CVE-2026-85720 in AsyncHttpClient
Summary
by MITRE • 09/17/2026
The AsyncHttpClient (AHC) library allows Java applications to easily execute HTTP requests and asynchronously process HTTP responses. From 2.0.0 until 2.16.1 and 3.0.12, a request using an HTTP proxy to reach an HTTPS origin can expose preemptive origin credentials because NettyRequestFactory and NettyRequestSender.sendRequestWithNewChannel attach Authorization to the plaintext CONNECT request before the TLS tunnel exists. Basic or Digest credentials and per-connection NTLM, Kerberos, or SPNEGO tokens intended for the origin are therefore visible to the proxy and to observers on the client-to-proxy hop. The tunneled request still receives origin Authorization after the tunnel is established, while Proxy-Authorization remains on CONNECT for its intended proxy recipient. This issue is fixed in versions 2.16.1 and 3.0.12.
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Analysis
by VulDB Data Team • 09/17/2026
The AsyncHttpClient library serves as a critical component in Java-based ecosystems, enabling developers to execute HTTP requests asynchronously with high performance and non-blocking I/O capabilities. While the library provides robust functionality for handling complex network interactions, including support for various authentication schemes such as Basic, Digest, NTLM, Kerberos, and SPNEGO, it contained a significant security flaw affecting versions from 2.0.0 up to 2.16.1 and version 3.0.12 of the library. This vulnerability specifically manifests in scenarios where an application utilizes an HTTP proxy to establish a connection with an HTTPS origin server. The core issue lies within the internal implementation details, particularly involving the NettyRequestFactory and NettyRequestSender classes which are responsible for managing the underlying network channels and sending requests.
The technical flaw stems from the order of operations during the establishment of a tunnel through an HTTP proxy to reach an HTTPS destination. When connecting via a proxy, the client must first send a CONNECT request over plaintext HTTP to instruct the proxy to open a TCP connection to the target host on port 443. Once this tunnel is established, TLS negotiation occurs within that encrypted channel. However, in the affected versions of AsyncHttpClient, the library incorrectly attaches the Authorization header intended for the origin server directly to this initial plaintext CONNECT request before the TLS tunnel exists. This means that credentials such as Basic or Digest authentication tokens, as well as per-connection NTLM, Kerberos, or SPNEGO security context tokens meant exclusively for the final destination are transmitted in clear text over the network segment between the client and the proxy server.
This behavior results in a severe exposure of sensitive preemptive origin credentials to unauthorized parties. Any observer positioned on the network path between the client and the HTTP proxy can intercept these plaintext headers, thereby gaining access to user names, password hashes, or active session tokens for the target service. This constitutes a classic case of credential leakage due to improper handling of authentication data during protocol negotiation phases. The vulnerability aligns with CWE-319, which describes the use of cleartext transmission of sensitive information, and falls under ATT&CK technique T1078, specifically relating to valid accounts being used inappropriately or credentials being intercepted before they can be secured by encryption. Although the library correctly sends a Proxy-Authorization header for authenticating with the proxy itself, it erroneously includes origin-specific Authorization headers in the same plaintext phase, violating the principle of least privilege and secure channel establishment.
The operational impact of this vulnerability is substantial, particularly for applications that rely on preemptive authentication to optimize performance by avoiding additional round-trips during TLS handshake negotiations. By exposing these credentials, attackers can perform credential stuffing attacks, session hijacking, or unauthorized access to backend services if the intercepted tokens are not rotated frequently enough. Even though the tunneled request itself receives the correct origin Authorization header after the tunnel is established and encrypted, the initial exposure remains a critical risk because network eavesdropping on the client-to-proxy hop does not require breaking TLS encryption; it only requires access to the unencrypted HTTP traffic preceding the CONNECT response. This makes exploitation relatively straightforward for any attacker with local network access or those capable of performing man-in-the-middle attacks at intermediate routing points.
To mitigate this risk, organizations using AsyncHttpClient must upgrade immediately to version 2.16.1 or later, and version 3.0.12 or later, where the issue has been resolved by ensuring that origin-specific Authorization headers are not attached to plaintext CONNECT requests. In addition to upgrading, developers should review their network architecture to ensure that client-to-proxy traffic is protected using mutual TLS or other encryption mechanisms if possible, although this does not replace the need for patching the library itself. Security teams should also audit logs for any potential exposure of credentials during the window between vulnerability discovery and remediation, particularly monitoring for unusual authentication failures or successful logins from unexpected IP addresses that might indicate exploitation attempts. Regular security assessments and dependency scanning tools can help identify such vulnerabilities in third-party libraries before they are exploited in production environments.