Enterprise Server: Incorrect Regular Expression
CVE-2026-4296 is a high-severity incorrect regular expression vulnerability in Enterprise Server from GitHub, with a CVSS 3.1 base score of 8.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, but only if a user takes part, for example by opening a link or file. The record lists version 3.20.1 as the first without the flaw, so updating Enterprise Server to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.43% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Enterprise Server to 3.20.1 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.
- Because the attack needs a user to take part, warn users about unexpected links and files related to this product until it is patched.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| GitHub | Enterprise Server | 3.14.0 through 3.14.25; 3.15.0 through 3.15.20; 3.16.0 through 3.16.16; 3.17.0 through 3.17.13; 3.18.0 through 3.18.7; 3.19.0 through 3.19.4; 3.20.0 up to, but not including, 3.20.1 | 3.20.1 |
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
- RequiredA user must take an action such as opening a file or link.
- 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:R/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 7.5 | High | CNA (github.com) |
| CVSS 3.1 | 8.8 | High | NVD |
Official description
An incorrect regular expression vulnerability was identified in GitHub Enterprise Server that allowed an attacker to bypass OAuth redirect URI validation. An attacker with knowledge of a first-party OAuth application's registered callback URL could craft a malicious authorization link that, when clicked by a victim, would redirect the OAuth authorization code to an attacker-controlled domain. This could allow the attacker to gain unauthorized access to the victim's account with the scopes granted to the OAuth application. This vulnerability affected all versions of GitHub Enterprise Server prior to 3.21 and was fixed in versions 3.20.1, 3.19.5, 3.18.8, 3.17.14, 3.16.17, 3.15.21, 3.14.26. This vulnerability was reported via the GitHub Bug Bounty program.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Incorrect Regular Expression
The product specifies a regular expression in a way that causes data to be improperly matched or compared.
How developers prevent this class of flaw
- Regular expressions can become error prone when defining a complex language even for those experienced in writing grammars. Determine if several smaller regular expressions simplify one large regular expression. Also, subject the regular expression to thorough testing techniques such as equivalence partitioning, boundary value analysis, and robustness. After testing and a reasonable confidence level is achieved, a regular expression may not be foolproof. If an exploit is allowed to slip through, then record the exploit and refactor the regular expression.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Advisories
- docs.github.com/en/[email protected]/admin/release-notes#3.14.26Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.15.21Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.16.17Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.17.14Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.18.8Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.19.5Release Notes
- docs.github.com/en/[email protected]/admin/release-notes#3.20.1Release Notes
Other vulnerabilities in Enterprise Server
- 7.7 High severity
- 8.8 High severity
- 9.8 Critical severity
- 8.8 High severity
- 7.5 High severity
- 7.4 High severity
Recent Incorrect Regular Expression vulnerabilities
- 7.5 High severity
- 8.8 High severity
- 9.3 Critical severity
- 7.5 High severity
- 9.8 Critical severity
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