CVE-2026-90679
Received Received - Intake

Forgejo ActivityPub Spoofing via Unverified Actor Identity

Publication date: 2026-09-13

Last updated on: 2026-09-13

Assigner: MITRE

Description
Forgejo 13.0.0 through 16.0.4, when "[federation] ENABLED = true" is set, has a spoofing issue that affects identity integrity but does not allow account takeover or content modification. It does not verify that the HTTP Signature on an incoming ActivityPub activity was produced by the key belonging to the actor named in the activity body. The signature verification in routers/api/v1/activitypub/reqsignature.go validates the request signature, but the inbox activity handlers subsequently read the acting identity from the attacker-controlled JSON body without binding it to the verified signing key. Additionally, the signed Digest header is not recomputed against the received request body. A remote attacker who hosts a single valid ActivityPub actor and keypair can therefore submit signature-valid activities attributed to any actor identity they name.

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Meta Information

Published
2026-09-13
Last Modified
2026-09-13
Generated
2026-09-13
AI Q&A
2026-09-13
EPSS Evaluated
N/A
NVD
CVE-2026-90679
EUVD
EUVD-2026-76944

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
forgejo forgejo From 13.0.0 (inc) to 16.0.4 (inc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-348 The product has two different sources of the same data or information, but it uses the source that has less support for verification, is less trusted, or is less resistant to attack.

Attack-Flow Graph

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Executive Summary

Forgejo versions 13.0.0 to 16.0.4 with federation enabled can be spoofed due to improper verification of ActivityPub HTTP signatures. The system checks the signature but later reads the actor identity from attacker-controlled JSON, allowing impersonation of any user without account takeover or content changes.

Detection Guidance

Check Forgejo logs for unexpected ActivityPub activities or signature validation failures. Monitor network traffic for unsigned or improperly signed requests to /api/v1/activitypub endpoints. Use tools like curl to inspect HTTP headers and request bodies for missing or invalid Digest headers and signatures.

Impact Analysis

An attacker could impersonate users in federated instances, potentially misleading others about the origin of activities like posts or comments. This undermines trust in identity integrity but does not grant access to accounts or modify content directly.

Compliance Impact

This vulnerability could potentially impact compliance with GDPR and HIPAA by allowing spoofed ActivityPub activities that misrepresent user identities. Since the system fails to bind verified signing keys to actor identities in the JSON body, unauthorized activities may appear legitimate, compromising data integrity and audit trails. This could lead to non-compliance with requirements for accurate identity verification and logging.

Mitigation Strategies
  • Disable federation by setting [federation] ENABLED = false in Forgejo configuration.
  • Update Forgejo to a version beyond 16.0.4 where the issue is patched.
  • Review and restrict access to ActivityPub endpoints to trusted sources only.

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