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Vulnerabilities

Vulnerability · CVE-2026-45721

Algernon is a small self-contained pure-Go web server.

Published May 26, 2026 · catalog updated Jul 24, 2026 · Algernon is a small

Check if you run it

No confirmed exploitation is on record for CVE-2026-45721. EPSS is 0.74%. Act only if a listed version of Algernon is a small is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (Algernon is a small).
  • No version range is stored. A customer cannot check an installed version against this record.
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Critical9.0
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.74%
Remediation
Advisory link
Affected products
See sources for product scope

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.

No affected version ranges are stored yet. Use the vendor advisory before you decide this record does not apply.

How to fix CVE-2026-45721

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-45721. EPSS is 0.74%. Act only if a listed version of Algernon is a small is installed.

Which versions are affected?

Affected version ranges are not on this record. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-20: Improper Input Validation; CWE-426: Untrusted Search Path; CWE-552: Files or Directories Accessible to External Parties. EPSS 0.74%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. May 26, 2026 · NIST NVD

    Detected

    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 .

  2. Jul 24, 2026 · NIST NVD

    Updated

    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 .

Sources

  • CVE.orgVulnerability recordCVE-2026-45721

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-45721

    30-day exploitation probability 0.7%

    first.org
  • NIST NVDVulnerability recordCVE-2026-45721

    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

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.