Enterprise Server: Observable Timing Discrepancy
CVE-2026-77987 is a critical-severity observable timing discrepancy vulnerability in Enterprise Server from GitHub, with a CVSS 3.1 base score of 9.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 3.17.*, 3.18.*, 3.19.* as the first without the flaw, so updating Enterprise Server to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.88% chance of exploitation within 30 days, higher than 60% of scored CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Update Enterprise Server to 3.17.* or 3.18.* or 3.19.* or 3.20.*, whichever matches your release line, or a later version.
- Read the advisory on docs.github.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| GitHub | Enterprise Server | 3.17.0 up to, but not including, 3.17.*; 3.18.0 up to, but not including, 3.18.*; 3.19.0 up to, but not including, 3.19.*; 3.20.0 up to, but not including, 3.20.*; 3.21.0 up to, but not including, 3.21.*; 3.22.0 up to, but not including, 3.22.* | 3.17.*, 3.18.*, 3.19.*, 3.20.*, 3.21.*, 3.22.* |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 9.3 | Critical | CNA (github.com) |
| CVSS 3.1 | 9.8 | Critical | NVD |
Official description
A server-side request forgery (SSRF) vulnerability was identified in the notebook viewer of GitHub Enterprise Server. The notebook viewer validated the scheme and host of a user-supplied URL but did not validate the port, allowing requests to be directed to internal services listening on other ports of the same appliance. Response bodies were not returned to the requester, but response timing acted as an oracle that allowed instance secrets to be extracted character by character. An extracted secret could then be used in a separate interaction with an internal service to obtain remote code execution on the appliance. Exploitation required network access to the instance and was unauthenticated when private mode was disabled, or required any authenticated user when private mode was enabled. This vulnerability affected GitHub Enterprise Server versions 3.17 through 3.22 and was fixed in versions 3.22.1, 3.21.6, 3.20.8, 3.19.12, 3.18.15, and 3.17.21. This vulnerability was reported through the GitHub Bug Bounty program.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Observable Timing Discrepancy
Two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not.
More Observable Timing Discrepancy vulnerabilities (CWE-208)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Advisories
- docs.github.com/en/[email protected]/admin/release-notes#3.17.21Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.18.15Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.19.12Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.20.8Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.21.6Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.22.1Release Notes
Other vulnerabilities in Enterprise Server
- 7.7 High severity
- 8.8 High severity
- 8.8 High severity
- 7.5 High severity
- 7.4 High severity
- 7.5 High severity
Recent Observable Timing Discrepancy vulnerabilities
- 7.4 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.4 High severity
- 8.5 High severity
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