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CVE-2026-45721

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

9.0 CriticalFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
High
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
High
Integrity
High
Availability
High

Exploit likelihood

0.44%Up 0.2 points since May 27, 2026
90%50%10%1%

What each source reports

CVE List Score used 9.0

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

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.

Record
CVE-2026-45721
CVSS 3.1
9.0 CriticalCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Weakness
CWE-20, CWE-426, CWE-552
Updated
2026-11-02
EUVDEUVD-2026-31867 9.0

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.

Record
EUVD-2026-31867
CVSS 3.1
9.0 CriticalCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Updated
2026-11-02
GitHub AdvisoriesGHSA-xwcr-wm99-g9jc 9.0

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

Summary

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.

Details

Root cause — unbounded upward search in DirPage

// engine/dirhandler.go:170-183
// Serve handler.lua, if found in parent directories
var ancestor string
ancestor = filepath.Dir(dirname)
for range 100 { // a maximum of 100 directories deep
    filename = filepath.Join(ancestor, "handler.lua")
    if ac.fs.Exists(filename) {
        ac.FilePage(w, req, filename, luaDataFilename)
        return
    }
    if ancestor == "." {
        break
    }
    ancestor = filepath.Dir(ancestor)
}

dirname is the absolute path of the requested directory on disk, e.g. /srv/algernon/ when running with --prod (see engine/config.go:207). filepath.Dir("/srv/algernon") is /srv, then /, and filepath.Dir("/") returns / indefinitely. The break clause if ancestor == "." only fires for relative paths, so on every absolute server-root configuration the loop walks all the way to / and then spins on / for the remaining iterations until the 100 cap is hit. Each iteration calls ac.fs.Exists(<ancestor>/handler.lua).

For the canonical --prod invocation the candidate set is:

/srv/handler.lua
/handler.lua

For algernon /var/www/example.com:

/var/www/handler.lua
/var/handler.lua
/handler.lua

For algernon ~/site started by user alice:

/home/alice/handler.lua
/home/handler.lua
/handler.lua

The first match wins. The match is then dispatched through FilePage, which for .lua files routes to RunLua (engine/handlers.go:269) and runs the file in a pooled lua.LState with the full Algernon function map attached (engine/lua.go:30-112). Every dangerous primitive in the engine is reachable: shell-out via run3() (engine/basic.go:140-146, calling exec.Command("sh", "-c", ...)), arbitrary outbound HTTP via the httpclient module, the unsandboxed gopher-lua os/io/debug libraries, and the full permissions/userstate API.

Why the request is reachable unauthenticated

The permission middleware in RegisterHandlers runs before DirPage but only rejects requests whose req.URL.Path matches an admin/user prefix:

// engine/handlers.go:510-525
allRequests := func(w http.ResponseWriter, req *http.Request) {
    if ac.perm != nil {
        if ac.perm.Rejected(w, req) {
            sc := sheepcounter.New(w)
            ac.perm.DenyFunction()(sc, req)
            ac.LogAccess(req, http.StatusForbidden, sc.Counter())
            return
        }
    }
    ...

Rejected returns false for / because of rootIsPublic && path == "/" (vendor/.../permissionbolt/v2/permissionbolt.go:118). Anonymous GET / therefore reaches DirPage, hits the ancestor walk, and — if any handler.lua exists anywhere in the parent chain — executes it as the response…

Record
GHSA-xwcr-wm99-g9jc
CVSS 3.1
9.0 CriticalCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Weakness
CWE-20, CWE-426, CWE-552
Updated
2026-11-02
NVD 9.0

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.

Record
CVE-2026-45721
CVSS 3.1
9.0 CriticalCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Weakness
CWE-20, CWE-426, CWE-552
Updated
2026-11-02
OSV 9.0

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

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.

Record
CVE-2026-45721
CVSS 3.1
9.0 CriticalCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Weakness
CWE-20, CWE-426, CWE-552
Updated
2026-11-02
OSVGHSA-xwcr-wm99-g9jc 9.0

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

Summary

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.

Details

Root cause — unbounded upward search in DirPage

// engine/dirhandler.go:170-183
// Serve handler.lua, if found in parent directories
var ancestor string
ancestor = filepath.Dir(dirname)
for range 100 { // a maximum of 100 directories deep
    filename = filepath.Join(ancestor, "handler.lua")
    if ac.fs.Exists(filename) {
        ac.FilePage(w, req, filename, luaDataFilename)
        return
    }
    if ancestor == "." {
        break
    }
    ancestor = filepath.Dir(ancestor)
}

dirname is the absolute path of the requested directory on disk, e.g. /srv/algernon/ when running with --prod (see engine/config.go:207). filepath.Dir("/srv/algernon") is /srv, then /, and filepath.Dir("/") returns / indefinitely. The break clause if ancestor == "." only fires for relative paths, so on every absolute server-root configuration the loop walks all the way to / and then spins on / for the remaining iterations until the 100 cap is hit. Each iteration calls ac.fs.Exists(<ancestor>/handler.lua).

For the canonical --prod invocation the candidate set is:

/srv/handler.lua
/handler.lua

For algernon /var/www/example.com:

/var/www/handler.lua
/var/handler.lua
/handler.lua

For algernon ~/site started by user alice:

/home/alice/handler.lua
/home/handler.lua
/handler.lua

The first match wins. The match is then dispatched through FilePage, which for .lua files routes to RunLua (engine/handlers.go:269) and runs the file in a pooled lua.LState with the full Algernon function map attached (engine/lua.go:30-112). Every dangerous primitive in the engine is reachable: shell-out via run3() (engine/basic.go:140-146, calling exec.Command("sh", "-c", ...)), arbitrary outbound HTTP via the httpclient module, the unsandboxed gopher-lua os/io/debug libraries, and the full permissions/userstate API.

Why the request is reachable unauthenticated

The permission middleware in RegisterHandlers runs before DirPage but only rejects requests whose req.URL.Path matches an admin/user prefix:

// engine/handlers.go:510-525
allRequests := func(w http.ResponseWriter, req *http.Request) {
    if ac.perm != nil {
        if ac.perm.Rejected(w, req) {
            sc := sheepcounter.New(w)
            ac.perm.DenyFunction()(sc, req)
            ac.LogAccess(req, http.StatusForbidden, sc.Counter())
            return
        }
    }
    ...

Rejected returns false for / because of rootIsPublic && path == "/" (vendor/.../permissionbolt/v2/permissionbolt.go:118). Anonymous GET / therefore reaches DirPage, hits the ancestor walk, and — if any handler.lua exists anywhere in the parent chain — executes it as the response…

Record
GHSA-xwcr-wm99-g9jc
CVSS 3.1
9.0 CriticalCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Weakness
CWE-20, CWE-426, CWE-552
Updated
2026-11-02
OSVGO-2026-5772 No score

Algernon: handler.lua discovery walks parent directories above the server root in github.com/xyproto/algernon

Algernon: handler.lua discovery walks parent directories above the server root in github.com/xyproto/algernon

Record
GO-2026-5772
Updated
2026-11-02
OSVopenSUSE-SU-2026:21483-1 No score

Security update for govulncheck-vulndb

This update for govulncheck-vulndb fixes the following issues:

  • Update to version 0.0.20260723T184607 2026-07-23T18:46:07Z (jsc#PED-11136).

Go CVE Numbering Authority IDs added or updated with aliases:

  • GO-2026-4730 CVE-2026-33022 GHSA-cv4x-93xx-wgfj
  • GO-2026-4761 CVE-2026-33211 GHSA-j5q5-j9gm-2w5c
  • GO-2026-5102 CVE-2026-45021 GHSA-3vcp-chfh-f6r2
  • GO-2026-5272 CVE-2026-40938 GHSA-94jr-7pqp-xhcq
  • GO-2026-5486 CVE-2026-40924 GHSA-m2cx-gpqf-qf74
  • GO-2026-5542 CVE-2026-40090 GHSA-pj97-4p9w-gx3q
  • GO-2026-5630 CVE-2026-25542 GHSA-rmx9-2pp3-xhcr
  • GO-2026-5643 CVE-2026-40923 GHSA-rx35-6rhx-7858
  • GO-2026-5675 CVE-2026-41492 GHSA-vvf7-6rmr-m29q
  • GO-2026-5711 CVE-2026-40161 GHSA-wjxp-xrpv-xpff
  • GO-2026-5746 CVE-2026-41567 GHSA-x86f-5xw2-fm2r
  • GO-2026-5749 CVE-2026-41328 GHSA-x92x-px7w-4gx4
  • GO-2026-5837 CVE-2026-44840 GHSA-q2m9-6jp9-c6mc
  • GO-2026-5929 CVE-2026-44342 GHSA-26v7-h57m-gh9m
  • GO-2026-5930 CVE-2026-33655 GHSA-6qcr-qxgr-m7fv
  • GO-2026-5933 GHSA-3g4q-2f67-2gvh
  • GO-2026-5934 GHSA-59qp-cfj3-rp64
  • GO-2026-5935 GHSA-7856-g3gv-9wq8
  • GO-2026-5936 CVE-2026-53552 GHSA-26rh-24rg-j3vv
  • GO-2026-5937 CVE-2026-53508 GHSA-2jcc-mxv7-p3f9
  • GO-2026-5938 CVE-2026-53553 GHSA-4g5x-hcwm-82jw
  • GO-2026-5939 CVE-2026-10517 GHSA-698x-9w2p-7vvp
  • GO-2026-5940 CVE-2026-53572 GHSA-6w3m-4hhp-775q
  • GO-2026-5941 CVE-2026-53487 GHSA-gvhc-wv3v-7pf8
  • GO-2026-5942 CVE-2026-46600
  • GO-2026-5944 CVE-2026-50197 GHSA-659f-rgp5-w4wf
  • GO-2026-5945 CVE-2026-6815 GHSA-rmxx-v9rj-vpvg
  • GO-2026-5946 CVE-2026-52831 GHSA-v5px-423j-pf7p
  • GO-2026-5947 CVE-2026-53649 GHSA-xqhv-chqm-fhcc
  • GO-2026-5948 CVE-2026-50554 GHSA-588f-fvcv-xhvf
  • GO-2026-5949 CVE-2026-50553 GHSA-rqrh-8wpv-x7hh
  • GO-2026-5950 CVE-2026-53602 GHSA-339v-266x-79xr
  • GO-2026-5951 CVE-2026-10219 GHSA-6jm8-4fhr-5w64
  • GO-2026-5952 CVE-2026-49834 GHSA-9vcr-p3rj-q5q6
  • GO-2026-5953 CVE-2026-9094 GHSA-c9w5-qp6m-m395
  • GO-2026-5956 CVE-2026-55252 GHSA-h5g6-xmh4-hc37
  • GO-2026-5957 CVE-2026-54158 GHSA-5xfx-xj4h-5p7r
  • GO-2026-5958 CVE-2026-54068 GHSA-gcm7-57gf-953c
  • GO-2026-5959 CVE-2026-54072 GHSA-h29v-hj44-q8cv
  • GO-2026-5961 CVE-2026-54069 GHSA-hvr9-72v2-fff3
  • GO-2026-5962 CVE-2026-54088 GHSA-m93h-4hw7-5qcm
  • GO-2026-5963 CVE-2026-54067 GHSA-mvjr-vv3c-w4qv
  • GO-2026-5964 CVE-2026-54066 GHSA-p4m3-mgmm-c664
  • GO-2026-5965 CVE-2026-54070 GHSA-w7cg-whh7-xp28
  • GO-2026-5966 CVE-2026-54089 GHSA-xqp3-jq6g-x3qm
  • GO-2026-5967 CVE-2026-50551 GHSA-56mp-4f3v-fgj2
  • GO-2026-5968 CVE-2026-54174 GHSA-fpg8-7664-jc5q
  • GO-2026-5969 GHSA-g936-7jqj-mwv8
  • GO-2026-5970 CVE-2026-56852
  • GO-2026-5973 CVE-2026-54250 GHSA-jxr7-mqhw-9p98
  • GO-2026-5974 CVE-2026-44300 GHSA-wmj8-9953-vff5
  • GO-2026-5975 CVE-2026-50018 GHSA-42j2-w334-qxw7
  • GO-2026-5976 CVE-2026-50141 GHSA-g7mm-9vx7-jm7h
  • GO-2026-5977 CVE-2026-50013 GHSA-qrh4-p6v4-mrfg
  • GO-2026-5978 CVE-2026-50125 GHSA-qw5r-ppcg-f8rj
  • GO-2026-5979 CVE-2026-50006 GHSA-xrcf-6jh3-ggvx
  • GO-2026-5980 CVE-2026-50158 GHSA-2c7f-fxww-6w6c
  • GO-2026-5981 GHSA-mqxv-9rm6-w8qc
  • GO-2026-5982 GHSA-pqg7-v6wh-3pfp
  • GO-2026-5984 CVE-2026-53604 GHSA-2p2f-px33-4vv5
  • GO-2026-5985 GHSA-7rx3-5wx3-5v76
  • GO-2026-5986 CVE-2026-61699 GHSA-cm26-5974-52h8
  • GO-2026-5987 CVE-2026-55513 GHSA-g4x6-jcvr-9m3g
  • GO-2026-5988 CVE-2026-54628 GHSA-hwrq-8wxh-q4xv
  • GO-2026-5989 CVE-2026-55512 GHSA-m3cx-mwpg-32jg
  • GO-2026-5990 CVE-2026-54629 GHSA-mf78-3rpf-r784
  • GO-2026-5991 CVE-2026-53603 GHSA-q4vm-pq3q-8wgq
  • GO-2026-5992 CVE-2026-61549 GHSA-qf34-295c-26v8
  • GO-2026-5996 CVE-2026-54450 GHSA-pph6-vfjv-vpjw
  • GO-2026-5997 CVE-2026-54452 GHSA-xgch-x3mx-cm3c
  • GO-2026-5998 CVE-2026-54495 GHSA-398h-7f66-3h4p
  • GO-2026-5999 CVE-2026-10814 GHSA-jh6h-v6mp-h22v
  • GO-2026-6002 CVE-2026-58196 GHSA-pr64-jmmf-jp54
  • GO-2026-6003 CVE-2026-53714 GHSA-22xc-xg2r-9j7v
  • GO-2026-6004 CVE-2026-52833 GHSA-3v79-m2cg-89ww
  • GO-2026-6005 CVE-2026-53715 GHSA-8fv2-88gg-hm7q
  • GO-2026-6006 CVE-2026-53716 GHSA-cxpq-8v7q-cg56 *…
Record
openSUSE-SU-2026:21483-1
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
github.com/xyproto/algernon
Go
 through 1.17.6
1.17.7
github.com/xyproto/algernon
Go
 before 1.17.7
1.17.7
govulncheck-vulndb
openSUSE:Leap 16.0
From an advisory covering 1734 CVEs
 before 0.0.20260723T184607-160000.1.1
0.0.20260723T184607-160000.1.1

History

History
RecordedChangedSources
affected, facts
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-45721 is a critical vulnerability (CVSS 9.0) in Xyproto Algernon, published May 19, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.44%, the 38th percentile. Fixed in github.com/xyproto/algernon 1.17.7.

References

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