| ▲ | Firecracker VM Escape: PwnEscape Part 2(pwn.ai) |
| 130 points by SerCe 4 hours ago | 64 comments |
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| ▲ | scoodah 2 hours ago | parent | next [-] |
| I really wish they would have expanded on this paragraph in their article further: > The configuration used here had no jailer. Firecracker's production containment includes dropping privileges and restricting the process through namespaces and seccomp. A deployment using those controls requires a separate assessment of the privileges and resources available after compromising its VMM. If I'm understanding all of this correctly, a properly jailed and configured firecracker instance would greatly reduce the blast radius of this escape, and I feel as though it would have been good for them to explain what jailing and dropping privileges would do to help. I'd hope most production firecracker deployments would be doing this already. Not saying that this is any less severe and the root issue is not specific to Firecracker, but to a flaw in the kernel's network code -- they just used an insecure firecracker configuration to demonstrate it. However, I feel like some are taking this as a flaw with firecracker when its not. This is an issue in the kernel networking stack. |
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| ▲ | tptacek 2 hours ago | parent | next [-] | | That's a good catch. Yes, since they've only proven this up to the jailer boundary, you'd need a second and relatively powerful LPE vulnerability to get real host control out of this. | |
| ▲ | amluto an hour ago | parent | prev | next [-] | | Nope. The exploit compromises the host kernel. The researchers built an unconventional exploit chain in which they used the host kernel compromise to compromise the host firecracker userspace code, but I see no fundamental reason they couldn't have compromised something else in the host kernel. | | |
| ▲ | tptacek 35 minutes ago | parent [-] | | Wait, this is also presumably true, come to think of it. I'm a little fuzzy because they document the memory disclosure primitive carefully but not the memory writes they also got from it --- but whatever those are, they're hitting kernel memory? But the exploit they're describing here gets code execution out of Firecracker itself, not the kernel. |
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| ▲ | vluft 21 minutes ago | parent | prev [-] | | yup, the other firecracker escape recently (https://www.coinspect.com/blog/firecracker-escape/) popped the jailer via io_uring (tl;dr firecracker was filtering syscalls appropriately with seccomp, and was filtering the uring _they_ initiated, so attacker just got them to pop a new unfiltered uring, which is rather nice). | | |
| ▲ | tptacek 7 minutes ago | parent [-] | | Just for what it's worth, that escape is much more situational than PwnEscape (there have been other escapes that depended on nested KVM, as well). Pretty much everyone seems to have been exposed with PwnEscape, because everyone using Firecracker is also using virtio-net. |
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| ▲ | lrvick 3 hours ago | parent | prev | next [-] |
| I am more convinced by the day that the Nova Microhypervisor approach is correct. Do virtualization with a tiny formally verified microkernel and have complex kernels like Linux run underneath instead of letting a huge, complex, and brittle linux kernel and vm ecosystem control the hypervisor layer. https://hypervisor.org/ |
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| ▲ | eddythompson80 3 hours ago | parent | next [-] | | How is this different from Xen? It's the same approach as Xen or Hyper-V where the hypervisor is its own component, then you boot into a virtualized Dom0 or Root Partition (in Xen and Hyper-V terminology respectively) that can manage the hypervisor underneath it. This is different from KVM where your hypervisor runs in your Dom0/Root Partition. | | |
| ▲ | markasoftware 3 hours ago | parent | next [-] | | Xen had a generic guest to host vulnerability disclosed in the last few days: https://codeant.ai/security-research/xenbleed-a-grant-copy-c... | | |
| ▲ | eddythompson80 2 hours ago | parent | next [-] | | Yeah, all security design guarantees are as good as the implementation at the end of the day. All the fancy hashing and encryption math in the world isn't gonna save you from an erroneous `"" == ""` check or an invalid reference check. I was replying to OP saying it's the same "approach" as Xen, if your preference is "approach" based. It's fair though to point to Nova's smaller code base. It's becoming an important metric for me in general (how big is the code base). It's not a bullet proof thing, but having a smaller code base is a positive sign in 2026 for me. | | |
| ▲ | markasoftware 2 hours ago | parent [-] | | Novas core seems to be formally verified which rules out a vast amount of bug categories that can happen with xen, including this one | | |
| ▲ | eddythompson80 2 hours ago | parent [-] | | Interesting, I'll have to learn more about Nova. | | |
| ▲ | lrvick 2 hours ago | parent [-] | | See Udos most recent talk at 3mdeb Boot Mastery Conference. He is an ex Intel CPU engineer and the depth of his thought into the security design had me questioning my life choices. | | |
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| ▲ | ThePowerOfFuet 2 hours ago | parent | prev [-] | | Now compare to Linux, which is the point raised. Nobody said alternatives like Xen are perfect... but my oh my, some are much better in this regard. |
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| ▲ | lrvick 3 hours ago | parent | prev [-] | | This is a much lighter weight (~100k) alternative to Xen focused on baremetal TPM anchored remote attestation, formal verification, and trusted computing applications. Anyone considering Xen should probably also consider Nova depending on requirements. |
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| ▲ | tptacek 3 hours ago | parent | prev | next [-] | | I don't know much of anything about Nova but since the late 1990s this has also been the pitch for L4; there are several mature L4's. | | |
| ▲ | lrvick 3 hours ago | parent [-] | | Nova is L4 inspired, and interchangable with L4 kernels in Genode. | | |
| ▲ | tptacek 3 hours ago | parent [-] | | Yep, very similar, but L4 is a little bit more general purpose (despite being microscopically tiny) and a little more battle-tested. | | |
| ▲ | lrvick 3 hours ago | parent [-] | | Is there a well tested L4 hypervisor solution with hardware accelerated virtualization and raw TPM support formally verified to a similar or greater level than Nova with similar remote attestation and security features? SeL4 is the closest contender on my radar atm. | | |
| ▲ | tptacek 3 hours ago | parent [-] | | Most of the L4s have hypervisor situations. I don't know about TPM support. Generally the vibe I get from systems security people is a bit of eye-rolling about the formal verification of things like L4; the real win is just drastic attack surface reduction. And then, of course, L4s live in millions and millions of shipped products, which is a reason to consider L4 instead of something novel, but I'm not saying you should or shouldn't. | | |
| ▲ | lrvick 2 hours ago | parent [-] | | I for one do not want any code in the TCB that is not formally verified, tiny, and easy to audit with every line of code justified. SeL4 is the only open, mature, and formally verified L4 solution I am aware of that can run on modern servers, but IIRC given vCPUs are not first class objects and any low level CPU virtualization features are handled in user space, it comes at a real performance cost making it only suitable for single-core or non-performance-critical applications. Nova by contrast was built to handle the expensive and trust critical TPM and vCPU handling directly for high performance remotely attestable multi-core applications without having to reach for something as heavy as Xen, though formal verification is admittedly not as complete as SeL4 yet AFAICT, though lots of recent progress here. Those are pretty big tradeoffs, but if you or anyone else knows other viable xen alternatives this general shape with a tiny easily-auditable footprint, please chime in! | | |
| ▲ | tptacek 2 hours ago | parent [-] | | Like some of what I'm saying is just that if you tell a systems security person "we're using seL4 because of its formal verification", you're going to get eye rolls. Whatever else you want to say about it, the formally-verified configuration of seL4 doesn't actually do much, meaning whatever system you're building on top of it is going to be where all the security drama is anyways. I'm certainly not telling you to use Xen. I wouldn't use Xen for anything untrusted. | | |
| ▲ | lrvick 2 hours ago | parent [-] | | > the formally-verified configuration of seL4 doesn't actually do much As a systems security engineer myself I would disagree and know several others that would as well. All depends on your circles, applications, and threat model I suppose. Xen, Hyper-V, and KVM have all had major security flaws in the form of logic violations, spacial memory corruptions, use-after-free, and pointer corruptions that allowed hypervisor trust boundary violations. I expect most if not all of those classes of bugs could have been prevented by formal verification. Xen likely has a bad reputation with both of us because of a history of preventable flaws like these. In my applications every single daemon gets its own sandbox, which is a VM whenever possible. Some with network, some without network, different filesystem and vsock access for maximum damage control and minimal trust between daemons. The hypervisor is the most important security boundary to me especially when isolating user-controlled workloads. The more confidence I can get that it is correct the better. | | |
| ▲ | tptacek 2 hours ago | parent [-] | | You're responding to a comment where I wrote that I would never use Xen with an argument not to use Xen. All I'm saying is that "formal verification", at least as it exists in current systems, is not why I wouldn't use Xen. | | |
| ▲ | lrvick an hour ago | parent [-] | | > an argument not to use Xen Not my intent but perhaps I communicated poorly. It was an argument that the -reason- Xen has had so many bugs is because it was not built for correctness. Formal verification should be a bare minimum for a TCB component to know many classes of bugs that have plagued the most popular hypervisors cannot happen. We both clearly agree not to use Xen. Anyway, I appreciate the engagement on this all the same. Few even know alternatives to xen exist! | | |
| ▲ | tptacek 38 minutes ago | parent [-] | | Yeah, all I'm saying is that I agree with you about Xen, but that I don't agree about the centrality of formal methods --- at least on current projects with current verifications, things could get different now that AI lets us be more ambitious with verification --- in determining attack surface. Generally, the less your OS can do, apart from strictly hypervising guest operating systems, the safer you probably are. Hence why people do things like seL4. |
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| ▲ | traceroute66 3 hours ago | parent | prev [-] | | > the Nova Microhypevisor approach is correct To save people the time: "This code is experimental and not feature complete. If it breaks, you get to keep both pieces." | | |
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| ▲ | zX41ZdbW 3 hours ago | parent | prev | next [-] |
| I have a demo service that entirely relies on Firecracker VMs: ClickBench playground, https://benchmark.clickhouse.com/playground/ It hosts hundreds of database engines - many of them have vulnerabilities, and some of them, like Pandas, are basically open RCEs regardless. We can PoC the Firecracker VM Escape on that service. |
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| ▲ | gavinray 2 hours ago | parent [-] | | > "run SQL against 110+ databases"
This is one of the most useful web tools I've ever seen.I work on connectors for datasources (mostly DB's. Being able to quickly test things like sorting behavior and how the same function can be emulated across DB's would have made my life so much easier. |
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| ▲ | markasoftware 3 hours ago | parent | prev | next [-] |
| This is not a new vulnerability, it's just a description of how to exploit the PwnEscape vulnerability in practice on firecracker VMs (disclosed multiple weeks ago and discussed in some less upvoted posts on HN) |
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| ▲ | YetAnotherNick 3 hours ago | parent [-] | | > already discussed at length on HN Couldn't find it. Care to provide link. | | |
| ▲ | tptacek 3 hours ago | parent | next [-] | | The actual vulnerability got very little discussion. It's clever. Because Linux networking people are obsessed with checksum performance, virtio-net plumbs checksum offload metadata through the host/guest driver interface. Guests can thus deliver the host packets along with instructions on what to checksum. The vulnerability is a missing sanity check that allows checksum instructions to instruct the host to overwrite the header length field in the IPv4 header, rather than the checksum field. Crucially, this happens in the middle of packet processing, so the kernel code is forced to work with two different ideas of how long the IPv4 header is. All hell predictably breaks loose. | |
| ▲ | ytxm 3 hours ago | parent | prev | next [-] | | Fixed a month ago upstream https://github.com/torvalds/linux/commit/6785011f8b16abf40b1... https://www.pwn.ai/blog/pwnescape | | |
| ▲ | tptacek 2 hours ago | parent [-] | | Additional color: per the commit, the bug was found by Paulos Yibelo, who runs PwnAI (the author of this piece), and worked on security at AWS for 4 years prior. | | |
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| ▲ | markasoftware 3 hours ago | parent | prev [-] | | Ah maybe not on HN, I forget where I learned of it. Either way, multiple weeks ago: https://news.ycombinator.com/item?id=50108845 |
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| ▲ | nwilkens 2 hours ago | parent | prev | next [-] |
| I've been building Firehyve, which is a firecracker like microVM solution for illumos and FreeBSD.. https://github.com/TritonDataCenter/rshyve Running our microVM's inside of a jailed zone, at least gives me some peace of mind.. It's far from perfect, as a zone escape could be lurking.. but at least it's a layer of defense. |
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| ▲ | tptacek 2 hours ago | parent [-] | | As another commenter noted, the PoC in this article doesn't defeat the Firecracker jailer, which does essentially what a "zone" would do for you. In both cases, that's cold comfort, because even in an unprivileged sandbox environment you still have a huge kernel attack surface to contend with LPEs in. | | |
| ▲ | cyphar an hour ago | parent [-] | | I would've thought that you could actually have an incredibly strict seccomp policy for the jailer which would require you to find an LPE in something like open(2), which seems unlikely even in the "agent swarm" era. | | |
| ▲ | tptacek 39 minutes ago | parent [-] | | That's basically what the Firecracker jailer does. Always my thoughts turn to the waitid(2) vulnerability; for a little while, you could pass kernel addresses through it and whack arbitrary kernel memory, and nobody was ever going to seccomp "waitid" out. But yes: it's a meaningful defense, if not one you'd want to bet your service on. |
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| ▲ | denysvitali 3 hours ago | parent | prev | next [-] |
| Dupe: https://news.ycombinator.com/item?id=50381397 (Doesn't HN usually deduplicate links? I'm confused) |
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| ▲ | tptacek 3 hours ago | parent [-] | | Duplicates are generally accepted when there was no significant discussion on the original submission. | | |
| ▲ | denysvitali 2 hours ago | parent | next [-] | | I seem to remember there's a system in place to prevent duplicates (where the submission of a duplicate link results in the upvote of the original story), but maybe that's limited to the first 24h? Idk, HN works in mysterious ways | |
| ▲ | kvemkon 2 hours ago | parent | prev [-] | | This has negative impact on searching older discussions. Instead of 1 result for 1 URL (within few month) getting 2 or even more. | | |
| ▲ | tptacek 2 hours ago | parent [-] | | It's a settled enough part of HN that you'd do better emailing hn@ycombinator.com about it than you will litigating it in public. I don't care either way, I'm just explaining the behavior to you. |
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| ▲ | clhodapp 3 hours ago | parent | prev | next [-] |
| I posted a similar query on the Replit sandbox engine article, but.. How do people feel about the hardware isolation provided by microVMs versus the stronger end of software isolation techniques like gvisor? Right now, we put a huge amount of stock in "hardware isolation". If that starts falling to agents to any substantial degree, I wonder whether we might end up leaning more on patchable software isolation, similar to the rise of software RAID (though, in userspace this time). |
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| ▲ | Betelbuddy 3 hours ago | parent | prev | next [-] |
| https://aws.amazon.com/security/security-bulletins/2026-137-... |
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| ▲ | thisisauserid 3 hours ago | parent | prev | next [-] |
| Firecracker VMs run on AWS. Odd that the article doesn't mention that. |
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| ▲ | aghadge 3 hours ago | parent | prev | next [-] |
| it's really informative. At least someone are break it. |
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| ▲ | sscaryterry 3 hours ago | parent | prev [-] |
| This is pure LLM slop. |
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| ▲ | tptacek 2 hours ago | parent [-] | | You need something more interesting to say than this, because this is an article by the discoverers of this vulnerability, the first general-purpose Firecracker escape that doesn't depend on nested virtualization in recent memory, along with in-depth discussion of exploitability. It's inherently interesting and on-topic and there isn't an alternate source for the same content. |
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