# CVE-2026-40303: zrok allows unauthenticated DoS via unbounded memory allocation in striped session cookie parsing

> Unauthenticated attackers can cause denial of service in zrok by sending crafted cookies that trigger unbounded memory allocation, crashing the service.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Apr 2026
- **Updated:** 20 Apr 2026
- **Also known as:** GHSA-cpf9-ph2j-ccr9, GO-2026-5329

## What is CVE-2026-40303?

CVE-2026-40303 is a high-severity denial of service vulnerability in openziti zrok, published on 17 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

zrok is software for sharing web services, files, and network resources. Prior to version 2.0.1, endpoints.GetSessionCookie parses an attacker-supplied cookie chunk count and calls make([]string, count) with no upper bound before any token validation occurs. The function is reached on every request to an OAuth-protected proxy share, allowing an unauthenticated remote attacker to trigger gigabyte-scale heap allocations per request, leading to process-level OOM termination or repeated goroutine panics. Both publicProxy and dynamicProxy are affected. Version 2.0.1 patches the issue.

Weakness: CWE-789 (Memory Allocation with Excessive Size Value), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-40303?

6 open source packages ship an affected version, including github.com/openziti/zrok and github.com/openziti/zrok/v2. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/openziti/zrok` | Go | All versions | None |
| `github.com/openziti/zrok` | Go | All versions to 1.1.11 | None |
| `github.com/openziti/zrok` | Go | All versions to 1.1.11 | None |
| `github.com/openziti/zrok/v2` | Go | All versions | 2.0.1 |
| `github.com/openziti/zrok/v2` | Go | All versions | 2.0.1 |
| `github.com/openziti/zrok/v2` | Go | All versions | 2.0.1 |

Products: Netfoundry Zrok.

## How do I fix CVE-2026-40303?

Upgrade to a fixed version.

## Is CVE-2026-40303 being exploited?

There is no known exploitation of CVE-2026-40303 in the wild and no public exploit code.

## Other vulnerabilities in openziti zrok

- [CVE-2026-45568](https://insights.sbomify.com/CVE-2026-45568/): zrok Python ProxyShare can be used as an SSRF proxy through absolute URL paths (critical 9.9)
- [CVE-2026-42275](https://insights.sbomify.com/CVE-2026-42275/): zrok: WebDAV drive backend follows symlinks outside DriveRoot, enabling host filesystem read/write (high 8.7)
- [CVE-2026-45576](https://insights.sbomify.com/CVE-2026-45576/): zrok copy writes attacker-controlled WebDAV paths outside the destination root (high 8.3)
- [CVE-2026-40302](https://insights.sbomify.com/CVE-2026-40302/): zrok has reflected XSS in GitHub OAuth callback via unsanitized refreshInterval error rendering (medium 6.1)
- [CVE-2026-40304](https://insights.sbomify.com/CVE-2026-40304/): zrok's broken ownership check in DELETE /api/v2/unaccess allows non-admin to delete global frontend records (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40303 and fix it if it is affected.

What it is: CVE-2026-40303 is a high-severity denial of service vulnerability in openziti zrok, published on 17 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-40303 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/openziti/zrok (Go): affected all versions; no fix listed
- github.com/openziti/zrok (Go): affected all versions up to 1.1.11; no fix listed
- github.com/openziti/zrok (Go): affected all versions up to 1.1.11; no fix listed
- github.com/openziti/zrok/v2 (Go): affected all versions; fixed in 2.0.1
- github.com/openziti/zrok/v2 (Go): affected all versions; fixed in 2.0.1
- github.com/openziti/zrok/v2 (Go): affected all versions; fixed in 2.0.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-40303/ (Markdown: https://insights.sbomify.com/CVE-2026-40303/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40303
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40303
- OSV: https://osv.dev/vulnerability/CVE-2026-40303

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40303/
Summaries are generated and may contain mistakes; the official description above is the record.