CVE-2026-55430: Anthropic Python — MEDIUM

### Summary The workspace app proxy resolves the target app from `httpapi.RequestHost()` which prefers the `X-Forwarded-Host` header over the real `Host` header. No middleware strips `X-Forwarded-Host` before routing and the header is not browser-forbidden so client-side JavaScript can set it on...

Full CISO analysis pending enrichment.

What systems are affected?

Package	Ecosystem	Vulnerable Range	Patched
Anthropic Python	go	>= 2.34.0, < 2.34.2	`2.34.2`
3.7K 5.4K dependents Pushed 4d ago 95% patched ~3d to patch Full package profile →

Do you use Anthropic Python? You're affected.

How severe is it?

CVSS 3.1

5.8 / 10

EPSS

N/A

Exploitation Status

No known exploitation

Sophistication

N/A

What is the attack surface?

AV Network

AC High

PR Low

UI Required

S Changed

C High

I None

A None

What should I do?

Patch available

Update Anthropic Python to version 2.34.2

Which compliance frameworks are affected?

Compliance analysis pending. Sign in for full compliance mapping when available.

Frequently Asked Questions

What is CVE-2026-55430?

### Summary The workspace app proxy resolves the target app from `httpapi.RequestHost()` which prefers the `X-Forwarded-Host` header over the real `Host` header. No middleware strips `X-Forwarded-Host` before routing and the header is not browser-forbidden so client-side JavaScript can set it on `fetch()` calls. > **Note:** Practical exploitation requires subdomain app routing (wildcard hostname) enabled, a victim who visits the attacker's shared app and a deployment whose upstream proxy does not strip `X-Forwarded-Host`. ### Impact App session cookies are scoped to the wildcard parent domain so the browser attaches them to any app subdomain. An attacker who controls a shared workspace app can serve JavaScript that sends same-site requests with a forged `X-Forwarded-Host` pointing at a victim's private app. The server routes by the attacker-controlled header but authorizes with the victim's cookie which lets the attacker read the victim's private app responses. Subdomain app routing must be enabled and no upstream proxy may strip `X-Forwarded-Host`. ### Patches The fix trusts `X-Forwarded-Host` only from configured trusted proxies and otherwise resolves the routing host from the verified request host. The fix was backported to all supported release lines: | Release line | Patched version | |---|---| | 2.34 | [v2.34.2](https://github.com/coder/coder/releases/tag/v2.34.2) | | 2.33 | [v2.33.8](https://github.com/coder/coder/releases/tag/v2.33.8) | | 2.32 | [v2.32.7](https://github.com/coder/coder/releases/tag/v2.32.7) | | 2.29 (ESR) | [v2.29.17](https://github.com/coder/coder/releases/tag/v2.29.17) | ### Workarounds Place an upstream reverse proxy that strips or overwrites `X-Forwarded-Host` on untrusted requests. ### Resources - Fix: #26204 ### Credits Coder would like to thank Anthropic's Security Team (ANT-2026-22435) for independently disclosing this issue!

Is CVE-2026-55430 actively exploited?

No confirmed active exploitation of CVE-2026-55430 has been reported, but organizations should still patch proactively.

How to fix CVE-2026-55430?

Update to patched version: Anthropic Python 2.34.2.

What is the CVSS score for CVE-2026-55430?

CVE-2026-55430 has a CVSS v3.1 base score of 5.8 (MEDIUM).

What are the technical details?

Original Advisory

### Summary The workspace app proxy resolves the target app from `httpapi.RequestHost()` which prefers the `X-Forwarded-Host` header over the real `Host` header. No middleware strips `X-Forwarded-Host` before routing and the header is not browser-forbidden so client-side JavaScript can set it on `fetch()` calls. > **Note:** Practical exploitation requires subdomain app routing (wildcard hostname) enabled, a victim who visits the attacker's shared app and a deployment whose upstream proxy does not strip `X-Forwarded-Host`. ### Impact App session cookies are scoped to the wildcard parent domain so the browser attaches them to any app subdomain. An attacker who controls a shared workspace app can serve JavaScript that sends same-site requests with a forged `X-Forwarded-Host` pointing at a victim's private app. The server routes by the attacker-controlled header but authorizes with the victim's cookie which lets the attacker read the victim's private app responses. Subdomain app routing must be enabled and no upstream proxy may strip `X-Forwarded-Host`. ### Patches The fix trusts `X-Forwarded-Host` only from configured trusted proxies and otherwise resolves the routing host from the verified request host. The fix was backported to all supported release lines: | Release line | Patched version | |---|---| | 2.34 | [v2.34.2](https://github.com/coder/coder/releases/tag/v2.34.2) | | 2.33 | [v2.33.8](https://github.com/coder/coder/releases/tag/v2.33.8) | | 2.32 | [v2.32.7](https://github.com/coder/coder/releases/tag/v2.32.7) | | 2.29 (ESR) | [v2.29.17](https://github.com/coder/coder/releases/tag/v2.29.17) | ### Workarounds Place an upstream reverse proxy that strips or overwrites `X-Forwarded-Host` on untrusted requests. ### Resources - Fix: #26204 ### Credits Coder would like to thank Anthropic's Security Team (ANT-2026-22435) for independently disclosing this issue!

Weaknesses (CWE)

CWE-345 Insufficient Verification of Data Authenticity Primary CWE-441 Unintended Proxy or Intermediary ('Confused Deputy') Primary

CWE-345 — Insufficient Verification of Data Authenticity: The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.

Source: MITRE CWE corpus.