We hardened zizmor's GitHub Actions static analyzer

We hardened zizmor's GitHub Actions static analyzer - The Trail of Bits BlogPage content

In March 2026, attackers exploited a pull_request_target misconfiguration in the aquasecurity/trivy-action GitHub Action to exfiltrate organization and repository secrets, then used those credentials to backdoor LiteLLM on PyPI (see Trivy’s post-mortem for the full timeline). zizmor is a static analyzer that GitHub Actions users run to catch exactly these misconfigurations before they ship. When GitHub Actions added support for YAML anchors in September 2025, a small but high-value slice of the ecosystem started writing workflows that zizmor could only analyze on a best-effort basis. Over the past three months, Trail of Bits collaborated with the zizmor maintainers to bring zizmor’s anchor support up to full coverage. First, we fixed parsing bugs that caused crashes, produced wrong-location findings, and silently mishandled aliased values. Second, we surfaced deserialization edge cases that broke zizmor on otherwise valid workflows. Finally, we helped align zizmor’s expression evaluator with GitHub’s own Known Answer Tests. We validated all of this against a new corpus of 41,253 workflows from 6,612 high-value open-source repositories. The result: 20 filed issues, 15 merged pull requests. Building the test corpus To understand how anchors are used in CI today and to stress-test zizmor against the full variety of YAML it encounters in the wild, we built a corpus of real workflows. We used BigQuery’s GitHub dataset to identify the 10,000 most-starred repositories created between 2022 and 2025, filtered to the 6,612 that use GitHub Actions, and downloaded every workflow file. That gave us 41,253 YAML files. Figure 1: Building a testing corpus When we ran zizmor against the corpus, it crashed on 45 of the 41,253 workflows. That’s a low rate, but each crash means a bug in zizmor. How anchors are used in the wild zizmor’s anchor support was deliberately limited, and for good reason. YAML anchors make workflows non-local: an alias defined in one place changes behavior elsewhere in the file. This complicated zizmor’s parsing model, and adoption was rare enough that the zizmor maintainers reasonably discouraged anchor use. In our corpus, only 43 of the 41,253 workflows use YAML anchors (roughly 0.1%), but those 43 include some of the most foundational projects in open source: Bitcoin Core PHP OpenSSL However, anchors are a supported feature, and their use will likely grow over time. We found two common patterns. The first is reusing steps across jobs , as Bitcoin Core’s CI does: jobs: runners: steps: - &ANNOTATION_PR_NUMBER name: Annotate with pull request number run: | if [ "${{ github.event_name }}" = "pull_request" ]; then echo "::notice ..." fi

test-each-commit: steps: - *ANNOTATION_PR_NUMBER - uses: actions/checkout@v6Figure 2: Reuse step definitionThe second pattern is pinning action versions once . For instance, Home Assistant’s CI defines the action reference (with its SHA hash) using an anchor, then reuses it wherever the same action appears: jobs: lint: steps: - uses: &actions-setup-python actions/setup-python@a309ff8b42... # later in the same workflow: - uses: *actions-setup-pythonFigure 3: Reuse action definitionFour anchor handling bugs found and fixed When we started, four anchor patterns from these workflows broke zizmor. Aliases in sequences were incorrectly flattened. When a YAML alias appeared inside a sequence (like a list of steps), zizmor’s internal path representation spread the alias contents rather than treating it as a single element. This caused zizmor to crash or produce findings pointing at the wrong location in the file. (Fixed in #1557) Anchor prefixes leaked into values.

foo: [&name v, *x]Figure 4: Anchor prefix leakIn YAML flow sequences, anchor prefixes like &name weren’t stripped from resolved values. Given the snippet in Figure 4, looking up the first element of foo would return &name v instead of v, causing any step that consumed the node value to fail. (Fixed in #1562) Duplicate anchors caused a crash. The YAML spec allows redefining an anchor name (the last definition wins). zizmor’s YAML layer assumed anchor names were unique and panicked on duplicates. (Fixed in #1575) The template-injection audit crashed on aliased run values. When a YAML alias was used as a scalar run: value, the audit didn’t expect the indirection and failed. (Fixed in #1732) To prevent future regressions, we also added integration tests covering anchor patterns found in real workflows (#1682) and updated the anchor documentation (#1788). What else the corpus surfaced Running zizmor against the full test corpus also surfaced bugs that had nothing to do with anchors. Deserialization edge cases. GitHub Actions accepts YAML constructs that zizmor’s workflow model didn’t anticipate: if: 0 (an integer where a string is expected),...

We hardened zizmor's GitHub Actions static analyzer

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