AWS CodeBuild Flaw Exposed Critical Supply Chain Risk Through Unanchored Regex

A critical misconfiguration in AWS's CodeBuild service exposed a fundamental vulnerability in how cloud providers secure their software supply chains. Security firm Wiz discovered that missing characters in webhook filters could have enabled complete takeover of AWS's own GitHub repositories, potentially affecting every AWS environment worldwide.

The Vulnerability: CodeBreach

The flaw, which Wiz researchers dubbed "CodeBreach," existed in CodeBuild, AWS's managed continuous integration service that commonly connects to GitHub repositories. The root cause was surprisingly simple: two missing characters in the webhook filters that are supposed to defend against untrusted pull requests.

Webhook filters act as gatekeepers, determining which events should trigger automated builds. AWS used an ACTOR_ID filter - a regular expression pattern designed to allow only trusted GitHub user IDs to trigger builds. However, the regex was unanchored, meaning it lacked the start (^) and end ($) characters that enforce exact matching.

Without these anchors, the regex engine doesn't search for a string that perfectly matches the pattern, but merely one that contains it. This meant any GitHub user ID containing an approved maintainer's ID as a substring could bypass the filter entirely.

Exploitation: Manufacturing Trust

The Wiz team demonstrated how attackers could exploit this blind spot. They registered new GitHub user IDs containing existing maintainer IDs by leveraging GitHub Apps' automated app creation feature. By sending 200 automated requests, they successfully captured a user ID that bypassed the ACTOR_ID filter for the AWS SDK for JavaScript repository.

With this foothold, they submitted a seemingly routine pull request containing a malicious NPM package dependency. This payload executed in the build environment and extracted GitHub credentials, which they then used to escalate privileges and create a repository administrator account. This account could push code to the main branch, approve pull requests, and exfiltrate repository secrets.

Scope and Impact

The potential attack surface was enormous. The compromised library is used in the AWS Console itself - what Wiz calls "the central nervous system of the cloud." According to Wiz, 66 percent of cloud environments include the JavaScript SDK, making this a supply chain attack that could have dwarfed SolarWinds in scale.

An attacker could have injected malicious code into the SDK just before its weekly release, infecting all downstream users. The researchers also gained admin privileges to several other repositories, including what appeared to be AWS's private mirrors.

AWS Response

AWS responded quickly after Wiz's disclosure in August, fixing the core issue within 48 hours and implementing additional protections in September. The company published a security bulletin and conducted extensive audits of all public build environments, reviewing logs to confirm no other actors had exploited the vulnerability.

In a statement, AWS maintained that the configuration flaw had "no impact" on any customer environment or AWS service. The company implemented additional remediations including protections for all build processes containing GitHub tokens or other credentials in memory.

Industry-Wide Problem

The vulnerability exposes a systemic issue in CI/CD security. As Wiz researcher Yuval Avrahami noted, "This vulnerability exploits a blind spot in CI/CD security, not a flaw unique to AWS."

The same risk exists across GitHub Actions, Jenkins, and other cloud CI services. Every major cloud provider and tech company faces this exact challenge in their open-source supply chains. The fundamental problem is granting excessive privileges to external contributors through automated CI/CD builds.

Why This Matters

This attack vector requires surprisingly low technical expertise. It uses standard developer workflows - forking repositories and submitting pull requests - rather than sophisticated exploits. Any intermediate developer could execute it, making it attractive to both nation-state actors and cybercriminal syndicates.

The real challenge is stealth: crafting payloads that look benign during code review. Once in control of repositories, attackers could inject backdoors to harvest credentials from millions of applications, exfiltrate sensitive data, or manipulate cloud infrastructure undetected.

Broader Implications

This incident highlights how modern software development's reliance on automated CI/CD pipelines creates new attack vectors. The same automation that enables rapid development also concentrates risk: a single misconfigured webhook can expose the entire software supply chain.

Organizations using CI/CD services should audit their webhook filters for proper anchoring and review the privileges granted to automated build processes. The principle of least privilege applies not just to runtime environments but to the build infrastructure itself.

The Wiz research team's discovery likely prevented a supply chain attack that could have compromised countless cloud environments. As Avrahami noted, this could have given attackers code execution in the very interface administrators use to manage their entire infrastructure - a far more direct path to cloud compromise than traditional network breaches.

For security teams, the lesson is clear: CI/CD configurations require the same rigorous review as application code. The tools that accelerate development can also accelerate attacks when misconfigured.