CVE-2025-65015: Hsiaoming Joserfc DOS Vulnerability

CVE-2025-65015 Overview

CVE-2025-65015 is a resource exhaustion vulnerability affecting joserfc, a Python library that provides implementations of several JSON Object Signing and Encryption (JOSE) standards. The vulnerability exists in how the library handles exception messages when processing oversized JWT tokens. When ExceededSizeError exceptions are raised, they embed non-decoded JWT token parts in the exception message, potentially causing Python logging systems to process and store arbitrarily large, attacker-controlled payloads.

Critical Impact

Attackers can exploit misconfigured web servers to send arbitrarily large bearer tokens, causing Python logging systems and diagnostic tools to process extremely large log messages containing full JWT payloads, leading to denial of service through resource exhaustion.

Affected Products

• hsiaoming joserfc versions 1.3.3 to before 1.3.5
• hsiaoming joserfc versions 1.4.0 to before 1.4.2

Discovery Timeline

• 2025-11-18 - CVE-2025-65015 published to NVD
• 2026-01-15 - Last updated in NVD database

Technical Details for CVE-2025-65015

Vulnerability Analysis

This vulnerability is classified as CWE-770 (Allocation of Resources Without Limits or Throttling). The core issue stems from the library's error handling behavior when processing oversized JWT tokens. When the joserfc.jwt.decode() operation encounters a JWT that exceeds size limits, it raises a joserfc.errors.ExceededSizeError() exception with the full JWT payload embedded in the exception message.

The attack scenario requires a web application environment where the production-grade web server is either misconfigured or absent, allowing attackers to bypass typical HTTP request size limitations. In such configurations, an attacker can craft HTTP requests with arbitrarily large bearer tokens in the authorization headers. When the Python application attempts to decode these tokens using joserfc, the library loads the entire payload into memory before performing size validation checks.

At this stage, the library cannot prevent or reject the oversized payload since it's already fully loaded into memory. The exception message containing the full payload then propagates to Python logging frameworks or diagnostic tools like Sentry, which may attempt to process, format, and store these extremely large log entries.

Root Cause

The root cause is the design decision to embed raw JWT token parts directly into exception messages without truncation or sanitization. When the library validates claims and signature payload sizes, it raises exceptions that include the complete payload content. Since memory allocation occurs before validation, the library lacks the ability to prevent resource consumption once an oversized token is received. This creates a condition where attacker-controlled data of arbitrary size flows directly into logging infrastructure.

Attack Vector

The attack leverages network-accessible endpoints that process JWT authentication tokens. An attacker targets applications where:

1. The web server layer does not enforce strict HTTP header size limits
2. The Python application uses joserfc for JWT decoding
3. The application logs exceptions or uses diagnostic tools that capture error details

By sending HTTP requests with extremely large bearer tokens in the Authorization header, the attacker triggers the size validation exception. The exception message, containing the full attacker-supplied payload, then flows into logging systems. This can exhaust disk space, memory, or processing capacity of logging infrastructure, potentially causing denial of service to both the application and downstream log aggregation systems.

The vulnerability allows an attacker to amplify their impact by targeting the logging infrastructure rather than the application directly, potentially affecting monitoring capabilities and incident response.

Detection Methods for CVE-2025-65015

Indicators of Compromise

• Unusually large HTTP Authorization headers in web server access logs
• Abnormally large log entries containing base64-encoded or JWT-formatted data
• Rapid disk space consumption in log directories
• Memory exhaustion events in Python application processes or logging services
• Sentry or similar diagnostic tools reporting processing failures due to oversized payloads

Detection Strategies

• Monitor HTTP request header sizes at the web server or load balancer level for anomalously large Authorization headers
• Implement alerting on log file growth rates and sizes to detect sudden spikes
• Configure application performance monitoring to track memory usage patterns in Python processes handling authentication
• Review joserfc version deployments across the environment to identify vulnerable installations

Monitoring Recommendations

• Set up log aggregation alerts for entries exceeding normal size thresholds
• Monitor Python application resource consumption metrics during authentication flows
• Implement rate limiting and size restrictions at the web application firewall layer
• Track disk I/O and storage utilization on log storage systems

How to Mitigate CVE-2025-65015

Immediate Actions Required

• Upgrade joserfc to version 1.3.5 or 1.4.2 depending on your current version branch
• Configure web server or reverse proxy to enforce HTTP header size limits
• Implement request size validation at the application gateway before requests reach the Python application
• Review and configure log rotation policies to prevent disk exhaustion from large log entries

Patch Information

The vulnerability has been addressed in joserfc versions 1.3.5 and 1.4.2. Users on the 1.3.x branch should upgrade to version 1.3.5, while users on the 1.4.x branch should upgrade to version 1.4.2. Security patches are available through the GitHub Release Version 1.3.5 and GitHub Release Version 1.4.2. Technical details of the fix can be reviewed in the GitHub Security Advisory GHSA-frfh-8v73-gjg4.

Workarounds

• Deploy a production-grade web server (nginx, Apache) with properly configured request header size limits in front of Python applications
• Implement middleware to validate and reject oversized Authorization headers before they reach JWT processing code
• Configure logging frameworks to truncate exception messages above a certain threshold
• Use structured logging with size limits on individual field values

# Example nginx configuration to limit header buffer size
# Add to http or server block
large_client_header_buffers 4 8k;
client_header_buffer_size 1k;