CVE-2026-55103: Auth Bypass (CVSS 8.6)

Description

Discourse ActivityPub adds ActivityPub support to Discourse. Prior to commit 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5, inbound ActivityPub processing does not sufficiently verify that the actor authenticated by the signed request matches the Activity actor and owns the referenced federated post. A remote attacker can exploit these missing identity and ownership checks to edit, delete, or reply to federated posts without the required permissions. This vulnerability is fixed with commit 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5.

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CVSS v3.1 Score

8.6

HIGH

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

Weakness Type (CWE)

CWE-345 CWE-345

CWE-863 Incorrect Authorization

References

Other References

https://github.com/discourse/discourse-activity-pub/commit/1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5 https://github.com/discourse/discourse-activity-pub/security/advisories/GHSA-jw39-rch6-mhwf

Frequently Asked Questions

What is CVE-2026-55103? +

Discourse ActivityPub adds ActivityPub support to Discourse. Prior to commit 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5, inbound ActivityPub processing does not sufficiently verify that the actor authenticated by the signed request matches the Activity actor and owns the referenced federated post. A remote attacker can exploit these missing identity and ownership checks to edit, delete, or reply to federated posts without the required permissions. This vulnerability is fixed with commit 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5. It has a CVSS v3.1 base score of 8.6 (HIGH).

How severe is CVE-2026-55103? +

CVE-2026-55103 has a CVSS v3.1 score of 8.6 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

How do I check if I'm vulnerable to CVE-2026-55103? +

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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