Vulnerability in github.com/xyproto/algernon (CVE-2026-45721)
Summary
Vulnerability in github.com/xyproto/algernon (CVE-2026-45721). Successful exploitation can lead to full system takeover. Exploitable via DirPage. Mitigation: upgrade to 1.17.7 or later.
Original description
The description as published by the source.
Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, when Algernon is asked for any URL path that resolves to a directory without an index file, DirPage walks upward through parent directories — past the configured server root — looking for a file named handler.lua to execute as the request handler. The loop terminates only after 100 ancestor steps or when filepath.Dir returns ., so on any absolute server-root path the search reaches the filesystem root (/ on Unix, drive letter on Windows). The first handler.lua it finds is loaded into the Lua interpreter with the full Algernon API exposed — including run3(), httpclient, os.execute, io.popen, PQ, MSSQL, raw filesystem access, and the userstate database. Any process that can write handler.lua anywhere in a parent directory of the server root obtains pre-authenticated remote code execution on the next HTTP request. This is reachable without authentication — the lookup happens before the permission check returns a hit (the perm system only gates URL prefixes, not the handler-resolution step), and any URL pointing at a directory without an index triggers the walk. On a fresh stock Algernon install the request GET / is enough. This vulnerability is fixed in 1.17.7.
Source: GitHub Security Advisories (Global) NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-45721 has been found in github.com/xyproto/algernon.
Attackers can target a specific entry point like DirPage over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 9.0/10.
What to do: upgrade github.com/xyproto/algernon to 1.17.7 or later.
If unsure, ask your IT team or search "github.com/xyproto/algernon CVE-2026-45721" on the vendor's site.
CVE-2026-45721 (github.com/xyproto/algernon) — CWE-20 / CVSS v3 9.0
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: DirPage / handler.lua / filepath.Dir / httpclient
Patched: 1.17.7 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-20) exists in github.com/xyproto/algernon. Attackers reach the vulnerable code path via DirPage without authentication.
Check the vendor advisory for the affected versions of github.com/xyproto/algernon. Attack surface: DirPage / handler.lua / filepath.Dir / httpclient.
Severity: Critical (CVSS 9.0/10). Successful exploitation can lead to full system takeover.
Update to 1.17.7.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'github.com/xyproto/algernon' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
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1Identify exposure identify
grep -r 'github.com/xyproto/algernon' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
github.com/xyproto/algernonを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade github.com/xyproto/algernon to 1.17.7ステージング環境で 1.17.7 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
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3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- advisory github.com
- advisory github.com
- advisory nvd.nist.gov
- package github.com
- web github.com