CVE-2026-53507 in oasdiff-action
Summary
by MITRE • 08/31/2026
oasdiff-action is a GitHub Action that detects breaking changes in OpenAPI specs and post a review on every pull request. Before version 0.0.51, the oasdiff actions resolved external $refs in the OpenAPI spec by default (allow-external-refs: true). When an action runs on a pull request whose spec is attacker-controlled — most importantly fork pull requests on public repositories — a $ref in that spec is fetched/read on the runner with no interaction required, enabling SSRF and disclosure of structured files on the runner. This issue has been patched in version 0.0.51.
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Analysis
by VulDB Data Team • 08/31/2026
The vulnerability identified within oasdiff-action prior to version 0.0.51 represents a critical server-side request forgery (SSRF) flaw rooted in the default configuration of external reference resolution. GitHub Actions are automated workflows that execute code on hosted runners, which often have access to internal network resources and sensitive file systems. The oasdiff-action is designed to analyze OpenAPI specifications for breaking changes by comparing different versions of a specification document. In its pre-patch state, the action defaulted to resolving external references, indicated by the allow-external-refs parameter being set to true. This design choice assumes that all referenced resources are trusted and safe to fetch from arbitrary URLs provided in the OpenAPI definition files.
The technical flaw manifests when an attacker controls the content of a pull request targeting a public repository using this action. Specifically, forked repositories on GitHub allow contributors who do not have direct write access to the main branch to submit changes that trigger CI/CD workflows. By crafting an OpenAPI specification file containing malicious external $ref directives pointing to internal IP addresses or local service endpoints, an attacker can force the GitHub Actions runner to initiate HTTP requests to these restricted destinations. Because this process occurs automatically upon the creation of the pull request without requiring any manual approval or interaction from repository maintainers, it constitutes a blind SSRF attack vector. The runner acts as a proxy for the attacker, leveraging its network permissions and potential access to internal metadata services such as those found in cloud environments like AWS EC2 instance metadata service (IMDSv1).
The operational impact of this vulnerability is severe, encompassing both data exfiltration and unauthorized network reconnaissance. By directing requests to local or internal endpoints, an attacker can retrieve sensitive structured files stored on the runner's file system if accessible via localhost services. Furthermore, in cloud-hosted environments, such SSRF can lead to the disclosure of instance metadata, which often contains temporary security credentials, IAM role information, and other configuration details that could be leveraged for further lateral movement or privilege escalation within the organization's infrastructure. This effectively bypasses network segmentation controls intended to isolate CI/CD runners from internal production networks, turning a standard code review tool into an attack vector against the broader enterprise architecture.
This vulnerability aligns with CWE-918, which describes Server-Side Request Forgery (SSRF) flaws where web applications fetch remote resources without validating user-supplied URLs. It also maps to MITRE ATT&CK technique T1498.002, Network Denial of Service via Reflection Amplification, although in this context the primary concern is data disclosure rather than denial of service, fitting more closely with reconnaissance patterns such as T1615, Group Policy Discovery, or general network scanning techniques enabled by SSRF capabilities. The lack of input validation on external references allows attackers to probe internal networks and extract sensitive information that should remain inaccessible from untrusted sources.
To mitigate this risk, organizations must immediately upgrade oasdiff-action to version 0.0.51 or later, where the default behavior has been corrected to prevent unauthorized external reference resolution. For environments running older versions, it is imperative to explicitly configure the action with allow-external-refs set to false unless there is a verified business need for such functionality and strict whitelisting of allowed domains can be enforced. Additionally, security teams should audit their GitHub Actions workflows to ensure that no other actions or scripts are processing untrusted OpenAPI specifications without proper sanitization of external references. Implementing network-level controls on runners, such as restricting outbound traffic from CI/CD environments to only known and necessary endpoints, provides an additional layer of defense against SSRF exploitation attempts.