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CVE-2026-46430

Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS

4.3 MediumFix available

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, the SSE event server bound to 0.0.0.0:5553 on Linux/macOS by default because the platform-dependent host default in engine/flags.go:39-46 set host = "" for non-Windows, and utils.JoinHostPort("", ":5553") resolves to ":5553". This vulnerability is fixed in 1.17.7.

Attack profile

How easy it is to exploit

Attack vector
Adjacent network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
Low
Integrity
None
Availability
None

Exploit likelihood

0.20%Up 0.2 points since May 27, 2026
90%50%10%1%

What each source reports

CVE List Score used 4.3

Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, the SSE event server bound to 0.0.0.0:5553 on Linux/macOS by default because the platform-dependent host default in engine/flags.go:39-46 set host = "" for non-Windows, and utils.JoinHostPort("", ":5553") resolves to ":5553". This vulnerability is fixed in 1.17.7.

Record
CVE-2026-46430
CVSS 3.1
4.3 MediumCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-668, CWE-1188
Updated
2026-11-02
EUVDEUVD-2026-31869 4.3

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, the SSE event server bound to 0.0.0.0:5553 on Linux/macOS by default because the platform-dependent host default in engine/flags.go:39-46 set host = "" for non-Windows, and utils.JoinHostPort("", ":5553") resolves to ":5553". This vulnerability is fixed in 1.17.7.

Record
EUVD-2026-31869
CVSS 3.1
4.3 MediumCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-gj84-924c-48fx 4.3

Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS

Summary

The SSE event server bound to 0.0.0.0:5553 on Linux/macOS by default because the platform-dependent host default in engine/flags.go:39-46 set host = "" for non-Windows, and utils.JoinHostPort("", ":5553") resolves to ":5553" — a Go http.Server.Addr of ":5553" listens on every interface. On Windows the same code chose "localhost", binding loopback only.

The result was a platform split where the OS Algernon's dev workflow is most often used on (Linux/macOS) got the network-exposed default, and only Windows users got the loopback-safe one. A LAN peer with no developer interaction could connect to <dev-laptop-ip>:5553 and read the file-change stream.

This advisory covers the bind-address default in isolation. The fix is independent of authentication (#2a) and CORS (#2b) — switching the default to loopback can be done without touching either.

Details

Root cause — platform-dependent host default in handleFlags

// engine/flags.go:39-46  (1.17.6)
host := ""
if runtime.GOOS == "windows" {
    host = "localhost"
    // Default Bolt database file
    ac.defaultBoltFilename = filepath.Join(serverTempDir, "algernon.db")
    // Default log file
    ac.defaultLogFile = filepath.Join(serverTempDir, "algernon.log")
}

// engine/config.go:388-391  (1.17.6, finalConfiguration)
if ac.eventAddr == "" {
    ac.eventAddr = utils.JoinHostPort(host, ac.defaultEventColonPort)
}

Result tabulated:

| Platform | host | eventAddr after JoinHostPort | Effective bind | |---|---|---|---| | Linux | "" | ":5553" | 0.0.0.0:5553 (all interfaces) | | macOS | "" | ":5553" | 0.0.0.0:5553 (all interfaces) | | Windows | "localhost" | "localhost:5553" | 127.0.0.1:5553 (loopback) |

The same host value also governs the main web server bind, so the platform split affects both ports. The web-server bind on Linux/macOS is a separate (defensible) design decision — a server intended to be reachable; the SSE port is not such a service and inherited the same default by accident.

Why this is an independent finding

The fix is a single line: change the default host value, or change the eventAddr default specifically, to "localhost" regardless of platform. No change to authentication or CORS is required to close the network-reach half of the original bundled advisory. A LAN peer can no longer connect — the listener is unreachable from another host — even if the SSE handler still has no authentication and still returns Allow-Origin: *.

PoC (against 1.17.6 on Linux/macOS)

# Operator's laptop on a hotel/cafe/office WiFi:
algernon -a /path/to/project
# => SSE listener bound to 0.0.0.0:5553

# Any peer on the same subnet:
$ curl -sN http://<dev-laptop-ip>:5553/sse
id: 0
data: /path/to/project/secret-notes.md

id: 1
data: /path/to/project/.env.local

No interaction from the developer is required. The peer needs network reach and nothing else.

Impact

  • Confidentiality: medium. LAN-bounded continuous information disclosure of filenames and edit timing.
  • Integrity: none.
  • Availability: none directly.

The CVSS vector uses AV:A (adjacent network) to model the LAN-only reach. The vector for a misconfigured deployment behind a NAT-less or routed network would shift to AV:N and rise to 5.3.

Suggestions to fix

Primary fix — pick localhost as the SSE default on every platform.

// engine/flags.go -- platform-independent default for the event listener
// (keep the existing platform split for the WEB server if desired, but
// not for the event server)
host := "localhost"

Or, more surgically:

// engine/config.go -- finalConfiguration
if ac.eventAddr == "" {
    ac.eventAddr = utils.JoinHostPort("localhost", ac.defaultEventColonPort)
}

An operator who genuinely wants LAN-reachable SSE can pass --eventserver 0.0.0.0:5553 explicitly and accept the consequences.

**Stronger fix — eliminate the…

Record
GHSA-gj84-924c-48fx
CVSS 3.1
4.3 MediumCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-668, CWE-1188
Updated
2026-11-02
NVD 4.3

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, the SSE event server bound to 0.0.0.0:5553 on Linux/macOS by default because the platform-dependent host default in engine/flags.go:39-46 set host = "" for non-Windows, and utils.JoinHostPort("", ":5553") resolves to ":5553". This vulnerability is fixed in 1.17.7.

Record
CVE-2026-46430
CVSS 3.1
4.3 MediumCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-668, CWE-1188
Updated
2026-11-02
OSV 4.3

Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, the SSE event server bound to 0.0.0.0:5553 on Linux/macOS by default because the platform-dependent host default in engine/flags.go:39-46 set host = "" for non-Windows, and utils.JoinHostPort("", ":5553") resolves to ":5553". This vulnerability is fixed in 1.17.7.

Record
CVE-2026-46430
CVSS 3.1
4.3 MediumCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-1188, CWE-668
Updated
2026-11-02
OSVGHSA-gj84-924c-48fx 4.3

Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS

Summary

The SSE event server bound to 0.0.0.0:5553 on Linux/macOS by default because the platform-dependent host default in engine/flags.go:39-46 set host = "" for non-Windows, and utils.JoinHostPort("", ":5553") resolves to ":5553" — a Go http.Server.Addr of ":5553" listens on every interface. On Windows the same code chose "localhost", binding loopback only.

The result was a platform split where the OS Algernon's dev workflow is most often used on (Linux/macOS) got the network-exposed default, and only Windows users got the loopback-safe one. A LAN peer with no developer interaction could connect to <dev-laptop-ip>:5553 and read the file-change stream.

This advisory covers the bind-address default in isolation. The fix is independent of authentication (#2a) and CORS (#2b) — switching the default to loopback can be done without touching either.

Details

Root cause — platform-dependent host default in handleFlags

// engine/flags.go:39-46  (1.17.6)
host := ""
if runtime.GOOS == "windows" {
    host = "localhost"
    // Default Bolt database file
    ac.defaultBoltFilename = filepath.Join(serverTempDir, "algernon.db")
    // Default log file
    ac.defaultLogFile = filepath.Join(serverTempDir, "algernon.log")
}

// engine/config.go:388-391  (1.17.6, finalConfiguration)
if ac.eventAddr == "" {
    ac.eventAddr = utils.JoinHostPort(host, ac.defaultEventColonPort)
}

Result tabulated:

| Platform | host | eventAddr after JoinHostPort | Effective bind | |---|---|---|---| | Linux | "" | ":5553" | 0.0.0.0:5553 (all interfaces) | | macOS | "" | ":5553" | 0.0.0.0:5553 (all interfaces) | | Windows | "localhost" | "localhost:5553" | 127.0.0.1:5553 (loopback) |

The same host value also governs the main web server bind, so the platform split affects both ports. The web-server bind on Linux/macOS is a separate (defensible) design decision — a server intended to be reachable; the SSE port is not such a service and inherited the same default by accident.

Why this is an independent finding

The fix is a single line: change the default host value, or change the eventAddr default specifically, to "localhost" regardless of platform. No change to authentication or CORS is required to close the network-reach half of the original bundled advisory. A LAN peer can no longer connect — the listener is unreachable from another host — even if the SSE handler still has no authentication and still returns Allow-Origin: *.

PoC (against 1.17.6 on Linux/macOS)

# Operator's laptop on a hotel/cafe/office WiFi:
algernon -a /path/to/project
# => SSE listener bound to 0.0.0.0:5553

# Any peer on the same subnet:
$ curl -sN http://<dev-laptop-ip>:5553/sse
id: 0
data: /path/to/project/secret-notes.md

id: 1
data: /path/to/project/.env.local

No interaction from the developer is required. The peer needs network reach and nothing else.

Impact

  • Confidentiality: medium. LAN-bounded continuous information disclosure of filenames and edit timing.
  • Integrity: none.
  • Availability: none directly.

The CVSS vector uses AV:A (adjacent network) to model the LAN-only reach. The vector for a misconfigured deployment behind a NAT-less or routed network would shift to AV:N and rise to 5.3.

Suggestions to fix

Primary fix — pick localhost as the SSE default on every platform.

// engine/flags.go -- platform-independent default for the event listener
// (keep the existing platform split for the WEB server if desired, but
// not for the event server)
host := "localhost"

Or, more surgically:

// engine/config.go -- finalConfiguration
if ac.eventAddr == "" {
    ac.eventAddr = utils.JoinHostPort("localhost", ac.defaultEventColonPort)
}

An operator who genuinely wants LAN-reachable SSE can pass --eventserver 0.0.0.0:5553 explicitly and accept the consequences.

**Stronger fix — eliminate the…

Record
GHSA-gj84-924c-48fx
CVSS 3.1
4.3 MediumCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-1188, CWE-668
Updated
2026-11-02
OSVGO-2026-5401 No score

Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS in github.com/xyproto/algernon

Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS in github.com/xyproto/algernon

Record
GO-2026-5401
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-2026-46430 is a medium vulnerability (CVSS 4.3) in Xyproto Algernon, published May 20, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.20%, the 9th percentile. Fixed in github.com/xyproto/algernon 1.17.7.

References

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