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CVE-2026-45721— Algernon: handler.lua discovery walks parent directories above the server root

Quick assessment

Affected
xyproto algernon
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

algernon是Alexander F. Rødseth个人开发者的一个 Web 服务器。 Algernon 1.17.7之前版本存在安全漏洞，该漏洞源于目录请求时向上遍历父目录查找handler.lua文件，可能导致任意进程在父目录中写入handler.lua时获得预认证远程代码执行。

CVSS 9.0 · Critical EPSS 0.74% · P54

Affected Version Matrix 1

VendorProduct Version RangeStatus
xyproto algernon < 1.17.7 affected
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I. Basic Information for CVE-2026-45721

Vulnerability Information
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Vulnerability Title
Algernon: handler.lua discovery walks parent directories above the server root
Source: CVE Program / CVE List V5
Vulnerability Description
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: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
输入验证不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
algernon 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
algernon是Alexander F. Rødseth个人开发者的一个 Web 服务器。 Algernon 1.17.7之前版本存在安全漏洞，该漏洞源于目录请求时向上遍历父目录查找handler.lua文件，可能导致任意进程在父目录中写入handler.lua时获得预认证远程代码执行。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
xyproto algernon < 1.17.7 -

II. Public POCs for CVE-2026-45721

# POC Description Source Link Shenlong Link
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Qwen3.6-35B-A3B · 9460 chars
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III. Intelligence Information for CVE-2026-45721

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Vendor Advisories for CVE-2026-45721 (1)

Same Patch Batch · xyproto · 2026-05-26 · 7 CVEs total

CVE-2026-48126 8.2 HIGH Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent
CVE-2026-45728 7.5 HIGH Algernon: Single-file mode unconditionally enables debug mode
CVE-2026-46430 4.3 MEDIUM Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS
CVE-2026-46431 4.3 MEDIUM Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *
CVE-2026-43981 Algernon: Race Condition in handle() shared LState
CVE-2026-43982 Algernon: Path traversal file write via savein()

IV. Related Vulnerabilities

Same product: algernon
Same vendor: xyproto
Same weakness: CWE-20

V. Comments for CVE-2026-45721

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