CVE-2025-65637 is a denial-of-service (DoS) vulnerability found in the Go logging library github.com/sirupsen/logrus, specifically in the Entry.Writer() method. The vulnerability occurs when logging a single-line payload exceeding 64KB without newline characters. Internally, logrus uses Go's bufio.Scanner to read input, which has a maximum token size limit. When this limit is exceeded, the scanner returns a "token too long" error, causing the writer pipe to close unexpectedly. This closure renders the Writer() unusable, leading to application unavailability due to the inability to log further data. The affected versions include all versions prior to 1.8.3, 1.9.0, and 1.9.2. The issue was resolved in versions 1.8.3, 1.9.1, and 1.9.3 and later by implementing input chunking, allowing the writer to continue functioning even if an error occurs during scanning. This vulnerability does not require authentication or user interaction to exploit, but the attacker must be able to supply a large single-line log entry, which may be feasible in applications that log user-controlled input. No known exploits have been reported in the wild as of the publication date. The vulnerability primarily impacts availability by causing denial-of-service conditions in applications relying on vulnerable logrus versions for logging.

For European organizations, the primary impact of CVE-2025-65637 is application unavailability due to denial-of-service conditions triggered by malformed log entries. Organizations using Go applications that incorporate vulnerable versions of logrus for logging could experience service disruptions if attackers supply large single-line payloads to the logging interface. This could affect web services, APIs, or backend systems that rely on logrus for structured logging. The impact is particularly significant for critical infrastructure, financial services, and technology companies where uptime and reliability are paramount. Additionally, the inability to log further events may hinder incident response and forensic investigations during or after an attack. While the vulnerability does not directly compromise confidentiality or integrity, the loss of availability can disrupt business operations and damage reputation. European organizations with automated logging pipelines or those exposing logging interfaces to external inputs are at higher risk. The absence of known exploits reduces immediate risk, but the vulnerability should be addressed proactively to prevent future exploitation.

To mitigate CVE-2025-65637, European organizations should: 1) Upgrade all instances of logrus to versions 1.8.3, 1.9.1, 1.9.3, or later, where the vulnerability is fixed. 2) Audit application code to identify where Entry.Writer() is used and ensure that logging inputs are sanitized or limited in size, especially avoiding large single-line payloads without newline characters. 3) Implement input validation or rate limiting on user inputs that may be logged to prevent attackers from sending excessively large log entries. 4) Monitor application logs and error reports for signs of bufio.Scanner errors or unexpected writer closures that may indicate attempted exploitation. 5) Incorporate fallback logging mechanisms or alerting to detect and respond to logging failures promptly. 6) Educate developers about safe logging practices, emphasizing the risks of logging untrusted input without size or format constraints. 7) For critical systems, consider deploying runtime application self-protection (RASP) or Web Application Firewalls (WAFs) that can detect and block anomalous payloads targeting logging interfaces.