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 v3.1 Score

6.3

MEDIUM

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

EPSS — Exploit Prediction

0.0004

Probability of exploitation

0.11%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-918 Server-Side Request Forgery (SSRF)

References

Other References

https://github.com/LemmyNet/lemmy/releases/tag/0.19.18 https://github.com/LemmyNet/lemmy/security/advisories/GHSA-3jvj-v6w2-h948 https://github.com/LemmyNet/lemmy/security/advisories/GHSA-3jvj-v6w2-h948

Frequently Asked Questions

What is CVE-2026-42180? +

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. It has a CVSS v3.1 base score of 6.3 (MEDIUM).

How severe is CVE-2026-42180? +

CVE-2026-42180 has a CVSS v3.1 score of 6.3 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0004, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-42180? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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