Misskey Dev
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JSON-LD signature validation in Misskey's ActivityPub federation layer accepts fraudulent federated activities due to a TOCTOU flaw where the JSON-LD parsing context used during signature verification is not shared with the context used during subsequent message processing. Affected are all self-hosted Misskey instances running versions 12.37.0 through 2026.5.3. A remote, unauthenticated attacker operating or spoofing a federated server can craft payloads that pass signature checks yet resolve to malicious content during processing, resulting in high integrity impact across the instance and potentially propagating to federated peers; no public exploit or CISA KEV listing has been identified at time of analysis.
Signature validation bypass in Misskey (versions 12.37.0 through prior to 2026.5.4) allows an attacker to have spoofed ActivityPub activities accepted as authentic. The flaw resides in the JSON-LD signature validation and compaction logic (CWE-347), enabling forgery of federated actions such as posts or actions attributed to other actors/instances. The CVSS 4.0 base score is 9.2 (Critical) with a network, unauthenticated, low-complexity vector emphasizing integrity impact; no public exploit has been identified at time of analysis, and it is not listed in CISA KEV.
Authentication replay in Misskey before 2026.6.0 lets an attacker reuse a valid TOTP code that should be single-use, because UserAuthService fails to invalidate a code after its first successful use within the same time step. An adversary who concurrently captures a victim's password and one TOTP value can replay that code to authenticate, enabling unauthorized actions up to account takeover. No public exploit has been identified at time of analysis and it is not listed in CISA KEV; the vendor fixed the flaw in release 2026.6.0.
Malformed theme application in the Misskey web client (versions 8.63.0 through pre-2026.5.4) triggers uncontrolled recursion (CWE-674), causing the browser client to slow down significantly or crash entirely. Unauthenticated network-reachable attackers can craft and distribute a malicious theme that, when applied by a victim user, produces a client-side denial of service. No public exploit code has been identified at time of analysis and this vulnerability is not listed in CISA KEV; active exploitation is not confirmed.
Insufficient permission checks in Misskey's Direct Messages (Chat) feature expose DM data points to authenticated users who lack authorization to access them, across versions 2025.3.2 through 2026.5.3. The root cause is CWE-639 (Authorization Bypass Through User-Controlled Key), where the server fails to validate account permissions before returning DM-related data via API. No public exploit or CISA KEV listing exists; the vendor patched this in version 2026.5.4 alongside four additional CVEs.
Unauthorized data disclosure via the Server Announcements API in Misskey affects all instances running versions 2024.5.0 through 2026.5.3, allowing remote callers to read restricted announcement data without appropriate authorization. The flaw stems from CWE-285 (Improper Authorization) - the API endpoint fails to enforce privilege checks before returning data, making restricted content accessible to actors who should not see it. No public exploit code or CISA KEV listing exists at time of analysis, but the network-accessible, no-authentication-required vector makes this straightforwardly reachable on any exposed instance.
Server-Side Request Forgery in Misskey's UrlPreviewService allows remote unauthenticated attackers to coerce the platform's backend into initiating outbound HTTP connections toward loopback addresses, RFC-1918 private ranges, and link-local networks. The root cause is that IP address validation is applied only after the TCP connection has already been established, meaning the outbound request reaches the internal target before the check rejects it. Per vendor disclosure, response data is not believed to be returned to the attacker, limiting the impact to blind SSRF with potential for triggering internal service side effects. No public exploit identified at time of analysis, and a fix is available in version 2026.6.0.
JSON-LD signature validation in Misskey's ActivityPub federation layer accepts fraudulent federated activities due to a TOCTOU flaw where the JSON-LD parsing context used during signature verification is not shared with the context used during subsequent message processing. Affected are all self-hosted Misskey instances running versions 12.37.0 through 2026.5.3. A remote, unauthenticated attacker operating or spoofing a federated server can craft payloads that pass signature checks yet resolve to malicious content during processing, resulting in high integrity impact across the instance and potentially propagating to federated peers; no public exploit or CISA KEV listing has been identified at time of analysis.
Signature validation bypass in Misskey (versions 12.37.0 through prior to 2026.5.4) allows an attacker to have spoofed ActivityPub activities accepted as authentic. The flaw resides in the JSON-LD signature validation and compaction logic (CWE-347), enabling forgery of federated actions such as posts or actions attributed to other actors/instances. The CVSS 4.0 base score is 9.2 (Critical) with a network, unauthenticated, low-complexity vector emphasizing integrity impact; no public exploit has been identified at time of analysis, and it is not listed in CISA KEV.
Authentication replay in Misskey before 2026.6.0 lets an attacker reuse a valid TOTP code that should be single-use, because UserAuthService fails to invalidate a code after its first successful use within the same time step. An adversary who concurrently captures a victim's password and one TOTP value can replay that code to authenticate, enabling unauthorized actions up to account takeover. No public exploit has been identified at time of analysis and it is not listed in CISA KEV; the vendor fixed the flaw in release 2026.6.0.
Malformed theme application in the Misskey web client (versions 8.63.0 through pre-2026.5.4) triggers uncontrolled recursion (CWE-674), causing the browser client to slow down significantly or crash entirely. Unauthenticated network-reachable attackers can craft and distribute a malicious theme that, when applied by a victim user, produces a client-side denial of service. No public exploit code has been identified at time of analysis and this vulnerability is not listed in CISA KEV; active exploitation is not confirmed.
Insufficient permission checks in Misskey's Direct Messages (Chat) feature expose DM data points to authenticated users who lack authorization to access them, across versions 2025.3.2 through 2026.5.3. The root cause is CWE-639 (Authorization Bypass Through User-Controlled Key), where the server fails to validate account permissions before returning DM-related data via API. No public exploit or CISA KEV listing exists; the vendor patched this in version 2026.5.4 alongside four additional CVEs.
Unauthorized data disclosure via the Server Announcements API in Misskey affects all instances running versions 2024.5.0 through 2026.5.3, allowing remote callers to read restricted announcement data without appropriate authorization. The flaw stems from CWE-285 (Improper Authorization) - the API endpoint fails to enforce privilege checks before returning data, making restricted content accessible to actors who should not see it. No public exploit code or CISA KEV listing exists at time of analysis, but the network-accessible, no-authentication-required vector makes this straightforwardly reachable on any exposed instance.
Server-Side Request Forgery in Misskey's UrlPreviewService allows remote unauthenticated attackers to coerce the platform's backend into initiating outbound HTTP connections toward loopback addresses, RFC-1918 private ranges, and link-local networks. The root cause is that IP address validation is applied only after the TCP connection has already been established, meaning the outbound request reaches the internal target before the check rejects it. Per vendor disclosure, response data is not believed to be returned to the attacker, limiting the impact to blind SSRF with potential for triggering internal service side effects. No public exploit identified at time of analysis, and a fix is available in version 2026.6.0.