CVE-2026-87858: Temporal Server completion callback source header can direct attacker-chosen requests to the internal frontend with administrator authorization
Temporal Technologies, Inc. · Temporal Server
No fix listed · Affected: Temporal Technologies, Inc. Temporal Server · Exploitation: unlikely (under 1%)
Not in CyberSIXT coverage yet: no article names this CVE. This page shows the public record.
Higher than 56% of scored CVEs
DescriptionCVE.org
Temporal Server decided whether a Workflow completion callback was internal by reading a caller-supplied HTTP header. An authenticated caller holding only write permission in a single namespace could attach a completion callback whose URL host matched the configured callback address allowlist, whose URL path was any Temporal HTTP API route, and whose header map contained a non-empty header named source. When the History service delivered that callback, the non-empty source header caused it to re-target the request at the local frontend client and rewrite only the scheme and host, preserving the caller's path, query, and request body. Where an internal frontend is deployed with its HTTP API enabled, that client resolves to the internal frontend, which authorizes every request as a system administrator without requiring authentication information. The result is that the server performs an attacker-chosen state-changing HTTP POST against its own administrative API on the caller's behalf, in namespaces where the caller has no permission. The caller never needs network access to the internal frontend, because the History service makes the request. Confirmed effects include terminating Workflows in other namespaces, registering namespaces, modifying another namespace's configuration, and deleting another namespace and its Workflows. The affected routing logic is present in both the HSM and CHASM callback delivery implementations. This description and the CVSS score in this record describe releases 1.30.0 and later, where any non-empty source header is sufficient. Releases 1.25.0 through 1.29.7 are affected by a narrower form of the same defect in which the header must exactly match a configured cluster ID, a UUID that a namespace-scoped caller cannot read through the API. The consequence once that match occurs is the same, but the attack is materially harder and scores lower.
Affected products2
From the CVE record. A product or version that is not listed is unknown, not unaffected.
- Temporal Technologies, Inc. · Temporal Serverfrom 1.30.0 before 1.30.7; from 1.31.0 before 1.31.3
- Temporal Technologies, Inc. · Temporal Serverfrom 1.25.0 up to and including 1.29.7
On the Atlas
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- Exploitation
- Unlikely (under 1%). EPSS under 1%, or no score yet.
- CISA KEV
- Not listed
- Public check
- No public nuclei template
- Published
- September 2026
CoverageNo articles yet
No CyberSIXT article names CVE-2026-87858 yet, so there is no coverage timeline, action brief or change log for it. Everything on this page comes from the public CVE record, CISA KEV, EPSS and the nuclei template index. Search the archive or see the CVEs in the news.