Executive Summary

This vulnerability in LearnHouse affects the course thumbnail upload endpoint, where file-type restrictions are only enforced on the client side. An attacker can bypass these checks by uploading a valid image to pass client validation, then intercepting and replacing the file content with malicious files such as SVGs containing JavaScript or server-side scripts (.py, .php). Because the server does not validate the file content and stores these files in a web-accessible location, this leads to stored cross-site scripting (XSS) attacks and potential remote code execution (RCE). This means attackers can execute arbitrary JavaScript in users' browsers or possibly run malicious code on the server, compromising security. [1, 2]

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Impact Analysis

This vulnerability can impact you by allowing attackers to upload malicious files that execute arbitrary JavaScript in users' browsers (Stored XSS), leading to session theft, phishing, and credential harvesting. Additionally, if server-side scripts are uploaded and executed, attackers could gain remote code execution (RCE) capabilities, potentially compromising the entire server, exfiltrating data, and moving laterally within your network. This poses significant security risks to both users and the system. [1, 2]

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Detection Guidance

This vulnerability can be detected by monitoring and inspecting network traffic to the endpoint `/api/v1/courses/{course_id}/thumbnail` for suspicious file uploads that bypass client-side restrictions. Specifically, look for PUT requests where the uploaded file content does not match the expected image MIME types (PNG, JPG/JPEG) but instead contains SVG files with embedded JavaScript or server-side scripts like .py or .php. Commands to detect this could include using tools like `tcpdump` or `Wireshark` to capture HTTP PUT requests, and then extracting and analyzing the multipart payloads for unexpected file types or suspicious content. For example, using `tcpdump` to capture traffic on port 80 or 443: `tcpdump -i any -s 0 -w capture.pcap port 80 or port 443` followed by inspecting the capture with Wireshark or using `strings` and `grep` on the capture file to find suspicious payloads. Additionally, scanning the server's upload directories for files with unexpected extensions or embedded scripts can help detect exploitation. [1, 2]

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Mitigation Strategies

Immediate mitigation steps include implementing proper server-side validation of uploaded files to verify the actual file content and MIME type using magic bytes rather than relying solely on client-side checks. Restrict uploads strictly to allowed image formats (e.g., PNG, JPG/JPEG) and reject any files that do not conform. Additionally, sanitize and validate file names and contents before storage. If possible, disable or restrict execution permissions on the upload directories to prevent execution of uploaded scripts. Monitoring and blocking suspicious upload attempts at the network or application firewall level can also help. Since the vendor has not provided a patch, applying these controls and reviewing uploaded files for malicious content are critical to reduce risk. [1, 2]

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