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

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No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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.

Source published

8 May 2026 19:29:04 UTC

Source modified

13 May 2026 17:18:23 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 6.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L

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.

Source published

May 8, 2026, 7:29:04 PM (timezone not stated)

Source modified

May 13, 2026, 5:18:23 PM (timezone not stated)

Observed here

7 Nov 2026 09:32:16 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 6.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L

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.

Source published

8 May 2026 20:16:31 UTC

Source modified

26 Oct 2026 18:10:00 UTC

Observed here

27 Oct 2026 11:12:01 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 6.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L

### Summary 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. ### Details The entry point is the normal post creation API. The user-controlled `url` field is accepted, normalized with `diesel_url_create()`, and only validated with `is_valid_url()`. That validation allows `http` and `https` but does not implement internal address rejection. The post creation flow then schedules Webmention delivery for public communities. This creates a direct source-to-sink path from an externally supplied post URL to a server-side outbound HTTP request. Core vulnerable code path: ```rust // crates/api_crud/src/post/create.rs let url = diesel_url_create(data.url.as_deref())?; if let Some(url) = &url { is_url_blocked(url, &url_blocklist)?; is_valid_url(url)?; } ``` ```rust // crates/utils/src/utils/validation.rs pub fn is_valid_url(url: &Url) -> LemmyResult<()> { let is_valid = ["http", "https", "magnet"].contains(&url.scheme()); if !is_valid { Err(LemmyErrorType::InvalidUrl)? } Ok(()) } ``` ```rust // crates/api_crud/src/post/create.rs if community.visibility == CommunityVisibility::Public { let post = inserted_post.clone(); let url = url.clone(); spawn_try_task(async move { if let Some(url) = url { Webmention::new(post.ap_id.clone().into(), url.into()).send().await?; } Ok(()) }); } ``` These snippets matter because they show that the attacker controls `CreatePost.url`, the only validation is scheme-level, and the resulting URL is later used for server-side Webmention delivery. ### PoC _Complete instructions, including specific configuration details, to reproduce the vulnerability._Prerequisites: - The attacker has a valid low-privileged account. - The attacker can post to a public community. Practical reproduction flow: 1. Run an HTTP listener on an internal or loopback-reachable address from the Lemmy server's perspective, such as `127.0.0.1:8081`. 2. Authenticate as a normal user. 3. Submit a post to a public community with `url` set to the internal target. 4. Observe the Lemmy API return a normal post creation response. 5. Observe the internal HTTP listener receive a request from the Lemmy server shortly afterwards. Complete PoC: ```http POST /api/v3/post HTTP/1.1 Host: victim.example Authorization: Bearer <low-priv-jwt> Content-Type: application/json { "name": "wm-ssrf", "community_id": 1, "url": "http://127.0.0.1:8081/", "body": null, "alt_text": null, "honeypot": null, "nsfw": false, "language_id": null, "custom_thumbnail": null } ``` Outcome: - The API returns a successful `post_view` response. - The Lemmy server later issues an outbound request toward `http://127.0.0.1:8081/` as part of Webmention processing. ### Impact An authenticated user can use the application server as a blind SSRF primitive against internal HTTP services. This can expose internal network reachability, trigger internal webhooks or administrative endpoints, and expand the attack surface beyond the public deployment boundary. Because the sink is reached after ordinary user content submission, the issue is practical to exploit in real deployments where normal users can post to public communities.

Source published

24 Apr 2026 15:22:49 UTC

Source modified

13 May 2026 13:56:07 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L

Publisher rating · MODERATE