CVE-2025-53890 Overview
CVE-2025-53890 is an unsafe JavaScript evaluation vulnerability in pyLoad, an open-source Download Manager written in pure Python. The vulnerability exists in pyLoad's CAPTCHA processing code and allows unauthenticated remote attackers to execute arbitrary code in the client browser and potentially the backend server. This code injection vulnerability (CWE-94) requires no user interaction or authentication and can result in session hijacking, credential theft, and full system remote code execution.
Critical Impact
Unauthenticated attackers can achieve remote code execution on both client browsers and backend servers through malicious CAPTCHA payloads, potentially leading to complete system compromise.
Affected Products
- pyLoad versions prior to 0.5.0b3.dev89
- pyLoad installations without commit 909e5c97885237530d1264cfceb5555870eb9546
Discovery Timeline
- 2025-07-15 - CVE-2025-53890 published to NVD
- 2025-07-15 - Last updated in NVD database
Technical Details for CVE-2025-53890
Vulnerability Analysis
The vulnerability stems from unsafe JavaScript evaluation within pyLoad's CAPTCHA processing functionality. When pyLoad processes CAPTCHA challenges, the application fails to properly sanitize or validate input before passing it to JavaScript evaluation contexts. This allows attackers to inject malicious JavaScript code that executes within the application's security context.
The attack surface is particularly dangerous because it requires no authentication—any remote attacker can craft malicious requests targeting the CAPTCHA processing endpoint. The vulnerability impacts both the client-side browser environment (enabling cross-site scripting attacks) and potentially the server-side backend (enabling remote code execution), depending on how the evaluated code is processed throughout the application stack.
Successful exploitation can lead to session hijacking through cookie theft, credential harvesting via injected keyloggers or phishing overlays, and in worst-case scenarios, full remote code execution on the server hosting pyLoad.
Root Cause
The root cause is improper input validation (CWE-94 - Improper Control of Generation of Code) in the CAPTCHA processing logic. The application uses unsafe JavaScript evaluation functions such as eval() or similar constructs to process CAPTCHA-related data without proper sanitization, allowing attacker-controlled input to be executed as code.
Attack Vector
The attack is network-based and can be executed remotely without any authentication or user interaction. An attacker can craft malicious CAPTCHA payloads containing JavaScript code and submit them to the vulnerable pyLoad instance. The server processes these payloads through the unsafe evaluation mechanism, executing the attacker's code.
The attack flow involves sending crafted requests to pyLoad's CAPTCHA handling endpoints, where the malicious payload is processed through the vulnerable code path. The vulnerability's network accessibility and lack of authentication requirements make it highly exploitable in exposed pyLoad installations.
Detection Methods for CVE-2025-53890
Indicators of Compromise
- Unusual JavaScript execution patterns in pyLoad logs or associated web server logs
- Unexpected outbound network connections from the pyLoad server
- Modified session tokens or evidence of session hijacking
- Presence of injected scripts or suspicious CAPTCHA-related requests in access logs
Detection Strategies
- Monitor web application logs for malformed or suspicious CAPTCHA-related requests containing JavaScript code patterns
- Implement Web Application Firewall (WAF) rules to detect and block JavaScript injection attempts in request parameters
- Deploy network intrusion detection systems (NIDS) with signatures for code injection payloads
- Review pyLoad application logs for error messages related to script evaluation failures
Monitoring Recommendations
- Enable verbose logging for pyLoad's CAPTCHA processing module
- Configure alerting for any eval-related errors or unexpected script execution
- Monitor server resource utilization for anomalies that may indicate active exploitation
- Implement egress filtering and monitor for suspicious outbound connections from the pyLoad host
How to Mitigate CVE-2025-53890
Immediate Actions Required
- Upgrade pyLoad to version 0.5.0b3.dev89 or later immediately
- If immediate patching is not possible, restrict network access to pyLoad to trusted networks only
- Review logs for any evidence of prior exploitation attempts
- Consider temporarily disabling CAPTCHA-related functionality until patching is complete
Patch Information
The vulnerability has been addressed in commit 909e5c97885237530d1264cfceb5555870eb9546, which is included in pyLoad version 0.5.0b3.dev89. The patch can be reviewed in GitHub Pull Request #4586 and the GitHub Commit. Additional details are available in the GitHub Security Advisory GHSA-8w3f-4r8f-pf53.
Workarounds
- Restrict network access to pyLoad instances using firewall rules to limit exposure to trusted IP addresses only
- Place pyLoad behind a reverse proxy with request filtering capabilities to block suspicious payloads
- Implement Content Security Policy (CSP) headers to mitigate client-side JavaScript injection impacts
- Consider running pyLoad in an isolated environment or container to limit the impact of potential exploitation
# Example: Restrict pyLoad access to local network only using iptables
iptables -A INPUT -p tcp --dport 8000 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 8000 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
