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The Unreasonable Effectiveness of Vex in NixOS

blog.kammel.dev48 points1 comments
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Explains how machine-readable VEX (Vulnerability Exploitability eXchange) statements can be used to scale vulnerability triage by encoding whether a given system is actually exposed to a reported CVE. It contrasts naive SBOM-based scanning (syft + grype) that produces many matches - including Go module alerts like GO-2026-5970/6629 - against language-aware reachability analysis (govulncheck) that can show a program does not call vulnerable symbols. It then shows the harder case of whole operating-system images: a Debian-based container yields hundreds of vulnerability hits, and manual triage of a single CVE (CVE-2007-2768, an sshd/PAM opie timing/info leak) requires checking sshd settings, keyboard-interactive auth, and whether an opie PAM module is present - work that is tedious to repeat on every change.

Demonstrates that NixOS makes this repeatable and automatable by codifying system configuration and package composition so conditional VEX statements can be generated from the Nix build. Using sbomnix to produce an SBOM, a Nix expression can evaluate conditions (absence of opie package, KbdInteractiveAuthentication == false, no PAM opie rules) and emit a vex.json with status "not_affected" and justification "vulnerable_code_not_present." Integrating that VEX into grype reduces triaged matches (e.g., drops the specific CVE), and the author has open-sourced nixos-vex while noting it should not be treated as a high-trust data source yet.

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