WikiLambda’s abstractwiki_run_fragment API accepted a caller-supplied Abstract Wikipedia fragment and sent it to the configured evaluator without checking the permission required for unsaved fragments. The related abstractwiki_fetch_section path performed that check, but the run-fragment path did not.
Technical details
An anonymous caller could submit a fragment containing a native implementation. MediaWiki forwarded the fragment to the configured WikiLambda evaluator, crossing the permission boundary intended to restrict execution of unsaved code.
The report demonstrates unauthorized code execution in the evaluator service. It does not establish code execution on the MediaWiki host or a WASM sandbox escape.
Reproduction
In a local WikiLambda setup, point the evaluator client at a loopback capture service and send an anonymous abstractwiki_run_fragment request containing a fragment with a harmless marker. The capture service receives the caller-supplied implementation. The linked PoC is limited to a loopback MediaWiki API and is intended for a local test setup.
Impact
Unauthenticated users could submit unsaved native code to a WikiLambda evaluator. The demonstrated impact is unauthorized evaluator execution; the affected evaluator is WASM-sandboxed.
References
WikiLambda's `abstractwiki_run_fragment` API accepted a caller-supplied Abstract Wikipedia fragment and sent it to the configured evaluator without checking the permission required for unsaved fragments. The related `abstractwiki_fetch_section` path performed that check, but the run-fragment path did not.
## Technical details
An anonymous caller could submit a fragment containing a native implementation. MediaWiki forwarded the fragment to the configured WikiLambda evaluator, crossing the permission boundary intended to restrict execution of unsaved code.
The report demonstrates unauthorized code execution in the evaluator service. It does not establish code execution on the MediaWiki host or a WASM sandbox escape.
## Reproduction
In a local WikiLambda setup, point the evaluator client at a loopback capture service and send an anonymous `abstractwiki_run_fragment` request containing a fragment with a harmless marker. The capture service receives the caller-supplied implementation. The linked PoC is limited to a loopback MediaWiki API and is intended for a local test setup.
## Impact
Unauthenticated users could submit unsaved native code to a WikiLambda evaluator. The demonstrated impact is unauthorized evaluator execution; the affected evaluator is WASM-sandboxed.
## References
- [CVE record](https://www.cve.org/CVERecord?id=CVE-2026-103446)
- [Public report repository](https://github.com/BomboBombone/CVE-2026-103446)
- [Phabricator report](https://phabricator.wikimedia.org/T435086)
- [WikiLambda fix on Gerrit](https://gerrit.wikimedia.org/r/1332761)
- [Research index](/about/)
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