Algernon: handler.lua discovery walks parent directories above the server root

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Summary

CVECVE-2026-45721
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-05-26 17:16:47 UTC
Updated2026-05-26 19:26:42 UTC
DescriptionAlgernon 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.

Risk And Classification

Primary CVSS: v3.1 9 CRITICAL from [email protected]

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H

EPSS: 0.002080000 probability, percentile 0.430880000 (date 2026-05-27)

Problem Types: CWE-20 | CWE-426 | CWE-552 | CWE-20 CWE-20: Improper Input Validation | CWE-426 CWE-426: Untrusted Search Path | CWE-552 CWE-552: Files or Directories Accessible to External Parties

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary9CRITICALCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
3.1CNADECLARED9CRITICALCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Xyproto Algernon affected < 1.17.7 Not specified

References

ReferenceSourceLinkTags
github.com/xyproto/algernon/security/advisories/GHSA-xwcr-wm99-g9jc 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com
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