CVE-2025-25306: Misskey Auth Bypass Vulnerability

CVE-2025-25306 Overview

CVE-2025-25306 is a critical Origin Validation Error vulnerability in Misskey, an open source, federated social media platform. The patch for a previous vulnerability (CVE-2024-52591) did not sufficiently validate the relation between the id and url fields of ActivityPub objects. This incomplete fix allows an attacker to forge an object where they claim authority in the url field even if the specific ActivityPub object type requires authority in the id field, potentially enabling impersonation or content manipulation across the federated network.

Critical Impact

Attackers can bypass ActivityPub object authority validation to forge objects and claim false authority, potentially compromising the integrity of federated social interactions and enabling impersonation attacks across connected instances.

Affected Products

• Misskey versions prior to 2025.2.1
• Federated instances running vulnerable Misskey deployments
• ActivityPub implementations relying on Misskey's object validation

Discovery Timeline

• 2025-03-10 - CVE-2025-25306 published to NVD
• 2025-11-26 - Last updated in NVD database

Technical Details for CVE-2025-25306

Vulnerability Analysis

This vulnerability is classified as CWE-346 (Origin Validation Error) and represents an incomplete security fix for the earlier CVE-2024-52591. The core issue lies in the ActivityPub protocol implementation within Misskey, specifically in how the platform validates the relationship between the id and url fields of incoming ActivityPub objects.

In the ActivityPub protocol, different object types have varying requirements for authority verification. Some object types require that authority be established through the id field, which should contain a canonical identifier controlled by the originating server. However, the incomplete patch allowed attackers to bypass this validation by crafting malicious objects that claim authority through the url field instead.

This creates a significant security gap in federated environments, as malicious actors can impersonate users or content from other instances by forging ActivityPub objects that pass Misskey's validation checks but contain falsified authority claims.

Root Cause

The root cause stems from an incomplete validation implementation in the ActivityPub object processing logic. When the patch for CVE-2024-52591 was applied, it addressed authority validation for the id field but failed to properly correlate this validation with the url field. The two fields were not sufficiently validated in relation to each other, allowing inconsistent authority claims to slip through the security checks.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker operating a malicious ActivityPub-compatible server can craft specially constructed ActivityPub objects that:

1. Set the id field to a legitimate-looking identifier
2. Override the url field with a value claiming authority from a different origin
3. Submit these forged objects to a vulnerable Misskey instance through standard federation protocols

Since Misskey participates in the Fediverse (federated network of social media platforms), a successful exploitation could propagate malicious content or impersonated identities across multiple connected instances. The vulnerability allows attackers to forge objects without requiring any special privileges, making it particularly dangerous in open federation environments.

Detection Methods for CVE-2025-25306

Indicators of Compromise

• Unexpected ActivityPub objects with mismatched id and url authority origins
• Incoming federated content where the claimed origin does not match the actual source server
• Anomalous activity patterns suggesting impersonation of users from other instances
• Federation logs showing objects with inconsistent authority claims between fields

Detection Strategies

• Monitor ActivityPub inbox logs for objects where the id field domain differs from the url field domain
• Implement logging to flag incoming federated objects with authority inconsistencies
• Review federation peer relationships for unexpected content attribution
• Deploy anomaly detection on user activity patterns to identify potential impersonation

Monitoring Recommendations

• Enable verbose logging for ActivityPub object processing operations
• Monitor federation traffic for unusual patterns or volumes from specific remote instances
• Establish baseline metrics for normal federated activity to detect deviations
• Implement alerts for objects failing enhanced authority validation after patching

How to Mitigate CVE-2025-25306

Immediate Actions Required

• Upgrade Misskey to version 2025.2.1 or later immediately
• Review federation logs for any suspicious ActivityPub objects received prior to patching
• Consider temporarily restricting federation with untrusted instances until patch is applied
• Notify connected federated instances about the vulnerability and encourage coordinated upgrades

Patch Information

Misskey has released version 2025.2.1 which addresses this vulnerability by implementing proper validation of the relationship between id and url fields in ActivityPub objects. The fix ensures that authority claims are consistently validated across both fields based on the requirements of each specific object type.

For patch details, refer to the GitHub Release 2025.2.1 and the GitHub Security Advisory GHSA-6w2c-vf6f-xf26.

Workarounds

• If immediate patching is not possible, consider temporarily disabling federation features
• Implement network-level filtering to block ActivityPub requests from untrusted origins
• Deploy a reverse proxy with custom rules to validate ActivityPub object authority before forwarding to Misskey
• Monitor and manually review incoming federated content for signs of forgery until the patch is applied

# Upgrade Misskey to patched version
cd /path/to/misskey
git fetch --tags
git checkout 2025.2.1
pnpm install
pnpm run build
pnpm run migrate
# Restart Misskey service
systemctl restart misskey