# CVE-2026-121880: RStudio Server / Posit Workbench localhost port-proxy does not enforce per-session port ownership


- **Severity:** Medium 4.7 (`CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/E:P/AU:Y/R:A/V:D/RE:M/U:Amber`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 6 Nov 2026
- **Updated:** 6 Nov 2026
- **Also known as:** GHSA-45mx-vf84-77ww

## What is CVE-2026-121880?

CVE-2026-121880 is a medium-severity vulnerability in Posit Workbench, published on 6 Nov 2026. 

### Official description

Authorization bypass through user-controlled key in the localhost port-proxy (/p/<token>/) in Posit Software, PBC RStudio Server and Posit Workbench before 2026.08.0 allows remote authenticated users to reach web applications and other services bound to 127.0.0.1 under a different user's session, and to view their live rendered content and data, via a proxy URL constructed from the attacker's own port token for an arbitrary port number. The port token is a documented obfuscation measure and not an authorization boundary, and the proxy does not verify that the requested port belongs to the requesting user's session.

NOTE: exploitation requires an authenticated account on the same multi-user server and a target service listening on localhost at the time of access. Impact may be greater on hosts that co-locate additional privileged localhost services, and the bypass is not prevented by allow-shell=false; no setting restricts the port-proxy without disabling all application proxying.

Weakness: CWE-862 (Missing Authorization), CWE-639 (Authorization Bypass Through User-Controlled Key)

## Which versions are affected by CVE-2026-121880?


## How do I fix CVE-2026-121880?

No fix has been published for CVE-2026-121880 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-121880 being exploited?

There is no known exploitation of CVE-2026-121880 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-121880 and fix it if it is affected.

What it is: CVE-2026-121880 is a medium-severity vulnerability in Posit Workbench, published on 6 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-121880 in the wild and no public exploit code.

Affected product: Posit Workbench. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-121880/ (Markdown: https://insights.sbomify.com/CVE-2026-121880/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-121880
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-121880
- OSV: https://osv.dev/vulnerability/CVE-2026-121880

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-121880/
Summaries are generated and may contain mistakes; the official description above is the record.