CVE-2026-21697 is a race condition vulnerability classified under CWE-362 affecting the axios4go Go HTTP client library prior to version 0.6.4. The root cause is the unsynchronized mutation of a global shared HTTP client instance named `defaultClient`. During concurrent HTTP requests executed via multiple goroutines (e.g., using GetAsync or PostAsync methods), the shared client's configuration properties—specifically `Transport`, `Timeout`, and `CheckRedirect`—are modified without proper synchronization mechanisms such as mutexes or atomic operations. This leads to a classic race condition where concurrent requests may interfere with each other's configurations, causing unpredictable behavior. For example, one request may inadvertently use another request's proxy settings or timeout values. This is particularly dangerous in scenarios where different requests require distinct proxy configurations or handle sensitive authentication credentials, tokens, or API keys. The vulnerability can result in confidentiality breaches (e.g., leaking tokens to unintended endpoints), integrity issues (e.g., requests being redirected or altered), and availability problems if requests fail due to misconfiguration. The vulnerability is remotely exploitable without authentication or user interaction, as it occurs during normal concurrent usage of the library. The CVSS 4.0 score of 8.2 reflects the high impact and ease of exploitation. The issue was publicly disclosed on January 7, 2026, and fixed in axios4go version 0.6.4. No known exploits have been reported in the wild to date, but the risk remains significant for affected deployments.

For European organizations, the impact of CVE-2026-21697 can be substantial, especially for those developing or deploying Go-based applications that rely on axios4go for HTTP client functionality with concurrent request patterns. The vulnerability can lead to unauthorized disclosure of sensitive data such as authentication tokens, API keys, or session credentials if requests inadvertently share or leak configuration details. This can facilitate further attacks like account takeover, data exfiltration, or lateral movement within networks. Additionally, misrouted or altered requests due to race conditions can disrupt service availability or cause data integrity issues. Organizations in sectors handling sensitive personal data (e.g., finance, healthcare, government) are particularly at risk due to regulatory compliance requirements under GDPR and other data protection laws. The vulnerability's remote exploitability without authentication increases the threat surface, potentially affecting cloud-native applications, microservices, and APIs widely used across European enterprises. Failure to patch could also damage organizational reputation and lead to legal liabilities in case of data breaches.

European organizations should immediately upgrade axios4go to version 0.6.4 or later, where the race condition has been fixed by introducing proper synchronization around the shared HTTP client configuration. For applications where upgrading is not immediately feasible, developers should implement temporary mitigations such as avoiding concurrent use of the global `defaultClient` instance by creating isolated HTTP client instances per request or goroutine. Additionally, review and refactor code to ensure that HTTP client configurations are immutable or properly synchronized when shared. Conduct thorough code audits to identify any other shared mutable state that could cause similar race conditions. Implement runtime monitoring and logging to detect anomalous request behaviors or configuration changes during concurrent execution. Finally, enforce strict access controls and secure handling of sensitive tokens and credentials to minimize impact if leakage occurs.