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TI-2026-30503 Aliases CVE-2026-42180GHSA-3jvj-v6w2-h948

Lemmy: SSRF in /api/v3/post via Webmention dispatch

Priority LOW 27CWE-918Published · Updated

Key facts

Attack vector
Network
Privileges required
Low
User interaction
None
Attack complexity
Low
Impact type
—
Confidentiality · integrity · availability
Low · Low · Low
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub.

Summary

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00184 7th percentileScored 2026-11-07
Fix Fixed lemmy (LemmyNet): 0.19.18lemmy_api_common (crates.io): 0.19.18

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
LemmyNetlemmybefore 0.19.180.19.18Patch availableCNA
—lemmy_api_common (crates.io)before 0.19.180.19.18Patch availableGHSA-3jvj-v6w2-h948
  • lemmy VendorLemmyNet Affected before 0.19.18 Fixed in 0.19.18 Patch available CNA
  • lemmy_api_common (crates.io) Vendor— Affected before 0.19.18 Fixed in 0.19.18 Patch available GHSA-3jvj-v6w2-h948

Remediation

  • Fix · CNA lemmy (LemmyNet) Fixed in 0.19.18
  • Fix · GHSA-3jvj-v6w2-h948 lemmy_api_common (crates.io) Fixed in 0.19.18 Advisory →

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-42180
  2. 2026-10-03 Alias added: GHSA-3jvj-v6w2-h948

References

Advisory 1

Other 4

  • TI-2026-23295 Lemmy: Unauthenticated SSRF via file_type query parameter injection in image endpoint CVSS 4.0HIGH 7.7
  • TI-2026-26073 Lemmy's Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid() CVSS 3.1MEDIUM 6.5
  • TI-2026-39555 Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed CVSS 4.0MEDIUM 6.4
  • TI-2026-39552 Lemmy: Lower-ranked federated moderator can remove higher-ranked moderators CVSS 3.1MEDIUM 6.5
  • TI-2026-30504 Lemmy: SSRF and internal image disclosure in post link metadata via unvalidated og:image CVSS 3.1MEDIUM 6.5
  • TI-2026-39553 Lemmy: Blocked users can edit private messages sent before the block CVSS 4.0MEDIUM 5.3
See all 10 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences