GitHub Enterprise Server EUVD-2026-40408

Exploitation requires a valid authenticated user account on the specific GitHub Enterprise Server instance - unauthenticated remote attackers cannot exploit this vulnerability. The authenticated account must hold read access to at least one repository on the instance, which serves as the comparison base in the crafted request; no access to the target private repository is needed or enforced. The Copilot pull request description feature must be enabled on the instance for the vulnerable endpoint to be reachable. Exploitation is therefore limited to internal threat actors - employees, contractors, or holders of compromised accounts - with any existing legitimate repository access. External attackers without an account on the instance are fully excluded. This vulnerability does not affect GitHub.com.

The CVSS 4.0 score of 6.0 (AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N) accurately captures the threat profile: network-reachable, low complexity once the endpoint behavior is known, but gated behind a specific attack requirement (AT:P) - the attacker must already possess an authenticated account with read access to at least one repository on the instance. The confidentiality impact is rated High (VC:H) because full source code diffs of arbitrary private repositories are exposed, which can include proprietary logic, embedded credentials, or sensitive configuration. Integrity and availability are not affected. No CISA KEV listing exists and no public proof-of-concept exploit has been identified, meaning widespread opportunistic exploitation is not confirmed. The insider threat and compromised-account scenarios represent the most realistic risk vectors - external unauthenticated attackers cannot exploit this. EPSS data was not provided, so a quantified exploitation probability cannot be stated. Despite the High confidentiality impact, the AT:P gating condition meaningfully reduces the exposed attack surface compared to an unauthenticated, no-prerequisites vulnerability.

A developer contractor on a GitHub Enterprise Server instance holds read access to a single project repository for their engagement. Aware of the Copilot diff summary endpoint behavior, they craft an API request specifying a cross-repository comparison range that targets a sensitive internal repository containing proprietary source code they have no authorized access to. The endpoint returns the full diff of the target repository without performing any authorization check, exposing the private codebase. No public exploit code is known, but the technique is straightforward for any technically capable authenticated user once the endpoint behavior is understood.