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5月7日，Hyunwoo Kim (V4bel) 披露了 Dirty Frag——两个 Linux 内核漏洞（CVE-2026-43284 和 CVE-2026-43500），这些漏洞允许非特权用户在自 2017 年以来发布的大多数 Linux 发行版上获得确定的 root 权限。微软在第二天确认了主动利用行为。我们在 Firecracker 微型虚拟机上构建 declaw.ai——用于 AI 代理的沙盒基础设施。我们运行着我们不编写且无法预测的不可信代码，因此当 Dirty Frag 出现时，我们的第一个问题是：我们的隔离边界是否牢固？我们在一颗故意未打补丁的内核上进行了测试。它保持了隔离。原因如下。该漏洞利用是一个页面缓存写入原语：它欺骗内核覆盖任何文件（/usr/bin/su, /etc/passwd）的内存内容，并提供 root 权限。完全确定，没有竞争条件。为什么这对于多租户平台很重要：页面缓存由整个机器共享。容器共享宿主内核，命名空间隔离、seccomp 和已删除的权限都是由该内核强制执行的。内核漏洞利用不需要逃逸容器——它在容器隔离存在的层之下运行。与 Dirty COW (2016) 和 Dirty Pipe (2022) 存在相同的结构性问题。在零日漏洞出现当天，在任何补丁存在之前，每个共享该内核的基于容器的沙盒都会暴露。打补丁是在事后关闭窗口；它无法提前关闭。我们在两种环境中运行了公开的 PoC（ESP 路径，CVE-2026-43284）。测试 1——容器沙盒（Docker，seccomp 开启，非特权 uid=1001，宿主内核 6.8.0）：非特权用户在 2 秒内获得 root 权限。Seccomp 处于活动状态，但没有帮助——所需的系统调用被配置文件允许。拥有 root 权限后，我们读取了 /etc/shadow、宿主内核启动参数和 Docker overlay2 路径。测试 2——Firecracker 微型虚拟机（未打补丁的客户内核，无 seccomp，以 root 身份启动，具有完全权限——有意比测试 1 更宽松）。该漏洞利用在客户机内部有效，但每次尝试访问宿主机的尝试都失败了：宿主内核不可见，宿主机进程不可见（客户机有自己的 kthreadd/kswapd），所有宿主机端口都关闭，只有虚拟块设备，没有宿主机硬件身份。它损坏的页面缓存属于客户机自己的内核，通过 EPT 映射到宿主内存的受限区域。不对称性是关键：微型虚拟机以比容器更高的权限启动，但仍然无法访问宿主机。重要的是软件授予的权限，而不是内核是否被共享。要逃逸 Firecracker，您需要在 VMM（约 5 万行 Rust 代码）或 KVM 中找到一个漏洞；谷歌的 kvmCTF 为客户机到宿主机的逃逸支付 25 万美元，并且只有一次被公开演示。如果您运行多租户的不可信代码，任何隔离提供商的问题是：如果沙盒内的代码获得 root 权限，它是否可以访问宿主机或其他租户？如果答案是“只要我们打了补丁”——这就是差距。PoC：https://github.com/V4bel/dirtyfrag 完整说明（命令 + 输出）：https://declaw.ai/blog/dirty-frag-microvm-isolation

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On May 7, Hyunwoo Kim (V4bel) disclosed Dirty Frag — two Linux kernel vulnerabilities (CVE-2026-43284 and CVE-2026-43500) that give unprivileged users deterministic root on most Linux distributions shipped since 2017. Microsoft confirmed active exploitation the next day.We build declaw.ai — sandboxing infrastructure for AI agents, on Firecracker microVMs. We run untrusted code we don't write and can't predict, so when Dirty Frag dropped our first question was: does our isolation boundary hold? We tested it on a deliberately unpatched kernel. It held. Here's why.The exploit is a page-cache write primitive: it tricks the kernel into overwriting the in-memory contents of any file (/usr/bin/su, /etc/passwd) and gives root. Fully deterministic, no race.Why it matters for multi-tenant platforms: the page cache is shared across the whole machine. Containers share the host kernel, and namespace isolation, seccomp, and dropped capabilities are all enforced by that kernel. A kernel exploit doesn't need to escape the container — it operates below the layer where container isolation exists. Same structural issue as Dirty COW (2016) and Dirty Pipe (2022). On the day a zero-day drops, before any patch exists, every container-based sandbox sharing that kernel is exposed. Patching closes the window after the fact; it can't close it in advance.We ran the public PoC (ESP path, CVE-2026-43284) in two environments.Test 1 — container sandbox (Docker, seccomp on, unprivileged uid=1001, host kernel 6.8.0): unprivileged user to root in under 2 seconds. Seccomp was active but didn't help — the required syscalls were permitted by the profile. With root we read /etc/shadow, host kernel boot params, and Docker overlay2 paths.Test 2 — Firecracker microVM (unpatched guest kernel, no seccomp, started as root with full capabilities — intentionally MORE permissive than test 1). The exploit worked inside the guest, but every attempt to reach the host failed: host kernel not visible, host processes invisible (the guest has its own kthreadd/kswapd), all host ports closed, only virtual block devices, no host hardware identity. The page cache it corrupted belongs to the guest's own kernel, mapped to a bounded region of host memory via EPT.The asymmetry is the point: the microVM started with more privilege than the container and still couldn't reach the host. What matters isn't what permissions the software grants — it's whether the kernel is shared. To escape Firecracker you'd need a bug in the VMM (~50K lines of Rust) or KVM; Google's kvmCTF pays $250K for a guest-to-host escape and only one has ever been publicly demonstrated.If you run untrusted code multi-tenant, the question for any isolation provider: if code inside the sandbox becomes root, can it reach the host or other tenants? If the answer is "as long as we're patched" — that's the gap.PoC: https://github.com/V4bel/dirtyfrag Full writeup (commands + output): https://declaw.ai/blog/dirty-frag-microvm-isolation

脏页漏洞：一个针对容器和微型虚拟机沙箱的内核零日漏洞