CVE-2026-35393: Arbitrary File Write/Upload in Go
// Executive Summary
Summary * POST multipart upload directory not sanitized | httpserver/updown.go:71-174
The CVE-2026-35393 vulnerability affects Go and represents a critical-severity Arbitrary File Write/Upload flaw with real-world consequences for organizations running affected versions.
Web Shell Upload
Attackers can upload web shells to gain persistent remote access to the server.
Configuration Tampering
Critical configuration files can be overwritten to alter system behavior or disable security controls.
Remote Code Execution Chain
Arbitrary file write can be chained with other vulnerabilities to achieve full code execution.
Follow these steps to determine whether your systems are exposed to CVE-2026-35393:
Detection Checklist
- Identify all instances of Go deployed in your environment (on-premises, cloud, and containerized deployments).
- Check the version of your Go installation — vulnerable versions are indicated in the CVE advisory.
- Review access logs for anomalous request patterns consistent with Arbitrary File Write/Upload exploitation attempts.
- Search for indicators of compromise (IOCs) including unexpected process spawns, unusual network connections, or modified configuration files.
- Run KENSAI's automated vulnerability scanner against your Go deployment to confirm exposure in minutes.
- Check your SIEM/IDS for alerts referencing this CVE or related attack signatures published by threat intelligence vendors.
Immediate Actions Required
- Apply the vendor patch immediately — update Go to the latest patched version as specified in the CVE advisory.
- If an immediate patch is not possible, implement temporary mitigations such as WAF rules to block exploit attempts.
- Isolate the affected system from the internal network until patched to prevent lateral movement by attackers.
- Enforce multi-factor authentication (MFA) on all accounts with access to the affected system as an additional defense-in-depth measure.
- Restrict network access to the vulnerable component using firewall rules, limiting exposure to trusted IP ranges only.
- Audit and rotate all credentials (passwords, API keys, tokens) that may have been accessible to an attacker if exploitation occurred.
- Enable comprehensive logging and alerting for the affected component to detect any ongoing exploitation attempts.
- Conduct a post-patch verification scan to confirm the vulnerability has been remediated before returning the system to production.
- Review related systems and dependencies that may be affected by the same vulnerability class.
What is CVE-2026-35393?
CVE-2026-35393 is a critical-severity Arbitrary File Write/Upload vulnerability affecting Go. It has a CVSS score of 9.8 and requires immediate attention from security teams.
How serious is CVE-2026-35393?
With a CRITICAL severity rating and CVSS score of 9.8, CVE-2026-35393 represents a significant security risk. Organizations using Go should treat remediation as a high priority.
Is CVE-2026-35393 actively exploited?
Security researchers have published details about this vulnerability. Monitor CISA's KEV catalog and your threat intelligence feeds for exploitation status updates. Apply the patch regardless of current exploitation status.
How do I fix CVE-2026-35393?
The primary remediation is to apply the vendor-supplied patch for Go. If patching is not immediately possible, implement temporary compensating controls such as WAF rules or network access restrictions while the patch is scheduled.
Does CVE-2026-35393 affect my organization?
If your organization uses Go, you should assess your exposure immediately. Check the specific affected versions in the CVE advisory and run an automated vulnerability scan to confirm your exposure status.