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Public proof of conceptFix available

CVE-2021-21423

A public proof of concept exists; no exploitation attested.

  • Published2021-04-06
  • Fixed in0.16.41
  • WeaknessCWE-527

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

`projen` is a project generation tool that synthesizes project configuration files such as `package.json`, `tsconfig.json`, `.gitignore`, GitHub Workflows, `eslint`, `jest`, and more, from a well-typed definition written in JavaScript. Users of projen's `NodeProject` project type (including any project type derived from it) include a `.github/workflows/rebuild-bot.yml` workflow that may allow any GitHub user to trigger execution of un-trusted code in the context of the "main" repository (as opposed to that of a fork). In some situations, such untrusted code may potentially be able to commit to the "main" repository. The rebuild-bot workflow is triggered by comments including `@projen rebuild` on pull-request to trigger a re-build of the projen project, and updating the pull request with the updated files. This workflow is triggered by an `issue_comment` event, and thus always executes with a `GITHUB_TOKEN` belonging to the repository into which the pull-request is made (this is in contrast with workflows triggered by `pull_request` events, which always execute with a `GITHUB_TOKEN` belonging to the repository from which the pull-request is made). Repositories that do not have branch protection configured on their default branch (typically `main` or `master`) could possibly allow an untrusted user to gain access to secrets configured on the repository (such as NPM tokens, etc). Branch protection prohibits this escalation, as the managed `GITHUB_TOKEN` would not be able to modify the contents of a protected branch and affected workflows must be defined on the default branch.

Timeline

dates in UTC
  1. 2021-04-0619:15CVE publishedNVD
  2. 2026-10-2319:23First public proof of conceptPoC-in-GitHub

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-21423?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-21423?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD · OSV
Projen project projen
BranchAffectedFixed in
0.x≥ 0.6.0, < 0.16.410.16.41
0.16< 0.16.410.16.41

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

  1. Update Projen project projen to 0.16.41

Advisories and patches

Detection

read 2026-10-23 19:23 UTC
Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
1 repository, unverifiedpvharmo2/gha-lab-b9842b12c0
PoC-in-GitHub

Sources

  • CNACVSS 3.16.8read 2026-10-23
  • NVDCVSS 3.18.1read 2026-10-23
  • NVDCVSS 2.05.5read 2026-10-23
  • CNAweakness (CWE)CWE-527read 2026-10-23
  • FIRST EPSSEPSS score1.4 %2026-10-23
  • PoC-in-GitHubproof of concept1read 2026-10-23
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
Published
2021-04-06
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-21423?

`projen` is a project generation tool that synthesizes project configuration files such as `package.json`, `tsconfig.json`, `.gitignore`, GitHub Workflows, `eslint`, `jest`, and more, from a well-typed definition written in JavaScript. Users of projen's `NodeProject` project type (including any project type derived from it) include a `.github/workflows/rebuild-bot.yml` workflow that may allow any GitHub user to trigger execution of un-trusted code in the context of the "main" repository (as opposed to that of a fork). In some situations, such untrusted code may potentially be able to commit to the "main" repository. The rebuild-bot workflow is triggered by comments including `@projen rebuild` on pull-request to trigger a re-build of the projen project, and updating the pull request with the updated files. This workflow is triggered by an `issue_comment` event, and thus always executes with a `GITHUB_TOKEN` belonging to the repository into which the pull-request is made (this is in contrast with workflows triggered by `pull_request` events, which always execute with a `GITHUB_TOKEN` belonging to the repository from which the pull-request is made). Repositories that do not have branch protection configured on their default branch (typically `main` or `master`) could possibly allow an untrusted user to gain access to secrets configured on the repository (such as NPM tokens, etc). Branch protection prohibits this escalation, as the managed `GITHUB_TOKEN` would not be able to modify the contents of a protected branch and affected workflows must be defined on the default branch.

Is CVE-2021-21423 actively exploited?

Active exploitation of CVE-2021-21423 has not been confirmed. Its EPSS score was 1.4% on 2026-10-23, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-21423?

CVE-2021-21423 has a CVSS v3 base score of 8.1 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N.

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