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Improper Input ValidationWeakness · CWE-20

CVE-2026-45721

CRITICAL · 9.0 CVSS v3.1 Published 2026-05-26
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
99/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Algernon versions before 1.17.7 contain a path traversal vulnerability where requesting any directory URL without an index file triggers upward directory traversal that searches parent directories for handler.lua files, potentially reaching the filesystem root. The first handler.lua found is executed with full Algernon API access including dangerous functions like run3(), os.execute, and io.popen, allowing pre-authentication remote code execution.

MitigationUpgrade to Algernon version 1.17.7 or later to address this vulnerability.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
High
Privileges
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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Algernon installation and version
    Locate the Algernon executable or service, then run 'algernon --version' or check the binary version metadata to determine the installed version number.
    Affected if Version is below 1.17.7 (e.g., 1.17.6, 1.17.5, etc.)
  2. Determine server root configuration
    Examine Algernon configuration files (algernon.conf, config.lua, or command-line arguments) to identify the server root directory being served.
    Affected if A server root is configured and the server is serving content.
  3. Check if directory serving is enabled
    Review Algernon configuration for settings related to directory listing or serving files without an index (e.g., 'dir', ' indexes', 'autoindex', or similar directives).
    Affected if Directory serving without index files is enabled, allowing the server to search for handler.lua.
  4. Audit parent directories for handler.lua
    Starting from the server root directory, examine all parent directories (parent, grandparent, etc.) for the presence of any handler.lua files. Use commands like 'ls -la ../../../handler.lua' or a filesystem search from root traversing downward.
    Affected if A handler.lua file exists in any parent directory of the server root and is writable by the process or attacker.

You are affected if running Algernon version 1.17.6 or earlier with directory serving enabled and a handler.lua file exists in any parent directory of your configured server root.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade to Algernon version 1.17.7 or later to address this vulnerability.

Recommended fix High confidence

1.17.7

  1. Upgrade Algernon to version 1.17.7 or later
  2. After upgrading, verify that directory traversal no longer occurs by requesting a directory path without an index file
  3. Confirm the handler.lua lookup is now restricted to the configured server root and does not traverse above it

Generated from the published advisory — verify against the referenced sources before acting.

Have this fixed Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-45721 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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