Blog: Cortex Security Audit Results

Tue, 07 Jul 2026 00:00:00 +0000

Introduction

The Cortex maintainer team is publishing the results of a third-party security audit of the Cortex codebase, completed on April 20, 2026.

Cortex is a horizontally scalable, multi-tenant, long-term storage backend for Prometheus. Multi-tenancy is enforced at the application layer via the X-Scope-OrgID header — every read and write path uses this tenant identifier to scope access to time-series data, rules, and alertmanager configurations. The integrity of this boundary is a critical security property.

Given the project’s role as the storage layer for multiple managed Prometheus offerings and large-scale self-hosted deployments, the maintainers requested a dedicated security engagement through the CNCF.

Engagement Details

Auditor: Quarkslab
Coordinator: OSTIF
Assessed commit: b4f5cfc37d83719040de7ca997ec125304a6b766
Methodology: Whitebox code review, static analysis, dynamic testing

Scope

The auditors operated from a threat model agreed upon with the maintainers prior to the engagement. The focus areas were:

Tenant boundary integrity — Can a tenant read, write, or delete another tenant’s series, rules, or alertmanager state? Are there code paths where X-Scope-OrgID propagation is missing or bypassable?
Cluster operations — Can an internal component (ingester, compactor, store-gateway) be induced into serving cross-tenant data? Are gossip/memberlist communications authenticated and integrity-checked?
Ingestion path — Remote write, PushStream gRPC, distributor validation, ingester replication.
Query path — Query frontend, querier, store-gateway block loading and filtering.
Configuration and secrets exposure — Runtime config endpoints, debug endpoints.

Findings

7 vulnerabilities identified — 6 Medium, 1 Low. No Critical or High severity issues.

ID	Title	Risk	Impact	Attack Vector
V01	Tenant impersonation via PushStream gRPC	Medium	High (cross-tenant write)	Authenticated tenant sending crafted gRPC stream with overridden org ID
V02	Stored XSS	Medium	Marginal	Malicious metric label values rendered in UI contexts
V03	Sensitive information leakage	Medium	Marginal	`/config` endpoint exposing storage credentials in plaintext
V04	Unbound Gzip decompression	Medium	Marginal (DoS)	Crafted compressed payload causing excessive memory allocation on distributor
V05	Uncontrolled memory allocation via protobuf histogram	Medium	Marginal (DoS)	Malformed histogram sample with extreme bucket count
V06	Unbounded read on gossip connections	Medium	Marginal (DoS)	Oversized gossip packet causing unbounded memory read in memberlist
V07	Gossip packet integrity check not enforced	Low	Negligible	Unauthenticated gossip messages accepted without HMAC validation

Fix Status

All 7 findings have verified fixes merged to master and shipped in Cortex v1.21.1 (released 2026-06-04).

Finding	Fix	PR
V01 — Tenant impersonation via PushStream gRPC	Reject `PushStream` requests where per-message `TenantID` diverges from authenticated caller; add HMAC-SHA256 stream auth via `-distributor.sign-write-requests-keys`	#7475
V02 — Stored XSS	Replace `text/template` with `html/template` in Alertmanager and Store Gateway status pages	#7512
V03 — Sensitive information leakage	Mask Swift, etcd, Redis, and HTTP basic-auth credentials on `/config` endpoint	#7473
V04 — Unbound Gzip decompression	Cap decompressed body via `-distributor.otlp-max-recv-msg-size` in `ParseProtoReader` and OTLP path	#7515
V05 — Uncontrolled memory allocation via protobuf histogram	Add `WrappedHistogram` with configurable size limit (`-validation.max-native-histogram-size-bytes`, default 16 KB)	#7570
V06 — Unbounded read on gossip connections	Add `-memberlist.packet-read-timeout`, `-memberlist.max-packet-size`, `-memberlist.max-concurrent-connections`	#7518
V07 — Gossip packet integrity check not enforced	Drop incoming TCP transport packets when digest verification fails	#7474

Upgrade to v1.21.1 or later to include all security fixes.

Auditor Assessment

From the report:

“Cortex benefits from a solid engineering foundation. The project shows good code quality, extensive test coverage, and strong observability features, all of which contribute to its maintainability and resilience.”

The report also recommended:

Tightening default configurations (e.g., enabling gossip encryption by default)
Centralizing input validation at ingestion boundaries rather than relying on per-component checks
Establishing a dependency update cadence for indirect transitive dependencies

Operator Action Required

If you operate a multi-tenant Cortex cluster:

Update to the v1.21.1 or later release. All fixes are included.
Review your /config endpoint exposure. If you expose Cortex’s HTTP API without authentication, V03 may have been exploitable in your environment.
Consider enabling memberlist encryption (-memberlist.encryption-enabled=true) if your gossip traffic traverses untrusted networks. V07 makes this particularly relevant.
Audit PushStream gRPC access. If you expose the distributor gRPC port to tenants directly (not behind an auth gateway), V01 was exploitable prior to the fix.

Resources

Acknowledgments

Quarkslab
OSTIF
CNCF
Cortex maintainers and community

Cortex – security