CVE-2026-76851 in GitHub
Summary
by MITRE • 09/01/2026
A Server-Side Request Forgery (SSRF) vulnerability was identified in GitHub Enterprise Server that allowed remote code execution on the instance. Insufficient network isolation allowed malicious pre-receive hook code to impersonate an internal service and redirect trusted internal requests to a privileged service, leading to elevated code execution. Exploitation required pre-receive hook networking to be enabled and either site administrator privileges or write access to a repository containing a configured pre-receive hook. This vulnerability affected all versions of GitHub Enterprise Server prior to 3.22 and was fixed in versions 3.17.20, 3.18.14, 3.19.11, 3.20.7, and 3.21.5. This vulnerability was reported via the GitHub Bug Bounty program.
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Analysis
by VulDB Data Team • 09/02/2026
A critical Server-Side Request Forgery (SSRF) vulnerability existed within GitHub Enterprise Server that ultimately facilitated remote code execution on the affected instance. The core technical flaw stemmed from insufficient network isolation between different components of the platform, specifically involving pre-receive hooks. Pre-receive hooks are scripts executed by Git upon receiving a push to a repository, allowing for custom validation or processing logic. In this vulnerable configuration, malicious actors could craft hook code that impersonated an internal service. By leveraging the SSRF weakness, this malicious code was able to redirect trusted internal requests toward privileged services within the GitHub Enterprise infrastructure. This redirection bypassed standard authentication and authorization checks because the target services treated the request as coming from a legitimate internal source rather than an external or untrusted entity.
The operational impact of this vulnerability was severe, resulting in full remote code execution on the server hosting the GitHub Enterprise instance. An attacker who successfully exploited this flaw could execute arbitrary commands with the privileges of the service account running the affected component. This level of access allows for complete compromise of the system, including potential lateral movement to other internal services, exfiltration of sensitive data such as source code and credentials, and disruption of availability through denial-of-service attacks or destruction of critical files. The vulnerability fundamentally undermined the trust boundary between user-controlled hook scripts and privileged backend services, turning a feature designed for repository governance into a vector for total system compromise.
Exploitation of this flaw required specific conditions to be met by an attacker. First, pre-receive hook networking had to be enabled on the target instance, which allows hooks to make outbound network requests. Second, the attacker needed either site administrator privileges or write access to a repository that contained a configured pre-receive hook. This means that while high-level administrative rights could directly trigger the exploit, even users with standard write permissions could achieve compromise if they controlled a repository equipped with such hooks and had previously injected malicious code into them. The vulnerability affected all versions of GitHub Enterprise Server prior to version 3.22. It was specifically patched in maintenance releases including versions 3.17.20, 3.18.14, 3.19.11, 3.20.7, and 3.21.5, ensuring that older legacy systems were also addressed through backported fixes rather than requiring a major version upgrade for all users.
From a classification perspective, this vulnerability aligns with CWE-918 Server-Side Request Forgery due to the ability to force the server to make requests on behalf of an attacker. It also relates to CWE-20 Improper Input Validation as the system failed to adequately validate or restrict the network destinations accessible from hook scripts. In terms of offensive security frameworks, this exploit maps to ATT&CK technique T1557 Adversary-in-the-Middle, where the attacker positions themselves between two communicating parties to intercept and potentially alter communications, specifically leveraging SSRF for lateral movement within a trusted environment. Additionally, it falls under privilege escalation vectors due to the transition from user-level hook execution to system-level code execution via impersonation of internal services.
Mitigation strategies primarily involve upgrading GitHub Enterprise Server to one of the fixed versions mentioned above. For organizations unable to immediately upgrade, disabling pre-receive hook networking is a critical temporary control if it does not disrupt essential operational workflows. Administrators should also enforce strict least-privilege principles regarding repository write access and ensure that only trusted personnel can configure or modify pre-receive hooks. Regular auditing of installed hooks and monitoring for unusual outbound network traffic from the application server can help detect potential exploitation attempts before they lead to full compromise. This vulnerability was responsibly disclosed through the GitHub Bug Bounty program, highlighting the importance of coordinated disclosure in maintaining platform security integrity.