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CVE-2026-45728

Algernon: Single-file mode unconditionally enables debug mode

7.5 HighFix available

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, when Algernon is invoked with a single file path instead of a directory, singleFileMode is set to true and debugMode is forcibly enabled. debugMode activates the PrettyError renderer, which on any Lua or template error response dumps the absolute path of the file that errored, complete byte contents of that file, and exception or parser error text. This response is served with HTTP 200 OK to whoever sent the request that triggered the error. Any client able to reach the server and able to provoke a runtime error in the served script obtains the full server-side source of that script and of any sibling Lua data file consulted during the request. This vulnerability is fixed in 1.17.7.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
High
Integrity
None
Availability
None

Exploit likelihood

0.30%Up 0.3 points since May 27, 2026
90%50%10%1%

What each source reports

CVE List Score used 7.5

Algernon: Single-file mode unconditionally enables debug mode

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, when Algernon is invoked with a single file path instead of a directory, singleFileMode is set to true and debugMode is forcibly enabled. debugMode activates the PrettyError renderer, which on any Lua or template error response dumps the absolute path of the file that errored, complete byte contents of that file, and exception or parser error text. This response is served with HTTP 200 OK to whoever sent the request that triggered the error. Any client able to reach the server and able to provoke a runtime error in the served script obtains the full server-side source of that script and of any sibling Lua data file consulted during the request. This vulnerability is fixed in 1.17.7.

Record
CVE-2026-45728
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-209, CWE-489, CWE-540, CWE-1188
Updated
2026-11-02
EUVDEUVD-2026-31868 7.5

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, when Algernon is invoked with a single file path instead of a directory, singleFileMode is set to true and debugMode is forcibly enabled. debugMode activates the PrettyError renderer, which on any Lua or template error response dumps the absolute path of the file that errored, complete byte contents of that file, and exception or parser error text. This response is served with HTTP 200 OK to whoever sent the request that triggered the error. Any client able to reach the server and able to provoke a runtime error in the served script obtains the full server-side source of that script and of any sibling Lua data file consulted during the request. This vulnerability is fixed in 1.17.7.

Record
EUVD-2026-31868
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-fwqx-8365-9983 7.5

Algernon: Single-file mode unconditionally enables debug mode

Summary

When Algernon is invoked with a single file path instead of a directory — the documented "quick demo" workflow (algernon foo.lua, algernon page.po2, algernon index.html, algernon mywebsite.alg) — singleFileMode is set to true and debugMode is forcibly enabled with no opt-out:

// engine/config.go:498-502
// Make a few changes to the defaults if we are serving a single file
if ac.singleFileMode {
    ac.debugMode = true
    ac.serveJustHTTP = true
}

debugMode activates the PrettyError renderer, which on any Lua or template error response dumps:

  1. The absolute path of the file that errored (Filename field of the error template).
  2. The complete byte contents of that file, HTML-escaped, with the offending line wrapped in <font style='color: red !important'>…</font>.
  3. The exception or parser error text — which in turn often quotes additional file content (Pongo2 errors include surrounding template lines; Lua tracebacks include argument values).

This response is served with HTTP 200 OK to whoever sent the request that triggered the error. There is no authentication, no rate limit specific to errors, no redaction, and no opt-out short of avoiding single-file invocations entirely. Any client able to reach the server and able to provoke a runtime error in the served script obtains the full server-side source of that script and of any sibling Lua data file consulted during the request.

This combines particularly badly with --prod not being effective: --prod sets productionMode = true and calls ac.debugMode = false inside finalConfiguration, but singleFileMode is computed after --prod in MustServe (line 499 vs finalConfiguration further down) and the forced debugMode = true happens before --prod's debugMode = false clamp runs — so even an operator who reasoned "I will pass --prod to be safe" gets debug-mode-on if they also pass a single Lua file. Operators routinely combine the two when running Algernon as a system unit (ExecStart=algernon --prod /etc/algernon/site.lua), unaware that single-file detection has overridden their hardening flag.

Details

Root cause 1 — single-file detection forces debugMode = true

// engine/config.go:441-502  (inside MustServe — abridged)
switch strings.ToLower(filepath.Ext(serverFile)) {
case ".md", ".markdown":
    ...
case ".zip", ".alg":
    ...
default:
    ac.singleFileMode = true
}
// ...
// Make a few changes to the defaults if we are serving a single file
if ac.singleFileMode {
    ac.debugMode = true
    ac.serveJustHTTP = true
}

Any single-file invocation whose extension is not .md/.zip/.alg lands in the default: branch and turns into singleFileMode = true, which then sets debugMode = true. That includes the natural quickstart inputs — .lua, .po2, .pongo2, .html, .amber, .tmpl, .jsx, .tl, .prompt — every file extension Algernon recognises as a server-renderable handler.

The .lua case has a follow-up at engine/config.go:536-548 that resets singleFileMode = false so the script can read sibling files, but debugMode has already been written to true and is not unset.

Root cause 2 — --prod's clamp runs after the forced enable, so it is the wrong direction

// engine/config.go:393-397  (finalConfiguration, called from MustServe)
// Turn off debug mode if production mode is enabled
if ac.productionMode {
    // Turn off debug mode
    ac.debugMode = false
}

This clamp is in finalConfiguration. finalConfiguration is invoked from MustServe after the single-file block (MustServe line 632: ac.finalConfiguration(ac.serverHost)). So the order is:

1. flag parsing       -> productionMode=true, debugMode=false
2. single-file detect -> debugMode = true     (overrides production)
3. finalConfiguration -> if productionMode { debugMode = false }

On paper step 3 wins. In…

Record
GHSA-fwqx-8365-9983
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-209, CWE-489, CWE-540, CWE-1188
Updated
2026-11-02
NVD 7.5

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, when Algernon is invoked with a single file path instead of a directory, singleFileMode is set to true and debugMode is forcibly enabled. debugMode activates the PrettyError renderer, which on any Lua or template error response dumps the absolute path of the file that errored, complete byte contents of that file, and exception or parser error text. This response is served with HTTP 200 OK to whoever sent the request that triggered the error. Any client able to reach the server and able to provoke a runtime error in the served script obtains the full server-side source of that script and of any sibling Lua data file consulted during the request. This vulnerability is fixed in 1.17.7.

Record
CVE-2026-45728
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-209, CWE-489, CWE-540, CWE-1188
Updated
2026-11-02
OSV 7.5

Algernon: Single-file mode unconditionally enables debug mode

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, when Algernon is invoked with a single file path instead of a directory, singleFileMode is set to true and debugMode is forcibly enabled. debugMode activates the PrettyError renderer, which on any Lua or template error response dumps the absolute path of the file that errored, complete byte contents of that file, and exception or parser error text. This response is served with HTTP 200 OK to whoever sent the request that triggered the error. Any client able to reach the server and able to provoke a runtime error in the served script obtains the full server-side source of that script and of any sibling Lua data file consulted during the request. This vulnerability is fixed in 1.17.7.

Record
CVE-2026-45728
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-1188, CWE-209, CWE-489, CWE-540
Updated
2026-11-02
OSVGHSA-fwqx-8365-9983 7.5

Algernon: Single-file mode unconditionally enables debug mode

Summary

When Algernon is invoked with a single file path instead of a directory — the documented "quick demo" workflow (algernon foo.lua, algernon page.po2, algernon index.html, algernon mywebsite.alg) — singleFileMode is set to true and debugMode is forcibly enabled with no opt-out:

// engine/config.go:498-502
// Make a few changes to the defaults if we are serving a single file
if ac.singleFileMode {
    ac.debugMode = true
    ac.serveJustHTTP = true
}

debugMode activates the PrettyError renderer, which on any Lua or template error response dumps:

  1. The absolute path of the file that errored (Filename field of the error template).
  2. The complete byte contents of that file, HTML-escaped, with the offending line wrapped in <font style='color: red !important'>…</font>.
  3. The exception or parser error text — which in turn often quotes additional file content (Pongo2 errors include surrounding template lines; Lua tracebacks include argument values).

This response is served with HTTP 200 OK to whoever sent the request that triggered the error. There is no authentication, no rate limit specific to errors, no redaction, and no opt-out short of avoiding single-file invocations entirely. Any client able to reach the server and able to provoke a runtime error in the served script obtains the full server-side source of that script and of any sibling Lua data file consulted during the request.

This combines particularly badly with --prod not being effective: --prod sets productionMode = true and calls ac.debugMode = false inside finalConfiguration, but singleFileMode is computed after --prod in MustServe (line 499 vs finalConfiguration further down) and the forced debugMode = true happens before --prod's debugMode = false clamp runs — so even an operator who reasoned "I will pass --prod to be safe" gets debug-mode-on if they also pass a single Lua file. Operators routinely combine the two when running Algernon as a system unit (ExecStart=algernon --prod /etc/algernon/site.lua), unaware that single-file detection has overridden their hardening flag.

Details

Root cause 1 — single-file detection forces debugMode = true

// engine/config.go:441-502  (inside MustServe — abridged)
switch strings.ToLower(filepath.Ext(serverFile)) {
case ".md", ".markdown":
    ...
case ".zip", ".alg":
    ...
default:
    ac.singleFileMode = true
}
// ...
// Make a few changes to the defaults if we are serving a single file
if ac.singleFileMode {
    ac.debugMode = true
    ac.serveJustHTTP = true
}

Any single-file invocation whose extension is not .md/.zip/.alg lands in the default: branch and turns into singleFileMode = true, which then sets debugMode = true. That includes the natural quickstart inputs — .lua, .po2, .pongo2, .html, .amber, .tmpl, .jsx, .tl, .prompt — every file extension Algernon recognises as a server-renderable handler.

The .lua case has a follow-up at engine/config.go:536-548 that resets singleFileMode = false so the script can read sibling files, but debugMode has already been written to true and is not unset.

Root cause 2 — --prod's clamp runs after the forced enable, so it is the wrong direction

// engine/config.go:393-397  (finalConfiguration, called from MustServe)
// Turn off debug mode if production mode is enabled
if ac.productionMode {
    // Turn off debug mode
    ac.debugMode = false
}

This clamp is in finalConfiguration. finalConfiguration is invoked from MustServe after the single-file block (MustServe line 632: ac.finalConfiguration(ac.serverHost)). So the order is:

1. flag parsing       -> productionMode=true, debugMode=false
2. single-file detect -> debugMode = true     (overrides production)
3. finalConfiguration -> if productionMode { debugMode = false }

On paper step 3 wins. In…

Record
GHSA-fwqx-8365-9983
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness
CWE-1188, CWE-209, CWE-489, CWE-540
Updated
2026-11-02
OSVGO-2026-5385 No score

Algernon: Single-file mode unconditionally enables debug mode in github.com/xyproto/algernon

Algernon: Single-file mode unconditionally enables debug mode in github.com/xyproto/algernon

Record
GO-2026-5385
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-2026-45728 is a high vulnerability (CVSS 7.5) 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 0.30%, the 23rd percentile. Fixed in github.com/xyproto/algernon 1.17.7.

References

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