CVE-2026-49406
Summary
When Deno was run in BYONM mode (nodeModulesDir: "manual"), the module resolver did not validate that a package's resolved entrypoint stayed within its node_modules/<pkg>/ directory. A malicious package.json whose main field contained .. segments was able to resolve to an arbitrary path on disk, and the resolver then read that file without consulting the --allow-read allowlist. This let a require("evil-pkg") call return the contents of a file that a direct Deno.readTextFileSync(...) call would have been blocked from reading.
Details
In BYONM mode, Deno resolved npm packages directly from a user-managed node_modules tree. Resolution of require("pkg") proceeded by reading pkg/package.json, taking the main field, joining it to the package directory, and loading the result as a module.
The path joined from main was not constrained to the package root. A package.json such as:
{ "main": "../../../secret.json" }
resolved to node_modules/pkg/../../../secret.json, escaping node_modules entirely. The BYONM permission check accepted any path that contained a node_modules component and did not reject .. traversal, so the resolved path was loaded without a read-permission check.
Because resolution loaded JSON entrypoints by parsing their contents and returning them through require, this exposed the contents of arbitrary .json files reachable by the OS user to the requiring code, even when --allow-read had been narrowed to a specific directory.
The same file accessed via Deno.readTextFileSync was correctly blocked. The bug was that module resolution did not enforce the same read-permission boundary that the filesystem APIs enforced.
Proof of concept
The reporter supplied a self-contained PoC. Layout:
/tmp/deno_byonm_poc/
├── app/
│ ├── deno.json (BYONM enabled)
│ ├── exploit.ts (require("evil-pkg"))
│ └── node_modules/
│ └── evil-pkg/
│ └── package.json (main: "../../../secret.json")
└── secret.json (outside --allow-read scope)
Run:
deno run --no-prompt --all
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Exploitation outlook
- Low exploitation risk0.17% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 7% of all EPSS-scored CVEs.
Advisory coverage (1)
External references
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[](https://www.csirts.com/cve/CVE-2026-49406)