Executive brief
A vulnerability in Nuclei's expression evaluation engine makes it possible for a malicious target server to inject and execute supported DSL expressions. This happens when HTTP response data containing helper/function syntax gets reused by multi-step templates. If the `-env-vars` / `-ev` option is explicitly enabled, this can expose host environment variables. That option is off by default, so standard configurations are not affected by the information disclosure risk.
**Affected Component**
The issue lives in `expressions.Evaluate()` at `pkg/protocols/common/expressions/` and in the unresolved-variable validation path (`hasLiteralsOnly()`).
**Description**
`expressions.Evaluate()` replaces placeholders first, then scans the substituted output for expressions. Because of this two-pass approach, response-derived values (including extractor output and response body content) can be reinterpreted as DSL/helper syntax on the second pass.
When `-env-vars` (`-ev`) is enabled, environment variables get 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 template request, resolve to actual environment variable values. This can expose sensitive host data like API keys, credentials, and tokens.
Without `-ev` enabled (the default), injected DSL expressions may still trigger helper functions such as `{{md5("test")}}`, but this has no meaningful security impact beyond unexpected behavior.
There is also a separate issue in `hasLiteralsOnly()`: it was evaluating helper expressions while deciding whether `{{...}}` contained unresolved variables, which caused validation logic to run side-effectful helpers even when the final request kept the value as a literal.
> [!NOTE] The `-env-vars` / `-ev` option is off by default. Users who have not explicitly turned it on are not affected by the information disclosure aspect of this vulnerability.
**Affected Users**
- **CLI users** running multi-step templates (with extractors or flow-based request chaining) that reuse response-derived values against untrusted or attacker-controlled targets, with the `-ev` flag enabled. - **SDK users** who have integrated Nuclei into platforms where `EnvironmentVariables` is set to `true` and scan targets are not fully trusted.
**Patches**
- The vulnerability is fixed in Nuclei v3.8.0. Upgrading to this version is strongly recommended. - Relevant fix references: #7221, #7321.
**Mitigation**
Upgrade to Nuclei v3.8.0. The updated evaluation logic now collects expressions from the original template text before placeholder substitution and only evaluates those template-authored expressions.
If you have `-ev` enabled, disable it when scanning untrusted targets to avoid environment variable disclosure.
**Workarounds**
If upgrading is not an option right now, make sure `-env-vars` / `-ev` is not enabled when running multi-step templates against untrusted targets.
**Acknowledgments**
Nuclei thanks @gnuletik for reporting this issue through responsible disclosure via security@projectdiscovery.io
Affected products
- go github.com/projectdiscovery/nuclei/v3