A server‑side request forgery flaw in GitHub Enterprise Server allowed an attacker to perform a timing side‑channel attack against the notebook rendering service, extracting sensitive environment variables one character at a time. The vulnerability arises because the notebook viewer follows HTTP redirects without revalidating the destination host when private mode is disabled, permitting an unauthenticated SSRF to internal services. By chaining the instance’s open redirect endpoint through an external redirect, an attacker can query an internal API with regex filters and measure response times to infer environment variable values. This reflects a classic CWE‑918 weakness where untrusted input is used to construct internal requests.

The flaw applies to all GitHub Enterprise Server releases prior to v3.21. It was fixed in the following patch releases: 3.14.26, 3.15.21, 3.16.17, 3.17.14, 3.18.08, 3.19.05, and 3.20.01. Users running older versions should check for and apply the latest update that includes this fix.

The CVSS score of 8.9 indicates a high‑severity vulnerability that could lead to the compromise of confidential configuration information. The EPSS score is not available, and the issue is not listed in CISA KEV, but the lack of public exploitation data does not reduce the potential threat for customers who are exposed. The attack requires: private mode disabled, the ability to chain the instance’s open redirect endpoint through an external redirect to an internal service, and a working regex API endpoint. Once these prerequisites are met, an attacker can launch a side‑channel attack, steadily revealing sensitive environment variables with a high degree of confidence within a reasonable time frame.