Executive brief
Nuclei is a vulnerability scanner used by security teams to automate security testing. When scanning untrusted web targets with the environment variable flag enabled, a malicious server can trick Nuclei into exposing sensitive host environment variables (such as API keys, credentials, and tokens) through response data that gets reinterpreted as template syntax. This exposure only occurs when users explicitly enable the environment variable feature, which is off by default.
Technical details
The vulnerability exists in Nuclei's DAST/fuzz payload evaluation pipeline (pkg/fuzz/parts.go) where response-derived data captured by internal extractors is evaluated multiple times instead of once. When the `-env-vars` / `-ev` flag is enabled, environment variables are merged into the template variable map. A malicious target can return response data containing expressions like `{{env_var_name}}` which, when reused in a subsequent fuzz step as substituted content, are re-evaluated and resolve to actual environment variable values. The root cause is that the previous CVE-2026-41645 fix enforced single-pass evaluation within one call but did not address callers that invoke evaluation multiple times on substituted output. The vulnerability requires multi-step templates chaining internal extractors into fuzz steps and the explicit `-ev` flag. The fix in Nuclei v3.10.0 ensures template text is rendered once through a shared rendering boundary.
Affected products
- Project Discovery Nuclei before 3.10.0
Timeline
- 2026-09-22: disclosed
- 2026-09-22: patched: Fixed in Nuclei v3.10.0
- 2025-12-30: other: Original CVE-2026-41645 fix attempt