Executive brief
Apache HttpComponents Client is an HTTP client library used by applications to communicate with web services. When the library encounters an invalid or unsupported Content-Encoding header in an HTTP response, it fails to properly release the network connection back to its connection pool. An attacker can exploit this by sending specially crafted responses to exhaust the connection pool, causing the application to run out of available connections and resulting in a denial of service.
Technical details
The vulnerability is a resource leak in Apache HttpComponents Client's classic I/O model (the async I/O model is unaffected). The root cause is improper exception handling in the Content-Encoding decoding logic—when the library encounters an invalid or unsupported Content-Encoding header value in an HTTP response, the connection is not returned to the connection manager's pool for reuse. An attacker on the network can trigger this leak by sending responses with malformed Content-Encoding headers, causing connections to accumulate and eventually exhaust the pool. This leads to denial of service as legitimate requests are unable to obtain connections. The fix is available in version 5.6.3 and later.
Affected products
- Apache HttpComponents Client 5.0-alpha1 through 5.6.2
Timeline
- 2026-07-31: disclosed: Vulnerability published to GitHub Advisory Database
- 2026-07-31: patched: Fix released in version 5.6.3