Defending CI/CD Pipelines with Build Application Firewalls
- [01] Attackers target build pipelines to inject malicious code, bypassing traditional static scanners that only inspect source code for known vulnerabilities.
- [02] Affected systems include automated CI/CD environments and build servers that lack granular runtime monitoring or network egress controls.
- [03] Defenders should implement build application firewalls to enforce zero-trust principles and monitor runtime behavior during the software compilation phase.
The Shift Toward Build-Time Protection
Traditional security measures for software development have historically focused on static analysis and software composition analysis. However, as the complexity of the Supply Chain Attack matures, defenders are realizing that scanning code before it is built is no longer sufficient. According to SecurityWeek, a new category of security tool known as the Build Application Firewall (BAF) is emerging to inspect runtime behavior inside the software build pipeline.
In high-profile incidents like the SolarWinds breach, attackers did not necessarily change the source code in the repository. Instead, they compromised the build environment itself, injecting malicious payloads during the compilation phase. This method renders static scanners ineffective because the ‘poison’ is introduced after the scan completes but before the final artifact is signed and distributed.
Monitoring Runtime Behavior in Build Pipelines
The fundamental premise of a BAF is to treat the build process as a dynamic execution environment that requires active monitoring. While a standard firewall might filter traffic at the edge of a network, a BAF operates within the ephemeral environment of a build runner. Security teams often ask how to prevent CI/CD supply chain attacks when existing scanners miss ephemeral malicious processes. The answer lies in enforcing strict policies on what a build process is allowed to do.
A BAF can identify unauthorized network connections, such as a compiler attempting to reach an external C2 server to download a second-stage payload. By establishing a baseline of expected behavior for a specific build job, the BAF can block any deviation, such as the execution of unexpected shell scripts or attempts to access sensitive environment variables that contain secrets.
Build Application Firewall vs Web Application Firewall
It is helpful to contrast these tools with more familiar technologies. Understanding build application firewall vs web application firewall differences is essential for architecting a modern Zero Trust environment. A Web Application Firewall (WAF) sits in front of a production application to filter incoming web traffic for attacks like XSS or SQL injection. Conversely, a BAF sits ‘behind the scenes’ in the development infrastructure, protecting the ‘factory’ that produces the software rather than the finished product itself.
Addressing Gaps in the MITRE ATT&CK Framework
Modern TTP sets used by APT groups frequently involve the exploitation of build-time resources. The MITRE ATT&CK framework highlights several techniques related to internal staging and automated exfiltration that occur within dev environments. Because build servers are often granted high privileges and broad network access to fetch dependencies, they are prime targets for Lateral Movement.
By monitoring runtime behavior in build pipelines, organizations can implement a finer level of control than what is possible with simple network segmentation. A BAF can ensure that a build process only communicates with known, trusted dependency repositories and prevents the exfiltration of credentials to unverified domains.
Defensive Recommendations for CI/CD Security
To effectively secure the software delivery pipeline, security professionals should prioritize the following actions:
- Implement Runtime Monitoring: Deploy tools that provide visibility into the processes, file system changes, and network activity occurring during every build execution.
- Enforce Egress Filtering: Restrict build runners so they can only communicate with a pre-approved list of internal and external endpoints.
- Adopt Zero Trust for Pipelines: Treat every build job as a potential source of compromise. Use short-lived, least-privileged credentials for all automated tasks.
- Integrate BAF into Existing Workflows: Ensure that the BAF provides telemetry that can be ingested by a SIEM or EDR to provide the SOC with visibility into dev-sec-ops health.
As the industry moves toward more resilient software development practices, the adoption of runtime build protection will likely become a standard requirement for organizations seeking to prevent the next major supply chain compromise.
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