Bug 2532645 (CVE-2026-90679)

Summary: CVE-2026-90679 forgejo: Forgejo: Identity spoofing via unverified ActivityPub signatures
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Forgejo when federation is enabled. This issue allows a remote attacker to spoof identities by submitting signature-valid ActivityPub activities attributed to any actor identity. The system fails to verify that the HTTP Signature on an incoming ActivityPub activity was produced by the key belonging to the actor named in the activity body, leading to compromised identity integrity.
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Bug Depends On: 2532933    
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Description OSIDB Bzimport 2026-09-13 04:01:19 UTC 
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 ActivityPubactivity 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.