{"answers":{"affected":"12 open source packages ship an affected version, including github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server and github.com/mattermost/mattermost-server/v5. It affects 1 product.","exploited":"There is no known exploitation of CVE-2026-6339 in the wild and no public exploit code.","fix":"Upgrade github.com/mattermost/mattermost/server/v8 to a fixed version for your release line: 8.0.0-20260327001745-7a339a6438f5, 11.4.4 or 11.5.2.","what":"CVE-2026-6339 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 18 May 2026. Any signed-in user can exploit it over the network, with no action from the victim."},"fix_prompt":"Check this codebase for CVE-2026-6339 and fix it if it is affected.\n\nWhat it is: CVE-2026-6339 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 18 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.\nExploitation: There is no known exploitation of CVE-2026-6339 in the wild and no public exploit code.\n\nAffected packages and fixed versions:\n- github.com/mattermost/mattermost/server/v8 (Go): affected all versions; fixed in 8.0.0-20260327001745-7a339a6438f5\n- github.com/mattermost/mattermost/server/v8 (Go): affected all versions; fixed in 8.0.0-20260327001745-7a339a6438f5\n- github.com/mattermost/mattermost/server/v8 (Go): affected all versions; fixed in 8.0.0-20260327001745-7a339a6438f5\n- github.com/mattermost/mattermost/server/v8 (Go): affected from 11.4.0; fixed in 11.4.4\n- github.com/mattermost/mattermost/server/v8 (Go): affected from 11.4.0; fixed in 11.4.4\n- github.com/mattermost/mattermost/server/v8 (Go): affected from 11.5.0; fixed in 11.5.2\n- github.com/mattermost/mattermost/server/v8 (Go): affected from 11.5.0; fixed in 11.5.2\n- github.com/mattermost/mattermost-server (Go): affected all versions; fixed in 5.3.2-0.20260327001745-7a339a6438f5\n- github.com/mattermost/mattermost-server (Go): affected all versions; fixed in 5.3.2-0.20260327001745-7a339a6438f5\n- github.com/mattermost/mattermost-server (Go): affected from 11.4.0+incompatible; fixed in 11.4.4+incompatible, 11.5.2+incompatible\n- github.com/mattermost/mattermost-server/v5 (Go): affected all versions; no fix listed\n- github.com/mattermost/mattermost-server/v6 (Go): affected all versions; no fix listed\n\nTask:\n1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.\n2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.\n3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.\n4. Run the tests and summarise what changed and why.\n5. If nothing in the project is affected, say so and change nothing.\n\nReference: https://insights.sbomify.com/CVE-2026-6339/ (Markdown: https://insights.sbomify.com/CVE-2026-6339/index.md)","id":"CVE-2026-6339","markdown":"https://insights.sbomify.com/CVE-2026-6339/index.md","name":"CVE-2026-6339","record":{"aliases":["GHSA-xvcx-mgpc-5xh3","GO-2026-5769"],"analysis":{"component_type":"web_application","fix_type":"patch","impacts":["data_read","security_bypass"],"network_reachable":"yes","non_default_config":false,"product_category":"Mattermost chat platform","requires_auth":"low_privilege","summary":"An authenticated user can force the reveal of burn-on-read messages without recipient consent due to missing origin validation on the reveal endpoint.","user_interaction":"not_required","vuln_class":"csrf"},"analysis_generated_by":"qwen3.8-flash-next","assigner":"Mattermost","cpe":[{"product":"mattermost_server","vendor":"mattermost"}],"cvss":{"score":4.3,"severity":"medium","source":"cna","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L","version":"3.1"},"cwes":[{"id":"CWE-346","name":"Origin Validation Error"}],"description":"Mattermost versions 11.5.x \u003c= 11.5.1, 11.4.x \u003c= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636","ecosystems":["Go"],"exploits":{"exploitdb":0,"first_seen":null,"github_poc":0,"max_poc_stars":null,"metasploit":0,"nuclei":0,"refs":[],"tier":"none_known"},"has_fix":true,"headline":"Missing request origin validation on burn-on-read reveal endpoint","id":"CVE-2026-6339","indexable":true,"kev":null,"kind":"cve","links":{"ecosystem":"Go","product":"mattermost:mattermost_server","products":["mattermost:mattermost_server"],"type":"csrf","year":"2026"},"n_packages":12,"n_references":1,"next":{"headline":"Memory Exhaustion via Malicious 7zip File Upload","id":"CVE-2026-6340"},"packages":[{"ecosystem":"Go","fixed":["5.3.2-0.20260327001745-7a339a6438f5"],"introduced":["0"],"last_affected":[],"package":"github.com/mattermost/mattermost-server","purl":null,"source":"GHSA-xvcx-mgpc-5xh3"},{"ecosystem":"Go","fixed":["5.3.2-0.20260327001745-7a339a6438f5"],"introduced":["0"],"last_affected":[],"package":"github.com/mattermost/mattermost-server","purl":"pkg:golang/github.com/mattermost/mattermost-server","source":"GHSA-xvcx-mgpc-5xh3"},{"ecosystem":"Go","fixed":["11.4.4+incompatible","11.5.2+incompatible"],"introduced":["11.4.0+incompatible","11.5.0+incompatible"],"last_affected":[],"package":"github.com/mattermost/mattermost-server","purl":"pkg:golang/github.com/mattermost/mattermost-server","source":"GO-2026-5769"},{"ecosystem":"Go","fixed":[],"introduced":["0"],"last_affected":[],"package":"github.com/mattermost/mattermost-server/v5","purl":"pkg:golang/github.com/mattermost/mattermost-server/v5","source":"GO-2026-5769"},{"ecosystem":"Go","fixed":[],"introduced":["0"],"last_affected":[],"package":"github.com/mattermost/mattermost-server/v6","purl":"pkg:golang/github.com/mattermost/mattermost-server/v6","source":"GO-2026-5769"},{"ecosystem":"Go","fixed":["8.0.0-20260327001745-7a339a6438f5"],"introduced":["0"],"last_affected":[],"package":"github.com/mattermost/mattermost/server/v8","purl":null,"source":"GHSA-xvcx-mgpc-5xh3"},{"ecosystem":"Go","fixed":["8.0.0-20260327001745-7a339a6438f5"],"introduced":["0"],"last_affected":[],"package":"github.com/mattermost/mattermost/server/v8","purl":"pkg:golang/github.com/mattermost/mattermost/server/v8","source":"GHSA-xvcx-mgpc-5xh3"},{"ecosystem":"Go","fixed":["8.0.0-20260327001745-7a339a6438f5"],"introduced":["0"],"last_affected":[],"package":"github.com/mattermost/mattermost/server/v8","purl":"pkg:golang/github.com/mattermost/mattermost/server/v8","source":"GO-2026-5769"},{"ecosystem":"Go","fixed":["11.4.4"],"introduced":["11.4.0"],"last_affected":[],"package":"github.com/mattermost/mattermost/server/v8","purl":null,"source":"GHSA-xvcx-mgpc-5xh3"},{"ecosystem":"Go","fixed":["11.4.4"],"introduced":["11.4.0"],"last_affected":[],"package":"github.com/mattermost/mattermost/server/v8","purl":"pkg:golang/github.com/mattermost/mattermost/server/v8","source":"GHSA-xvcx-mgpc-5xh3"},{"ecosystem":"Go","fixed":["11.5.2"],"introduced":["11.5.0"],"last_affected":[],"package":"github.com/mattermost/mattermost/server/v8","purl":null,"source":"GHSA-xvcx-mgpc-5xh3"},{"ecosystem":"Go","fixed":["11.5.2"],"introduced":["11.5.0"],"last_affected":[],"package":"github.com/mattermost/mattermost/server/v8","purl":"pkg:golang/github.com/mattermost/mattermost/server/v8","source":"GHSA-xvcx-mgpc-5xh3"}],"prev":{"headline":"HTTP request smuggling in Kong Enteprise Gateway","id":"CVE-2026-6338"},"products":["Mattermost"],"published":"2026-05-18","related":[{"cvss_score":9.9,"headline":"Account takeover on OAuth/OpenID-enabled servers","id":"CVE-2025-12419","kev_date_added":null,"published":"2025-11-27"},{"cvss_score":9.9,"headline":"Account Takeover via Code Exchange Endpoint","id":"CVE-2025-12421","kev_date_added":null,"published":"2025-11-27"},{"cvss_score":9.9,"headline":"Path Traversal Leading to RCE by Any Authenticated Mattermost User","id":"CVE-2025-4981","kev_date_added":null,"published":"2025-06-20"},{"cvss_score":9.9,"headline":"Arbitrary file read via block duplication in Mattermost Boards","id":"CVE-2025-20051","kev_date_added":null,"published":"2025-02-24"},{"cvss_score":9.9,"headline":"Arbitrary file read in Mattermost Boards via import \u0026 export board archive","id":"CVE-2025-25279","kev_date_added":null,"published":"2025-02-24"},{"cvss_score":9.8,"headline":"Mattermost Server: Insufficient Password-Reset Link Invalidation","id":"CVE-2016-11074","kev_date_added":null,"published":"2020-06-19"},{"cvss_score":9.8,"headline":"Mattermost Server allows attackers to gain privileges by accessing unintended API endpoints with users' credentials","id":"CVE-2017-18885","kev_date_added":null,"published":"2020-06-19"}],"sources":[{"class":"attribution","licence":"CVE Terms of Use (MITRE)","name":"CVE List","prefix":"CVE"},{"class":"attribution","licence":"CC BY 4.0","name":"GitHub Advisory Database","prefix":"GHSA"},{"class":"attribution","licence":"CC BY 4.0","name":"Go Vulnerability Database","prefix":"GO"},{"class":"attribution","licence":"US public information, NVD API terms","name":"NVD"},{"class":"attribution","licence":"CWE Terms of Use","name":"CWE (MITRE)"},{"class":"attribution","licence":"FIRST CVSS licence","name":"CVSS (FIRST)"}],"ssvc":{"automatable":"no","exploitation":"none","technical_impact":"partial"},"state":"PUBLISHED","updated":"2026-05-18","vendors":["Mattermost"]},"summary":"An authenticated user can force the reveal of burn-on-read messages without recipient consent due to missing origin validation on the reveal endpoint.","url":"https://insights.sbomify.com/CVE-2026-6339/"}