oasdiff-action is a GitHub Action that compares OpenAPI specifications and posts review comments on pull requests. Prior to version 0.0.51, the action resolved external $refs in the specification files by default, using the setting allow‑external‑refs: true. When a pull request contains a maliciously crafted spec, the action automatically fetches or reads the referenced URI on the CI runner without user interaction. This behavior allows an attacker to perform server‑side request forgery (SSRF) against internal or private resources and to read structured files on the runner, thereby leaking sensitive data.

The vulnerability affects users who run oasdiff-action version 0.0.50 or earlier in any GitHub Workflow that processes pull‑request events. The affected product is the oasdiff-action GitHub Action for OpenAPI spec diffing. The fix is available starting with version 0.0.51, which disables external ref resolution by default.

The vulnerability has a CVSS score of 8.3, indicating a high severity. EPSS data is not available, so the current exploitation probability cannot be estimated, and the issue is not listed in the CISA KEV catalog. The likely attack scenario involves an attacker forking a public repository, creating a pull request that contains an OpenAPI spec with a malicious $ref, and relying on the default external ref resolution to trigger an SSRF or file disclosure on the CI runner. No additional environmental prerequisites are noted beyond the use of the public action in a workflow that triggers on pull requests.