Security readout for executives and security teams
Plain-English summary
CVE-2021-39195 lets a logged-in Misskey user make the server fetch URLs through Upload from URL or remote attachment handling. On affected versions, this could expose information only reachable from the server's internal network. The vendor fixed it in Misskey 12.90.0.
Executive priority
Treat this as a high-priority confidentiality issue for any Misskey deployment below 12.90.0. Prioritize upgrade and egress restriction, especially where the server can reach sensitive internal systems.
Technical view
Affected Misskey versions before 12.90.0 contain CWE-918 SSRF in URL-based upload and remote attachment handling. CVSS 3.1 is 7.7: network reachable, low complexity, low privileges, no user interaction, scope changed, high confidentiality impact, no integrity or availability impact.
Likely exposure
Exposure is most likely on Misskey deployments running versions earlier than 12.90.0, especially internet-facing instances that allow authenticated users or federated remote attachments to trigger server-side fetches. Proxy-backed deployments need extra review because the advisory notes additional measures may be required.
Exploitation context
The bundle does not show CISA KEV listing or cited evidence of active exploitation. The attack class is SSRF: an attacker may use server-side fetch behavior to reach internal network resources that are not publicly exposed.
Researcher notes
Evidence names affected features, fixed version, CVSS vector, and proxy caveat. The supplied bundle does not specify exploit prevalence, detailed proxy hardening steps, or affected downstream forks. Validate findings against the GitHub advisory, fixing commit, and 12.90.0 changelog.
Mitigation direction
- Upgrade Misskey to version 12.90.0 or later.
- If using a proxy, review the vendor advisory for additional required measures.
- Restrict the Misskey host from accessing private network ranges unnecessarily.
- Limit egress paths to internal services, metadata endpoints, and administrative networks.
- Review current vendor guidance before relying on compensating controls.
Validation and detection
- Inventory all Misskey instances and record their running versions.
- Confirm no production instance is running a version earlier than 12.90.0.
- Identify whether Upload from URL or remote attachment handling is enabled.
- Review proxy and egress controls for private-network access from the Misskey host.
- Check logs for unusual URL-fetch activity around uploads or remote attachments.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-918: Information exposure and cloud metadata lookup
Information exposure and SSRF weaknesses can make discovery, cloud metadata, and credential material review relevant. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCloud metadata behavior lookup
The CVE wording references SSRF or metadata access, so cloud discovery and credential material review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-39195 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.7 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N3.14Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.7HighVector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/misskey-dev/misskey/security/advisories/GHSA-mqv7-gxh4-r5vfCVE reference · x_refsource_CONFIRM
- https://github.com/misskey-dev/misskey/commit/e1a8b158e04ad567d92d8daf3cc0898ee18f1a2eCVE reference · x_refsource_MISC
- https://github.com/misskey-dev/misskey/blob/develop/CHANGELOG.md#12900-20210904CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Server-Side Request Forgery (SSRF)
Server-Side Request Forgery (SSRF) represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.