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VulnerabilityPublished

CVE-2026-45721: xyproto algernon · Improper Input Validation

Affects xyproto algernonSource CVE Database V5

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Not known to be exploited · CVSS 9.0 Critical : upgrade to 1.17.7 as a priority.

Data as of 30 Oct 2026, 11:52 UTC

CVSS 3.1 base score
9.0Critical
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.4% chance of exploitation in 30 days – higher than 37% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 1.17.7
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 26 May 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.

What to do

Upgrade algernon and github.com/xyproto/algernon to 1.17.7.

Affected software

ProductAffectedFixed in
xyprotoalgernonAffected
  • <1.17.7
Fixed in
  • 1.17.7
Gogithub.com/xyproto/algernonpkg:golang/github.com/xyproto/algernonOther threatsAffected
  • <1.17.7
Fixed in
  • 1.17.7

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade Algernon to version 1.17.7 or later, where this vulnerability is fixed. There is no indication of any temporary or alternative mitigation. Until patched, prevent untrusted users from writing handler.lua files in any parent directories of the server root to avoid exploitation.

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.

References

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