Gitlab CVE-2026-3857

Unauthenticated attacker crafts malicious webpage or link containing GraphQL mutation requests. Victim (authenticated GitLab user) must visit the attacker's site while logged into GitLab CE/EE 17.10-18.10.0. Affects all versions: 17.10 before 18.8.7, 18.9 before 18.9.3, 18.10 before 18.10.1. No special GitLab configuration required.

This vulnerability presents a high real-world risk based on multiple converging signals. The CVSS score of 8.1 (High) reflects network-based exploitation (AV:N) with low complexity (AC:L), requiring no privileges (PR:N) but user interaction (UI:R), with high impact to confidentiality and integrity. The availability of a public proof-of-concept exploit on HackerOne (report 3584382) dramatically lowers the barrier to exploitation and indicates detailed attack methodology is publicly documented. While EPSS data is not provided, the combination of public exploit availability, high CVSS score, and the critical nature of GitLab in software development pipelines suggests elevated probability of exploitation attempts. The requirement for user interaction (UI:R) means exploitation requires social engineering, typically via phishing or watering hole attacks, which is a minor constraint for determined attackers. GitLab instances with many authenticated users represent high-value targets for supply chain attacks, making this a genuine priority for organizations using affected versions.

An attacker hosts a malicious website or sends a phishing email containing crafted HTML with embedded forms or JavaScript that target GitLab's GraphQL API endpoints. When an authenticated GitLab user visits this malicious page while still logged into their GitLab instance, the attacker's code automatically submits GraphQL mutations using the victim's browser, which includes valid session cookies. The attacker could add SSH keys to the victim's account, modify repository settings to disable security features, create malicious CI/CD pipeline configurations to inject code into builds, or exfiltrate sensitive project data. Since a public proof-of-concept exists on HackerOne (report 3584382), attackers have detailed exploitation guidance readily available.