本期作者
张智
哔哩哔哩高级安全工程师
1. 前言
1.1 背景
随着云原生技术的普及，其暴露出来的攻击面也被黑客们念念不忘，相关的攻击技术也跟着被“普及”，自动化漏洞利用攻击工具更是如雨后春笋般出现在GitHub开源平台，其中比较有代表性的如cdk-team/CDK。其是一款为容器环境定制的渗透测试工具，在已攻陷的容器内部提供零依赖的常用命令及PoC/EXP。集成Docker/K8s场景特有的 逃逸、横向移动、持久化利用方式，插件化管理。
在漏洞利用门槛如此低廉的今天，作为企业安全的建设者（搬砖人），除了考虑部署容器层面运行时检测平台，在k8s api-server层面，启用日志审计功能，也是一个成本低廉又高效发现入侵攻击的途径。
通过对api-server的日志进行审计分析，对于攻击者的信息收集行为，部署k8s cronjob后门、利用rbac做权限提升等持久化攻击行为都能及时的发现并输出告警。
1.2 kubernetes日志审计介绍
Kubernetes 审计功能提供了与安全相关的按时间顺序排列的记录集，记录单个用户、管理员或系统其他组件影响系统的活动顺序。 它能帮助集群管理员处理以下问题：
- 发生了什么？- 什么时候发生的？- 谁触发的？- 为什么发生?- 在哪观察到的？- 它从哪触发的？- 它将产生什么后果？
审计日志示例（图片来自参考[5]
如何开启日志审计
1、api-server命令行启动时，添加如下参数
--audit-policy-file=/etc/kubernetes/audit/audit-default-policy.yaml # 审计策略文件--audit-log-path=/data/log/audit/audit.log # kube-apiserver 输出的审计日志文件，此处以日志文件落地的方式做日志收集--audit-log-maxbackup=10 # kube-apiserver 审计日志文件的最大备份数量--audit-log-format=json #日志保存格式--audit-log-maxage=10 #日志最大保留时间--audit-log-maxsize=500 # 单个日志文件最大为500M
2、kubeadm启动时
修改api-server配置文件/etc/kubernetes/manifests/kube-apiserver.yaml，增加如下内容
--audit-policy-file=/etc/kubernetes/audit/audit-default-policy.yaml--audit-log-path=/data/log/audit/audit.log--audit-log-maxbackup=10--audit-log-format=json--audit-log-maxage=10--audit-log-maxsize=500
2. 攻击行为检测
2.1 分析流程
若攻击者通过各种手段拿到一个容器shell环境后，下一步必然是利用这个容器进行信息收集，获取更多敏感信息或机器资源，此时，CDK作为一款自动化的攻击工具，就让整个攻击过程如虎添翼，大大提高攻击效率。
作为甲方安全的守护者，从安全建设的角度，如何有效及时的发现攻击者的入侵行为，是一个无法避开的问题。本文通过观察cdk的攻击行为，从k8s日志审计的角度罗列一些入侵检测的常见规则。
目前，针对于日志的审计分析，我们落地方案的整个流程为：
注：下列场景，仅限于原生的CDK行为检测，如果攻击者具备开发能力，修改对应特征，可以绕过对应的检测规则。
2.2 信息收集
CDK在容器内主要收集以下信息：
工具在镜像内运行，其信息收集的结果执如下：
从图上可知，信息收集的整个过程，存在与api-server交互的仅有网络探测部分，总共会向api-server发送三条请求，分别为API-server、namespace、api信息探测。
2.2.1 探测API-server
audit.log
{"kind": "Event","apiVersion": "audit.k8s.io/v1","level": "Metadata","stage": "ResponseComplete","requestURI": "/","verb": "get","user": {"username": "system:anonymous","groups": ["system:unauthenticated"]},"sourceIPs": ["172.18.0.2"],"userAgent": "Go-http-client/1.1","responseStatus": {"metadata": {},"status": "Failure","reason": "Forbidden","code": 403},"requestReceivedTimestamp": "2023-02-02T08:29:12.189459Z","stageTimestamp": "2023-02-02T08:29:12.189553Z","annotations": {"authorization.k8s.io/decision": "forbid","authorization.k8s.io/reason": ""}}
2.2.2 列举namespace
audit.log
{"kind": "Event","apiVersion": "audit.k8s.io/v1","level": "Metadata","auditID": "41770e7a-1827-4a14-860f-a812d3db1647","stage": "ResponseComplete","requestURI": "/api/v1/namespaces","verb": "list","user": {"username": "system:serviceaccount:test:default","uid": "63b8dd88-88dd-4426-bdd1-7966906dc0d5","groups": ["system:serviceaccounts","system:serviceaccounts:test","system:authenticated"]},"sourceIPs": ["172.18.0.2"],"userAgent": "Go-http-client/1.1","objectRef": {"resource": "namespaces","apiVersion": "v1"},"responseStatus": {"metadata": {},"status": "Failure","reason": "Forbidden","code": 403},"requestReceivedTimestamp": "2023-02-02T08:29:12.205422Z","stageTimestamp": "2023-02-02T08:29:12.205485Z","annotations": {"authorization.k8s.io/decision": "forbid","authorization.k8s.io/reason": ""}}
2.2.3 探测可访问的api
跟namespace类似，只不过requestURI为/apis
2.2.4 总结
综上， 上述日志中可概述为，当cdk在做信息收集时，会向api-server请求3次，分别访问/，/apis，/api/v1/namespaces，可根据这些特征做告警规则
1、userAgent: Go-http-client/1.1 # CDK特定的userAgent，此时，该字段为主要特征2、responseStatus.code: 403 #默认serviceaccount无权限时 api-server返回的状态码3、requestURI: / # 访问根目录
2.3 漏洞利用
Exploit模块包含的功能：
2.3.1 容器逃逸
其中，容器逃逸层面，一般是不需要与api-server做交互的，也就不会留下日志。但部分容器逃逸手段是利用错误配置的pod在创建时实施攻击。因此，容器逃逸的检测必须前置到容器创建时，记录下应用请求的权限，然后结合运行时入侵检测做进一步监控。
当一个拥有privilege、sys_admin、network、ipc等特殊权限的pod创建时，它的日志记录是这样的。
audit.json
{"kind": "Event","apiVersion": "audit.k8s.io/v1","level": "RequestResponse","stage": "ResponseComplete","requestURI": "/api/v1/namespaces/testpods/pods","verb": "create","user": {"username": "kubernetes-admin","groups": ["system:masters","system:authenticated"]},"sourceIPs": ["172.18.0.1"],"userAgent": "kubectl1.16.15/v1.16.15 (darwin/amd64) kubernetes/2adc8d7","objectRef": {"resource": "pods","namespace": "testpods","name": "testpod","apiVersion": "v1"},"responseStatus": {"metadata": {},"code": 201},"responseObject": {"kind": "Pod","apiVersion": "v1","metadata": {"name": "testpod","namespace": "testpods","selfLink": "/api/v1/namespaces/testpods/pods/testpod","uid": "e717d204-7e6d-4608-998b-648a8667e8e1","resourceVersion": "13517","creationTimestamp": "2023-02-02T09:51:10Z","labels": {"creator": "zhiye","team": "teamf"}},"spec": {"volumes": [{"name": "rootfs","hostPath": {"path": "/","type": ""}}],"containers": [{"name": "trpe","image": "alpine","command": ["/bin/sh","-c","tail -f /dev/null"],"resources": {},"volumeMounts": [{"name": "default-token-mm6s8","readOnly": true,"mountPath": "/var/run/secrets/kubernetes.io/serviceaccount"}],"terminationMessagePath": "/dev/termination-log","terminationMessagePolicy": "File","imagePullPolicy": "Always","securityContext": {"capabilities": {"add": ["SYS_ADMIN"]},"privileged": true}}],"restartPolicy": "Always","terminationGracePeriodSeconds": 30,"dnsPolicy": "ClusterFirst","serviceAccountName": "default","serviceAccount": "default","hostNetwork": true,"hostPID": true,"hostIPC": true,"securityContext": {},"schedulerName": "default-scheduler","tolerations": [{"key": "node.kubernetes.io/not-ready","operator": "Exists","effect": "NoExecute","tolerationSeconds": 300},{"key": "node.kubernetes.io/unreachable","operator": "Exists","effect": "NoExecute","tolerationSeconds": 300}],"priority": 0,"enableServiceLinks": true},"status": {"phase": "Pending","qosClass": "BestEffort"}},"requestReceivedTimestamp": "2023-02-02T09:51:10.632436Z","stageTimestamp": "2023-02-02T09:51:10.660958Z","annotations": {"authorization.k8s.io/decision": "allow","authorization.k8s.io/reason": ""}}
从日志上可以看出，针对于错误配置导致的逃逸，我们可以关注以下几个日志字段,制定告警规则。
responseObject.spec.volumes # 检测敏感卷，是否挂载docker.sock等responseObject.spec.containers.volumeMounts # 检测敏感挂载，是否挂载docker.sock等responseObject.spec.containers.securityContext.capabilities.add # 是否使用SYS_ADMIN权限，（字段嵌套这么多层，真的得吐槽responseObject.spec.containers.securityContext.privileged # 检测是否为特权pod容器responseObject.spec.hostNetwork # 是否使用宿主机网络responseObject.spec.hostPID # 是否使用宿主机hostPIDresponseObject.spec.hostIPC # 是否共享宿主机内存responseObject.spec.serviceAccount # 是否使用特殊的serviceaccount 默认为default
2.3.2 网络探测
此功能为端口扫描+指纹识别，不涉及与API交互,未产生日志。
2.3.3 信息窃取
与api-server交互的为secrets、config和psp，如果是自动获取的话，cdk会发送两次请求，分别使用匿名账户和当前serviceaccout做list动作。
下边列举些主要特征
requestURI: /api/v1/secrets，requestURI: /api/v1/configmapsrequestURI: /apis/policy/v1beta1/podsecuritypoliciesuserAgent: Go-http-client/1.1user.username: "system:anonymous"responseStatus.code: 403"verb": "list"
2.3.4 权限提升
RBAC权限绕过
日志如下
{"kind": "Event","apiVersion": "audit.k8s.io/v1","level": "RequestResponse","auditID": "bfc643d6-8337-434e-9dec-ba41dd36bfa7","stage": "ResponseComplete","requestURI": "/api/v1/namespaces/kube-system/pods","verb": "create","user": {"username": "system:serviceaccount:test:default","uid": "63b8dd88-88dd-4426-bdd1-7966906dc0d5","groups": ["system:serviceaccounts","system:serviceaccounts:test","system:authenticated"]},"sourceIPs": ["172.18.0.3"],"userAgent": "Go-http-client/1.1","objectRef": {"resource": "pods","namespace": "kube-system","apiVersion": "v1"},"responseStatus": {"metadata": {},"status": "Failure","reason": "Forbidden","code": 403},"responseObject": {"kind": "Status","apiVersion": "v1","metadata": {},"status": "Failure","message": "pods is forbidden: User \"system:serviceaccount:test:default\" cannot create resource \"pods\" in API group \"\" in the namespace \"kube-system\"","reason": "Forbidden","details": {"kind": "pods"},"code": 403},"requestReceivedTimestamp": "2023-02-03T09:56:24.061825Z","stageTimestamp": "2023-02-03T09:56:24.061888Z","annotations": {"authorization.k8s.io/decision": "forbid","authorization.k8s.io/reason": ""}}
由上可发现如下特征：
Pod内serviceaccount无权限的情况requestURI: /api/v1/namespaces/kube-system/podsuserAgent: Go-http-client/1.1responseStatus.code: 403Pod内serviceaccount有权限的情况requestURI: /api/v1/namespaces/kube-system/podsresponseObject.metadata.selfLink: /api/v1/namespaces/kube-system/pods/cdk-rbac-bypass-create-podresponseObject.metadata.spec.containers.args: *cat /run/secrets/kubernetes.io/serviceaccount/token*verb: create
2.3.5 持久化
部署daemonset后门
源码是这样定义的（参考[6]）：
体现到日志中，有以下几个点：
objectRef.name: cdk-backdoor-daemonsetobjectRef.namespace: kube-systemresponseObject.metadata.selfLink: /apis/apps/v1/namespaces/kube-system/daemonsets/cdk-backdoor-daemonsetresponseObject.spec.template.spec.volumes.hostPath.path: /responseObject.spec.template.spec.containers.name: cdk-backdoor-podresponseObject.spec.template.spec.containers.securityContext[capabilities:ptivileged]:如图上所示responseObject.spec.template.spec.[hostNetwork|hostPID]: true
部署K8S CronJob
CDK源代码是这样定义的（参考[7]）：
体现在日志中，特征有以下几点：
requestURI: /apis/batch/v1beta1/namespaces/kube-system/cronjobsverb: createobjectRef.name: cdk-backdoor-cronjobresponseObject.matadata.name: cdk-backdoor-cronjobresponseObject.matadata.selfLink: /apis/batch/v1beta1/namespaces/kube-system/cronjobs/cdk-backdoor-cronjobresponseObject.spec.jobTemplate.spec.template.spec.containers.name: cdk-backdoor-cronjob-container
部署影子k8s api-server
在pod权限足够的情况下，通过创建shadow api-server做权限维持，详情见参考[4]
在非二开的情况下，通过k8s日志升级可检测以下几个字段
objectRef.name: *-shadow-*responseObject.metadata.labels.component: kube-apiservershadowresponseObject.spec.containers.command: "--secure-port=9444"
2.3.6 总结
因为漏洞利用部分，动作都普遍较大，因此可观测字段已不仅仅局限于userAgent,其特征均比较明显，极富有工具本身特色
2.4 API利用
2.4.1 Tool模块
此处需要关注的（跟api-server有交互的）为kcurl命令，此命令可借助高权限serviceaccount账户列举/使用k8s资源。
执行./cdk kcurl default get 'https://10.96.0.1:443/api/v1/nodes' ，日志内容如下：
audit.log
{"kind": "Event","apiVersion": "audit.k8s.io/v1","level": "Metadata","auditID": "418cafa5-2c1e-4fbf-b086-3d68e321d2bb","stage": "ResponseComplete","requestURI": "/api/v1/nodes","verb": "list","user": {"username": "system:serviceaccount:test:default","uid": "63b8dd88-88dd-4426-bdd1-7966906dc0d5","groups": ["system:serviceaccounts","system:serviceaccounts:test","system:authenticated"]},"sourceIPs": ["172.18.0.3"],"userAgent": "Go-http-client/1.1","objectRef": {"resource": "nodes","apiVersion": "v1"},"responseStatus": {"metadata": {},"code": 200},"requestReceivedTimestamp": "2023-02-06T09:51:51.790260Z","stageTimestamp": "2023-02-06T09:51:51.791075Z","annotations": {"authorization.k8s.io/decision": "allow","authorization.k8s.io/reason": "RBAC: allowed by ClusterRoleBinding \"defaultadmin\" of ClusterRole \"cluster-admin\" to ServiceAccount \"default/test\""}}
日志中annotations.authorization.k8s.io/reason给出了允许执行的原因。我们可以根据如下三个字段制定告警规则：
serviceaccount有权限的情况下：annotations.authorization.k8s.io/reasonannotations.authorization.k8s.io/decisionuserAgent: Go-http-client/1.1responseStatus.code：200无权限的情况下：userAgent: Go-http-client/1.1responseStatus.code：403
2.4.2 总结
因为tool模块需要借助高权限serviceaccount或token，因此，权限不足的情况下，api-server返回responseStatus.code: 403 的条目说明了api-server接受到了非预期请求，结合userAgent等信息便可输出可疑告警。
3. 不仅仅是CDK
3.1 CVE-2022-3172 K8S聚合
API（Aggregation API）SSRF漏洞.
聚合 API 实际上是在 kube-apiserver 中运行的，在新 API 注册之前，它并不会工作。如果要添加新的 API，则需要创建一个APIService 对象，用来申请 Kubernetes 中新的 URL 路径。注册成功后，当有发送到此路径中的请求，则会被转发到已经注册的 APIService 上。
APIService 可以将客户端的请求转发到任意的 URL 上，这就有可能会导致 Client 发送请求时，所携带的一些认证信息可能会被发送给第三方。
通过日志审计监控responseStatus.code字段来进行判断是否有出现重定向的情况，通过检测如下字段：
responseObject.code:302responseObject.code:301
3.2 使用不合规镜像创建pod
除了cdk这种常见的攻击手法，还有一些常见的异常行为需要我们关注，比如一般企业内，pod创建使用的容器均会从企业私有的镜像仓库中拉取，此时如果日志中出现了公共仓库的镜像，则可判断为异常，可关注以下字段
verb : createlevel:RequestResponseesponseObject.kind:PodrequestObject.spec.containers.image:镜像仓库地址
3.3 pod命令执行
对于kubectl exec命令进行监控，可对如下字段进行监控，注：命令执行的后续无法通过日志审计来进一步监控，需结合运行时检测进一步分析
objectRef.subresource:execobjectRef.subresource:attachuserAgent
4. 落地实践踩过的坑
1、主要为k8s日志类型比较多，且每种类型的字段名，字段数量均不一致，导致es在存储数据时存在索引内字段类型不一致无法解析存储的问题。
解决方案为对于字段不一致的obj，选择为不做深层次解析。（或者使用hdfs等存储方式，查询时对字段进行解析
2、日志量过大，导致api-server磁盘读写io过高
持续优化audit.yaml中的日志规则，对于其中的node/status,pod/status,coordination.k8s.io/leases等不做日志记录。
5. 总结
本文从k8s日志审计的角度，分析当使用cdk等自动化攻击时，能够从日志中获取到的信息，并给出通过这些信息可监控字段的告警示例。因为这部分字段都比较固定，完全可以通过机器学习提升告警准确率。抛砖引玉，希望后续可以看到更多日志的分析防御角度。
当然，CDK作为开源工具，这些特征都可以做关键字替换。因此，笔者认为功夫应该用到平时，加强k8s的基线管控，比如避免出现高serviceaccount权限、通过准入策略限制使用的docker镜像，并部署容器运行时入侵检测平台。让安全能力覆盖每个环节，才能保证集群的安全稳定。
6. 参考
[1] https://github.com/cdk-team/CDK
[2] https://www.cdxy.me/?p=839
[3] https://kubernetes.io/docs/tasks/debug/debug-cluster/audit/
[4] https://discuss.kubernetes.io/t/security-advisory-cve-2022-3172-aggregated-api-server-can-cause-clients-to-be-redirected-ssrf/21322
[5]https://github.com/tencentyun/qcloud-documents/blob/master/product/%E5%AD%98%E5%82%A8%E4%B8%8ECDN/%E6%97%A5%E5%BF%97%E6%9C%8D%E5%8A%A1/%E6%9C%80%E4%BD%B3%E5%AE%9E%E8%B7%B5/TKE%20%E5%AE%A1%E8%AE%A1%E6%97%A5%E5%BF%97%E5%88%86%E6%9E%90.md
[6] https://github.com/cdk-team/CDK/blob/main/pkg/exploit/k8s_backdoor_daemonset.go#LL35-L87C2
[7] https://github.com/cdk-team/CDK/blob/main/pkg/exploit/k8s_cronjob.go#LL34-L59C2
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