CVE-2026-27780: Gitea Auth Bypass Vulnerability

CVE-2026-27780 Overview

CVE-2026-27780 is a branch-protection bypass vulnerability in Gitea versions before 1.26.0. The flaw resides in the pre-receive hook input processing logic, where the server uses bufio.Scanner to parse incoming reference updates. When bufio.Scanner encounters an error, such as oversized input exceeding the default token buffer, the code does not fail closed. Attackers can craft oversized push payloads to bypass branch-protection checks. The vulnerability is classified under [CWE-863: Incorrect Authorization]. Successful exploitation lets attackers push commits to protected branches, undermining code review requirements and repository integrity controls.

Critical Impact
Attackers with push access can bypass branch protection rules on Gitea repositories, injecting unauthorized commits into protected branches without triggering enforcement checks.

Affected Products

Gitea versions prior to 1.26.0
Self-hosted Gitea instances processing pre-receive hooks
Repositories configured with branch protection rules

Discovery Timeline

2026-07-03 - CVE-2026-27780 published to NVD
2026-07-06 - Last updated in NVD database

Technical Details for CVE-2026-27780

Vulnerability Analysis

Gitea enforces branch protection by inspecting reference updates through a pre-receive hook. The hook reads the update list from standard input and evaluates each entry against configured policies such as required reviews, signed commits, and status checks. The parser wraps input in a bufio.Scanner and iterates line by line using Scanner.Scan().

The bufio.Scanner API returns false from Scan() both on end-of-input and on error. Callers must check Scanner.Err() to distinguish the two states. In vulnerable Gitea versions, the pre-receive handler treats a false return as a clean end of input and proceeds to approve the push. When the scanner aborts due to bufio.ErrTooLong on oversized lines, the enforcement loop exits before evaluating the offending references.

The fix landed in the GitHub Pull Request for Gitea and shipped with the Gitea Release Tag v1.26.0.

Root Cause

The root cause is a fail-open error handling pattern. The scanner loop does not consult Scanner.Err() after iteration ends, so scanner failures are indistinguishable from normal completion. Any condition that causes bufio.Scanner to abort, including oversized tokens, is silently swallowed and treated as a successful policy evaluation.

Attack Vector

An attacker with push access to a repository crafts a Git push containing a reference update line larger than the scanner buffer. The pre-receive hook begins processing input, encounters the oversized line, and the scanner terminates with an error. The enforcement code interprets the truncated iteration as a policy pass. The push proceeds to update protected branches without review, signing, or status-check enforcement.

See the Gitea Blog Release Announcement for release context and the linked pull request for the specific code change.

Detection Methods for CVE-2026-27780

Indicators of Compromise

Unexpected commits on protected branches from accounts that lack merge or bypass privileges.
Pre-receive hook log entries that terminate without an explicit accept or reject decision.
Git push events with abnormally large single-line payloads in reference update streams.
Force-push or fast-forward updates on protected branches without corresponding pull request approvals.

Detection Strategies

Correlate Gitea audit logs against branch protection policy events to find pushes that skipped enforcement.
Alert on bufio.Scanner or hook processor errors surfaced in Gitea server logs.
Compare commit SHA history on protected branches against merge records to identify commits with no associated pull request.
Monitor for Git operations that transfer unusually large pre-receive input relative to the repository baseline.

Monitoring Recommendations

Ingest Gitea application logs, SSH access logs, and Git hook stderr into a centralized log platform for retention and query.
Track authorship, committer, and pusher identity for every push to protected branches and flag mismatches.
Establish a baseline for pre-receive hook execution duration and payload size, then alert on outliers.
Enable Gitea webhook delivery for push and repository events and forward them to a SIEM for continuous review.

How to Mitigate CVE-2026-27780

Immediate Actions Required

Upgrade all Gitea instances to version 1.26.0 or later without delay.
Audit protected branches for unauthorized commits pushed before the upgrade and revert or quarantine as needed.
Rotate any secrets, deployment keys, or CI credentials that may have been exposed through unreviewed commits.
Restrict push access to protected branches to the minimum set of trusted users until patching is complete.

Patch Information

The vulnerability is fixed in Gitea 1.26.0. The patch, tracked in pull request 36963, updates the pre-receive hook input handler to check Scanner.Err() and abort the push when scanner errors occur. Release artifacts are available at the Gitea Release Tag v1.26.0.

Workarounds

Enforce server-side limits on push payload size at the reverse proxy or Git transport layer to reject oversized reference update streams.
Require signed commits and validate signatures out-of-band in continuous integration pipelines until the upgrade is applied.
Disable direct push access to protected branches and require merges through pull requests reviewed by administrators.
Increase logging verbosity on the Gitea hook processor to surface scanner errors for manual review.

bash

# Verify Gitea version and upgrade if needed
gitea --version

# Example upgrade steps for a systemd-managed Gitea instance
sudo systemctl stop gitea
sudo -u git cp /usr/local/bin/gitea /usr/local/bin/gitea.bak
sudo -u git wget -O /usr/local/bin/gitea \
  https://dl.gitea.com/gitea/1.26.0/gitea-1.26.0-linux-amd64
sudo chmod +x /usr/local/bin/gitea
sudo systemctl start gitea

# Confirm the running version is 1.26.0 or later
gitea --version