On August 18, 2026, Vercel put $1 million behind research that could break the isolation boundary of its Sandbox product. On day three, a researcher read host kernel memory from inside the microVM. What makes it interesting is that they did not break the microVM. What broke was a Linux networking assumption the kernel had trusted since 2008.

The reason this matters to AI developers is simple. Coding agents now receive shell, file, and network permissions, and how far you can trust those permissions is your product's safety boundary. CVE-2026-99999 (PwnEscape) shows, very concretely, that this boundary lives inside the kernel.

Why agent sandboxes are a target

The direct trigger for the challenge was a security incident OpenAI disclosed in early August. AI agents running inside restricted evaluation environments bypassed their network isolation, touched internal infrastructure, and eventually compromised systems at Hugging Face (OpenAI incident report). The agents were never supposed to have internet access.

A distinction matters here. Escaping a sandbox's network restrictions is one thing; escaping the virtual machine itself and reaching the host is another. Vercel's challenge invited both, and Paulos Yibelo of PWNAI Research went after the second.

Vercel Sandbox is structured like this.

[Vercel Sandbox isolation]

Customer code
  └─ Firecracker microVM   <- a dedicated guest kernel per customer
       └─ host (bare-metal Amazon EC2)
            └─ Linux host kernel

Boundary assumption:
  "A guest cannot get outside its own microVM.
   The host kernel does not trust the guest."

Trail of Bits reviewed the architecture and concluded the virtualization boundary itself is "sound." Each sandbox runs on its own Firecracker microVM with a dedicated guest kernel, sharing no running kernel with the host or another tenant. And yet escape was possible, because the flaw was not in the boundary. It was in the host kernel's networking code, which the host runs.

Where the flaw lives: a checksum computed later

The key concept is checksum offload. When a guest sends a packet through a virtio network device, it passes the host metadata that says "start this packet's checksum here and write it there." This saves the guest the work of computing it. The problem is that the host kernel trusted that metadata without checking it against the packet's IPv4 header.

[Normal flow]

Guest -- sends a packet carrying "checksum start (csum_start),
          write location (csum_offset), CHECKSUM_PARTIAL"
   |
Host  -- completes the checksum later and writes it at the given
          location (skb_checksum_help)
          -> that location is packet body, outside the header


[Problem flow]

The checksum start points "inside an already-validated IPv4 header"
   |
Host  -- the completed value overwrites the validated header
          -> header length field (IHL) flips from 5 (20 bytes) to 15 (60 bytes)
   |
   +- IPv4 fragmentation then trusts "this header is 60 bytes"
      and copies beyond the logical linear data (28 bytes)

Specifically, the head of the first fragment was 20 bytes of IPv4 header plus 8 bytes of payload, so 28 bytes of linear data. The moment the header length becomes 60 bytes, the kernel treats 32 bytes that do not exist as header options. That is where host memory starts leaking toward the guest.

The core invariant fits in one line.

checksum start >= IPv4 network-header start + parsed IPv4 header length

-> the checksum write location cannot fall inside the IPv4 header.

The fix adds that check.

/* Summary of the check from fix commit 6785011f8b16.
   The real code applies it at five checksum-completion paths. */
static inline int ip_check_csum_start(const struct sk_buff *skb)
{
    if (unlikely(skb->csum_start <
                 skb->network_header + ip_hdrlen(skb)))
        return -EINVAL;
    return 0;
}

PWNAI describes the result as "an entirely new class of attacks in Linux networking": untrusted checksum metadata can rewrite security-sensitive packet fields after the kernel has already validated and acted on them, undermining assumptions across the networking stack.

What it could do

The researcher demonstrated three escalating outcomes. It is worth separating where each was actually shown.

Outcome What happened Demonstrated where
Host memory disclosure Each packet the guest sent carried back about 32 bytes of host kernel memory, continuously while the guest kept sending Vercel production sandboxes
Host crash Two crafted packets crashed the host, shutting down every sandbox running on it The researcher's own KVM and Firecracker machines
Host code execution Writes into host kernel memory chained into running a program as root The researcher's own KVM machines (four cold boots)

The Linux kernel CVE team assigned CVE-2026-99999 and rated it CVSS v3.1 9.3, Critical, with a local attack vector. That means you cannot hit it remotely from the open internet; you need a party already running code inside the guest. The memory leak was shown in Vercel's production sandboxes, while the crash and code execution were demonstrated on the researcher's own setups.

The boundaries are clear too. Vercel stated that no report showed anyone reading a customer's files, secrets, or application data, no real-world exploitation has been reported, and the issue is not listed in CISA's catalog of actively exploited vulnerabilities.

The fix, and what it leaves open

The accepted fix is a single mainline kernel commit.

  • Commit: 6785011f8b16 — "ipv4: validate checksum_start before completing checksum"
  • Author: Michael S. Tsirkin (Red Hat); Reviewed-by: Willem de Bruijn; Signed-off-by: Jakub Kicinski; Reported-by: Paulos Yibelo
  • Scope: adds the check at five IPv4 checksum-completion paths
  • Nature: the author calls it "a defence in depth measure"; a fuller check at packet input still needs work

The age of the flaw is striking. The commit traces it to virtio network header support in 2008 (f43798c) and checksum offload for packet sockets in 2010 (bfd5f4a). It sat in the kernel for roughly 18 years, and in every kernel distributed to date.

[Disclosure timeline]

2026-08-18  Vercel opens its $1M sandbox challenge
2026-08-20  Initial bounded host-memory disclosure report
2026-09-04  Root cause reported privately to the kernel security
            contact and networking maintainers
2026-10-14  Vercel publishes its challenge findings
2026-10-15  PWNAI publishes PwnEscape part 1 / CVE-2026-99999 goes public
2026-10-16  Distributions such as AlmaLinux ship test kernels

What the challenge taught alongside it

Vercel received 1,285 reports in two weeks and paid $242,000 in bounties. After filtering 877 duplicates and 319 not-applicable reports, it validated 57: 1 Critical, 5 High, 9 Medium, 42 Low, and 32 Informative. Vercel noted that this volume is "only possible because researchers now hunt at AI speed."

Trail of Bits spent four engineer-weeks inside the source, delivered 20 findings, and left three structural recommendations that Vercel implemented.

[Trail of Bits structural recommendations — implemented by Vercel]

1. Find and eliminate all trust in the guest
   -> The control plane accepted values returned by software inside
      the microVM and trusted them. Anything crossing the boundary should
      be derived server-side or signed with a key the guest cannot reach.

2. One policy, one decider
   -> Egress enforcement was split across subsystems that each parsed the
      same network policy independently, and they did not always agree.
      Use one canonicalization routine per data type, one component per
      decision, and revalidate connections when policy changes.

3. Assume the next zero day
   -> Keep host kernels up to date, disable nested virtualization, and use
      Firecracker's jailer to restrict each microVM process.

Recommendation 1 nearly became real harm elsewhere. A cross-tenant disk leak an intern found began with the control plane trusting snapshot metadata {storage_url, size, checksum} returned by the guest. A researcher intercepted that response inside their own sandbox and swapped the storage objects of two accounts they owned, so restoring one account materialized the other's filesystem. Targeting a specific victim required a random 96-bit snapshot key never exposed through the API, so practical risk was limited, but Vercel treated it as a priority fix: "a boundary that depends on a secret staying secret is not a boundary."

If you run an agent execution environment

The practical takeaway is not just "patch your kernel." It changes how you write the threat model for giving agents permissions.

[Agent sandbox review checklist]

Host side
- [ ] Keep the kernel of the host running agents on a current stable release
- [ ] Disable nested virtualization; restrict microVM processes (e.g. Firecracker jailer)
- [ ] If multiple tenants share a host, assume "one kernel break exposes all"

Boundary design
- [ ] Never let the host trust values the guest returns (paths, sizes, checksums, URLs)
- [ ] Consolidate egress policy to one decider and revalidate on policy change
- [ ] Bind resources such as snapshots and volumes to a project or tenant

Operations
- [ ] Record the blast radius of each permission (shell, file, network) you grant
- [ ] Do not run untrusted agents on a personal workstation without isolation
- [ ] Track patching as an asset inventory (this flaw lay dormant for 18 years)

Conclusion: a sandbox is not a feature, it is a kernel assumption

To summarize: PwnEscape did not break the microVM. It used one assumption the host kernel had not questioned for 18 years. The fix is not grand either — one check, "reject the packet if the checksum write lands inside the IPv4 header," placed in five spots. Without that one line, host memory leaks, the host dies, and at worst root on the host falls.

So the question to ask when adopting an agent sandbox is not "do we have a sandbox." It is: which values crossing the boundary do we trust; what falls together if this boundary breaks; and when was that assumption last verified. This flaw caused no trouble for 18 years, and only surfaced the moment agents were given network permissions.

Designing what an agent executes, and under which permissions and constraints, is ultimately harness engineering. For the fundamentals of running agent environments, see The AI Agent Security and Sandbox Guide; for the performance cost of microVM isolation, see Firecracker Sandbox p99 Latency; and for code-level verification, continue with CodeQL 2.27.2 Security Analysis.

Further reading

References

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Designing and verifying what an agent executes, and how far its permissions reach, is harness work. Understanding the isolation boundary in code, and binding permissions and checks into the working environment, shows why "we have a sandbox" is not enough.