CVE-2026-96399 in Gitea
Summary
by MITRE • 10/06/2026
A repository's external issue tracker regular expression containing alternating capture groups could produce invalid slice indexes when Gitea rendered issue references, causing a runtime panic that terminated the Gitea process. A user who can edit a repository's external issue tracker settings could make any later rendering of matching content, such as viewing a README, crash the instance for all users.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified in this scenario stems from improper input validation and error handling within the regular expression processing logic used by Gitea to parse references in external issue trackers. Specifically, when an administrator configures an external issue tracker with a regular expression that contains alternating capture groups, the internal parsing mechanism fails to correctly calculate slice indices for extracting matched text segments. This miscalculation results in invalid memory access attempts during runtime execution. In Go-based applications like Gitea, such errors typically manifest as panic conditions because the language does not allow out-of-bounds array or slice indexing without explicit bounds checking that is bypassed here due to flawed logic assumptions about regex match lengths and group positions.
From a technical perspective, this flaw represents a classic case of improper input validation leading to a denial of service through application crash. The vulnerability allows any user with edit permissions for the repository's external issue tracker settings to inject or modify configuration parameters that trigger the faulty code path. Once such a misconfigured regular expression is saved, it does not immediately cause an error but rather lies dormant until specific rendering operations occur. These operations include viewing README files, displaying issue titles, or processing any content where Gitea attempts to resolve and render references against the configured external tracker pattern. When these events are triggered, the application encounters a runtime panic that terminates the entire process, effectively taking down the service for all connected users until manual intervention restarts it.
The operational impact of this vulnerability is significant due to its potential for remote denial of service with minimal prerequisites. An attacker does not need authentication or high-level privileges beyond repository edit access, which may be granted to many collaborators in open-source projects. The resulting crash affects the availability of the entire Gitea instance rather than just a single user session, creating a persistent disruption that requires administrative recovery actions. This aligns with CWE-20 Improper Input Validation and CWE-787 Out-of-bounds Write or Access, as the root cause lies in failing to validate regex group structures before using them for memory indexing operations. The attack vector is classified under ATT&CK T1499 Endpoint Denial of Service, specifically leveraging application-level flaws rather than resource exhaustion techniques like CPU or memory flooding.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. Administrators should immediately audit all repositories configured with external issue trackers to identify any regular expressions containing alternating capture groups that could trigger the bug. Upgrading Gitea to a patched version is the primary solution, as developers typically implement stricter bounds checking or regex validation in subsequent releases. Additionally, implementing input sanitization at the configuration entry point can prevent malformed patterns from being stored in persistent storage. Security teams should also consider deploying monitoring solutions that detect abnormal process termination rates and automatically restart services with limited downtime while ensuring proper logging of panic stack traces for forensic analysis. Regular security reviews of third-party integrations and external service configurations are essential to maintain resilience against similar logic-based vulnerabilities.