How severe is CVE-2026-5199?

No CVSS score has been assigned to CVE-2026-5199 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-5199?

CVE-2026-5199 affects the following packages: go.temporal.io/server (Go), go.temporal.io/server (Go). Ecosystems affected: Go.

How do I fix CVE-2026-5199?

Update go.temporal.io/server to 1.30.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-5199 is resolved across your whole dependency graph.

How do I detect CVE-2026-5199 in my Go dependencies?

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

How do I mitigate CVE-2026-5199 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.

How does O3 Security protect against CVE-2026-5199?

O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-5199 can be triaged on real exposure rather than presence alone.

Is CVE-2026-5199 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-5199 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-2026-5199?

CVE-2026-5199 has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 16th 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-2026-5199?

CVE-2026-5199 is classified as CWE-639 (CWE-639). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-5199 published, and has it been updated?

CVE-2026-5199 was published on April 1, 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.

CVE-2026-5199: server

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-2026-5199.

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs16th percentile — riskier than 16% of all scored CVEsHighest risk

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Real-World Exposure

2 pkgs affected

🐹go.temporal.io/server🐹go.temporal.io/server

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

A writer role user in an attacker-controlled namespace could signal, delete, and reset workflows or activities in a victim namespace on the same cluster. Exploitation requires the attacker to know or guess specific victim workflow ID(s) and, for signal operations, signal names. This was due to a bug introduced in Temporal Server v1.29.0 which inadvertently allowed an attacker to control the namespace name value instead of using the server's own trusted name value within the batch activity code. The batch activity validated the namespace ID but did not cross-check the namespace name against the worker's bound namespace, allowing the per-namespace worker's privileged credentials to operate on an arbitrary namespace. Exploitation requires a server configuration where internal components have cross-namespace authorization, such as deployment of the internal-frontend service or equivalent TLS-based authorization for internal identities.

This vulnerability also impacted Temporal Cloud when the attacker and victim namespaces were on the same cell, with the same preconditions as self-hosted clusters.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`go.temporal.io/server`	≥ 1.30.0-143.0&&< 1.30.3	1.30.3`go get go.temporal.io/server@v1.30.3`
🐹Go	`go.temporal.io/server`	all versions	1.29.5`go get go.temporal.io/server@v1.29.5`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for go.temporal.io/server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update go.temporal.io/server to 1.30.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-5199 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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-5199 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2026-5199. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

A writer role user in an attacker-controlled namespace could signal, delete, and reset workflows or activities in a victim namespace on the same cluster. Exploitation requires the attacker to know or guess specific victim workflow ID(s) and, for signal operations, signal names. This was due to a bug introduced in Temporal Server v1.29.0 which inadvertently allowed an attacker to control the namespace name value instead of using the server's own trusted name value within the batch activity code. The batch activity validated the namespace ID but did not cross-check the namespace name against the

O3 Security · Impact-Aware SCA

Is CVE-2026-5199 in your dependencies?

O3 Security finds CVE-2026-5199 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

Scan my dependencies How O3 SCA works

CVE-2026-5199 — server