How severe is CVE-2026-55430?

CVE-2026-55430 has a CVSS score of 5.8/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-55430?

CVE-2026-55430 affects the following packages: github.com/coder/coder/v2 (Go), github.com/coder/coder/v2 (Go), github.com/coder/coder/v2 (Go), github.com/coder/coder/v2 (Go). Ecosystems affected: Go.

How do I fix CVE-2026-55430?

Update github.com/coder/coder/v2 to 2.34.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55430 is resolved across your whole dependency graph.

How do I detect CVE-2026-55430 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/coder/coder/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-55430 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-55430 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-55430 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

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

CVE-2026-55430 has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 11th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-55430?

CVE-2026-55430 is classified as CWE-345 (CWE-345), CWE-441 (CWE-441). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-55430 published, and has it been updated?

CVE-2026-55430 was published on July 8, 2026 and was last updated on October 21, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2026-55430 — coder

MEDIUMFix: coder/coder#26204

CVE-2026-55430 is a medium-severity (CVSS 5.8) CWE-345 vulnerability in github.com/coder/coder/v2. A fix is available for github.com/coder/coder/v2 — see the affected versions and patch details below.

Coder's subdomain workspace app routing trusts unauthenticated X-Forwarded-Host header, enabling cross-app data access

Also known asGHSA-5g4w-3vw9-478wGO-2026-5917

Published

Jul 8, 2026

Updated

Oct 21, 2026

Affected

4 pkgs

Patched

4 / 4

Exploits

None indexed

Exploitation data as of Oct 26, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55430.

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs11th percentile — riskier than 11% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2026-55430 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 395,492 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

4 pkgs affected

🐹github.com/coder/coder/v2🐹github.com/coder/coder/v2🐹github.com/coder/coder/v2🐹github.com/coder/coder/v2

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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
2.33	v2.33.8
2.32	v2.32.7
2.29 (ESR)	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!

Affected Packages

4 total 4 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/coder/coder/v2`	≥ 2.34.0&&< 2.34.2	2.34.2`go get github.com/coder/coder/v2@v2.34.2`
🐹Go	`github.com/coder/coder/v2`	≥ 2.33.0&&< 2.33.8	2.33.8`go get github.com/coder/coder/v2@v2.33.8`
🐹Go	`github.com/coder/coder/v2`	≥ 2.30.0&&< 2.32.7	2.32.7`go get github.com/coder/coder/v2@v2.32.7`
🐹Go	`github.com/coder/coder/v2`	all versions	2.29.17`go get github.com/coder/coder/v2@v2.29.17`

Affected Products

1 product · 4 configurations

Application

codercoder

≥ 2.34.0 && < 2.34.2

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/coder/coder/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/coder/coder/v2 to 2.34.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55430 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### 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 dom