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Q1What is this vulnerability? (Essence + Consequences)
🚨 **Essence**: Apache Flink REST handler allows arbitrary file upload via directory traversal. 📉 **Consequences**: Attackers can write files to ANY location on the local filesystem via malicious HTTP headers.…
🛡️ **CWE**: CWE-23 (Path Traversal). 🔍 **Flaw**: The REST API handler fails to sanitize filenames in uploaded files. It allows `../` sequences to escape the intended upload directory.
👮 **Privileges**: Files are written with the privileges of the Flink process. 📂 **Data Access**: Can overwrite critical system files, config files, or deploy malicious scripts (JSP/Shell).…
⚡ **Threshold**: LOW. 🔑 **Auth**: Often requires NO authentication or minimal access to the REST API. ⚙️ **Config**: Exploits the default REST interface behavior. No complex setup needed.
Q6Is there a public Exp? (PoC/Wild Exploitation)
🔓 **Public Exp?**: YES. 📂 **PoCs**: Multiple GitHub repositories exist (e.g., `QmF0c3UK/CVE-2020-17518`, `rakjong/Flink-CVE-2020-17518-getshell`). 🌐 **Automation**: Included in Nuclei templates for mass scanning.
Q7How to self-check? (Features/Scanning)
🔍 **Check**: Scan for Apache Flink REST API endpoints. 📝 **Test**: Send crafted HTTP headers with `../` in filename fields. 📊 **Tool**: Use Nuclei or custom scripts to detect the directory traversal response.
Q8Is it fixed officially? (Patch/Mitigation)
🩹 **Fixed?**: YES. 📢 **Official**: Apache issued security advisories (FLINK-20875). 🚀 **Patch**: Upgrade to patched versions (e.g., 1.10.3+ or latest stable release). Check official Apache Flink security page.
Q9What if no patch? (Workaround)
🛑 **No Patch?**: 1. Block REST API access via Firewall/WAF. 2. Disable the REST endpoint if not needed. 3. Implement strict input validation on the filename parameter in the HTTP request.
Q10Is it urgent? (Priority Suggestion)
🔥 **Urgency**: HIGH. 🚨 **Priority**: P1. ⚠️ **Reason**: Easy exploitation, high impact (RCE), and public PoCs available. Immediate patching or mitigation is critical for exposed Flink instances.