xyproto algernon up to 1.17.6 API run3 permission

Summaryinfo

A vulnerability was found in xyproto algernon up to 1.17.6. It has been classified as critical. This issue affects the function run3 of the component API. The manipulation leads to permission. This vulnerability is traded as CVE-2026-45721. It is possible to initiate the attack remotely. There is no exploit available. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability, which was classified as critical, has been found in xyproto algernon up to 1.17.6. This issue affects the function run3 of the component API. The manipulation with an unknown input leads to a permission vulnerability. Using CWE to declare the problem leads to CWE-275. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-45721 since 05/13/2026. The exploitation is known to be difficult. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1222 according to MITRE ATT&CK.

Upgrading to version 1.17.7 eliminates this vulnerability.

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Productinfo

Vendor

• xyproto

Name

• algernon

Version

• 1.17.0
• 1.17.1
• 1.17.2
• 1.17.3
• 1.17.4
• 1.17.5
• 1.17.6

Website

• Product: https://github.com/xyproto/algernon/

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