 disclosure date, this effort was only partially successful, resulting in the present website (mdsattacks.com) presenting only the two independent RIDL and Fallout papers. See the timeline below for more information.

MDS attacks exploit hardware security flaws deep inside the processor and this makes malware that exploit MDS vulnerabilities very hard to distinguish from benign software. While anti-virus software could theoretically detect MDS exploits, it is very unlikely to happen in practice.

Intel has provided CPU microcode updates, and recommendations for mitigation strategies for operating system (and hypervisor) software. See Intel's Security Advisory for more details. We recommend you install the software updates provided by your operating system and/or hypervisor vendor.

In addition, we recommend disabling Simultaneous Multi-Threading (SMT), also known as Intel® Hyper-Threading Technology, which significantly reduces the impact of MDS-based attacks without the cost of more complex mitigations. Note that you might still be vulnerable despite disabling SMT, as MDS does not strictly rely on the presence of SMT.

A Virtual Machine (VM for short) is a software emulation of the computer's physical hardware. VMs are often used by clouds to allow customers to rent time on the cloud's physical hardware, rather than maintaining their own infrastructure. As clouds often run multiple VMs from different clients on the same physical hardware, it is important that cloud vendors maintain isolation between VM (security) boundaries.

To guarantee even stronger isolation (e.g., with protection against a compromised operating system), cloud providers can also run software inside secure (SGX) enclaves.

As shown in the RIDL paper, an attacker can exploit hardware security vulnerabilities inside the processor to break VM and SGX boundaries. However, countermeasures deployed by cloud providers resulting from our work should make MDS hard to exploit.

The kernel is the operating system core, and as such is responsible for most operations performed by the operating system. In particular, the kernel has access to all the data stored in the machine's memory, and to the secrets in it. As such, it is vital to properly isolate the kernel from other applications running on the machine.

Both the RIDL and Fallout attacks violate the kernel privacy by extracting information from within it. Moreover, both attacks can expose the kernel's location in the system's memory, simplifying other exploits.

RIDL and Fallout are related to, and inspired by, previous speculative execution attacks, including Spectre and Meltdown. In contrast to previous attacks, MDS attacks do not need to make assumptions about the memory layout of the victim data and do not depend on the processor cache.

This makes our attacks very difficult to mitigate. For instance, RIDL's ability to observe data used by other code running on the same physical core (hyperthread) is not trivial to mitigate without disabling the hyperthreading functionality entirely. However, note that both RIDL and Fallout can read recently accessed data across security boundaries even if hyperthreading is disabled.

A detailed analysis of the many existing speculative execution vulnerabilities, their relationship to each other and to RIDL and Fallout, can be found in the RIDL and Fallout papers.

We don't know whether malicious actors have abused these vulnerabilities in the wild. However, RIDL and Fallout show the potential impact is serious with several proof-of-concept exploits. For example, an attacker could launch an attack using malicious JavaScript in a web page or from a co-located Virtual Machine in the cloud (see our exploits and demos), allowing them to leak confidential data present on your system such as passwords and crypto keys.

Yes, similar to existing attacks, attackers can only mount our attacks in practical settings once they have the ability to execute (unprivileged) code on the victim machine. We could convince ourselves this is still an obstacle, but we should first be prepared to disable JavaScript (and similar) in the browser, abandon cloud computing, etc.

Detailed information about these in-flight buffers can be found in the papers above.

As a quick preview, the first diagram below shows the relevant parts of Intel's CPU pipeline and the relationship to the Line Fill Buffers, Load Ports, and Store Buffers used by our attacks. All data read/written from/to memory goes through some of these buffers.

The second diagram shows an example of how we leak data from these buffers. We perform a carefully crafted speculative load to an address the CPU is not immediately prepared to handle (e.g., resulting in a page fault). This lures the CPU into incorrectly pulling in-flight data from the target buffers, and then similarly to other speculative execution attacks, we leak this data using a Flush+Reload attack before the CPU realizes the mistake.

Intel CPU pipeline Attack overview

While performing some experiments on a rainy Amsterdam day, we realized we were able to leak data across security boundaries. We knew we could leak arbitrary in-flight (the "I" in RIDL) data but the cause was initially a big riddle to us. We came to know we were leaking from various CPU buffers only later...

It is also a reference to Intel's formal name for Meltdown: RCDL (related to RIDL, but instead leaking "C"ached data).

Internally, we referred to RIDL (and MDS in general) as the `cat pictures' vulnerability. :)

We started working on Fallout imminently after Meltdown, and Fallouts are typically a direct consequence of Meltdowns.

The following is the fine-grained vulnerability classification adopted by Intel, together with their CVSS score (their rating of the severity of the bug):

CVE-2018-12126: "Microarchitectural Store Buffer Data Sampling (MSBDS)" (CVSS score 6.5: Medium). See the Fallout paper for more information.

CVE-2018-12127: "Microarchitectural Load Port Data Sampling (MLPDS)" (CVSS score 6.5: Medium). See the RIDL paper for more information.

CVE-2018-12130: "Microarchitectural Fill Buffer Data Sampling (MFBDS)" (CVSS score 6.5: Medium). See the RIDL paper for more information.

CVE-2019-11091: "Microarchitectural Data Sampling Uncacheable Memory (MDSUM)" (CVSS score 3.8: Low). See the RIDL paper for more information.

Not really. And some of the researchers do share your same feelings.

VUSec in particular doesn't really like vulnerability logos. But we do like beer. So here is a MDS-branded one just in case:

MDS beer

Yes, with rights waived via CC0. Logos are designed by Marina Minkin. Click here for the logo in SVG format and here for the logo in PNG format.

MDS rack
Intel patents

Questions about RIDL and MDS vulnerabilities in general can be sent to [email protected].

Known Timeline

Based on the information shared with us by Intel and other finders.

  • TU Graz reports Meltdown UC to Intel
  • TU Graz reports Meltdown UC - Meltdown leaks from Uncacheable Memory - (identified as MDSUM) to Intel.

  • Microsoft reports to Intel
  • Microsoft independently reports a new issue (identified as MFBDS) to Intel on behalf of Giorgi Maisuradze (interning at MSR).
  • Bitdefender reports to Intel
  • Dan Horea Lutas' team at Bitdefender independently reports a L1TF mitigation bypass (identified as MFBDS) to Intel.
  • Volodymyr Pikhur reports to Intel
  • Volodymyr Pikhur independently reports a L1TF mitigation bypass (identified as MDSUM) to Intel.
  • VUSec reports RIDL to Intel
  • The RIDL authors from VU Amsterdam (VUSec) independently report the RIDL class of vulnerabilities (identified as MFBDS, and later also acknowledged as MLPDS and MDSUM) to Intel. Upon request, Intel communicates none of the existing (MFBDS) finders are interested in collaborating on a research paper with VUSec.
  • RIDL submitted @ IEEE S&P '19
  • The RIDL paper is submitted for publication to the IEEE Symposium on Security & Privacy, 2019.
  • Giorgi Maisuradze joins RIDL team
  • Giorgi Maisuradze and VUSec merge research efforts into the RIDL paper, after discussing common findings for the first time.
  • RIDL accepted @ IEEE S&P '19
  • The RIDL paper is accepted for publication at the IEEE Symposium on Security & Privacy, 2019.
  • Minkin, Genkin, and Yarom report to Intel
  • Minkin, Genkin, and Yarom report Fallout vulnerabilities (identified as MSBDS) to Intel.

    The finders would also like to acknowledge all the other authors on the Fallout paper.

  • VUSec and Volodymyr Pikhur share information
  • VUSec and Volodymyr discuss their findings for the first time.
  • VUSec and Dan Horea Lutas share information
  • VUSec and Dan discuss their findings for the first time.
  • TU Graz reports ZombieLoad to Intel
  • TU Graz reports ZombieLoad (identified as MFBDS) to Intel.
  • VUSec allowed to share by Intel
  • VUSec allowed to share findings with all academic finders by Intel. VUSec starts sharing information and making plans with the other teams, in an attempt to converge to a common website with a unified message and acknowledgements for all the finders. Unfortunately, as some teams felt unable to share their findings before the disclosure date, this effort was only partially successful, resulting in the present website (mdsattacks.com) presenting only the two independent RIDL and Fallout papers. See also FAQ#2.
  • VUSec, Genkin, and Yarom share information
  • VUSec, Genkin, and Yarom discuss their findings for the first time. VUSec shares the RIDL paper with Genkin and Yarom.
  • VUSec, Genkin, and Yarom share information
  • The Fallout paper is shared with VUSec.
  • Public disclosure of MDS attacks
  • The embargo is over after nearly a year and RIDL, Fallout, ZombieLoad, and MDS are now publicly disclosed.