Nuclei: Environment Variable Disclosure via Response-Derived Data in DAST/Fuzz Mode

A vulnerability in Nuclei's DAST/fuzz expression evaluation path allows a malicious target server to trigger disclosure of scanner-host environment variables when the -env-vars / -ev option is explicitly enabled.

This is an incomplete fix for CVE-2026-41645 / GHSA-jm34-66cf-qpvr. The original fix hardened expressions.Evaluate() to be single-pass within one call, but did not address callers that invoked evaluation multiple times on substituted output in the DAST/fuzz pipeline.

Affected Component

The issue is in the DAST/fuzz payload evaluation path (pkg/fuzz/parts.go) and the shared template rendering boundary. When a multi-step template captures response data via an internal extractor and reuses it in a subsequent fuzz step, the fuzz evaluator could treat the substituted response content as fresh template syntax on a second evaluation pass.

Description

In DAST/fuzz mode, payload evaluation previously ran expression substitution more than once on the same value. Response-derived content captured by an internal: true extractor in a prior protocol step could flow into a fuzz payload and be reinterpreted as DSL/helper syntax on a subsequent pass.

When -env-vars (-ev) 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, resolve to actual environment variable values. This can expose sensitive host data such as API keys, credentials, and tokens.

Without -ev enabled (the default), response-derived data may still cause other DSL helpers to run, but that behavior is not treated as a security issue and has no meaningful security impact beyond unexpected behavior.

[!NOTE] The -env-vars / -ev option is off by default. Users who have not explicitly enabled it are not affected by this vulnerability.

Affected Users

• CLI users running nuclei -dast (or fuzzing) with multi-step templates that chain an internal extractor into a subsequent fuzz step against untrusted targets, with the -ev flag enabled.
• SDK users who integrate Nuclei with the fuzz pipeline enabled, EnvironmentVariables set to true, and scan targets that are not fully trusted.