CVE-2024-7708: Resource Exhaustion via HTTP Connection Buffer Leak in Eclipse Jetty

The vulnerability exists within the HTTP connection handling implementation of Eclipse Jetty, specifically inside the HttpConnection.fillRequestBuffer() method. Eclipse Jetty utilizes an internal buffer recycling mechanism to minimize memory fragmentation and reduce garbage collection overhead during high-throughput network operations. This system depends on the proper allocation and subsequent reclamation of buffer instances.

In affected versions, network conditions or specific application behaviors can cause the connection layer to allocate a buffer but abandon it without triggering the appropriate release cycle. The attack surface is exposed to any remote client capable of establishing a TCP connection to the Jetty HTTP connector. No authentication or specific privileges are required to reach the vulnerable code path.

Because the resource leak occurs within the server's core network transport layer, the degradation is cumulative. Every instance of an abandoned buffer reduces the available capacity of the shared buffer pool. Under sustained or repeated invocation, the pool is depleted, preventing the server from accepting new connections and ultimately triggering an unrecoverable system failure.

The root cause of CVE-2024-7708 lies in the logical control flow of org.eclipse.jetty.server.HttpConnection.fillRequestBuffer(). To optimize memory performance, Jetty utilizes RetainableByteBuffer objects obtained from a central RetainableByteBufferPool. The connection handler is designed to acquire a buffer from this pool, read incoming network packets via the underlying endpoint, and release the buffer once parsing is complete or when the connection state transitions.

A memory leak occurs when the server attempts to read from a connection's endpoint and the operation returns zero bytes, or when a non-I/O exception is thrown during processing. For example, if a client sends request headers indicating a payload is forthcoming (such as a Content-Length or chunked transfer encoding) but the connection is closed or stalled before payload bytes arrive, the endpoint read returns zero. In this state, the original implementation branched past the cleanup code, failing to call releaseRequestBuffer().

Furthermore, the error handling framework in the vulnerable versions was overly restrictive. The try-catch block surrounding the endpoint fill operation only caught IOException. If an unhandled RuntimeException or other error occurred during packet decoding or header parsing, the execution skipped the recovery block entirely, leaving the acquired RetainableByteBuffer permanently allocated in the pool.

An analysis of the vulnerability patch in HttpConnection.java clarifies the precise location of the logical omission. The code block below compares the vulnerable handling with the corrected resource release logic.

// Vulnerable Flow vs Patched Flow in HttpConnection.java
 
                // Prior logic: only added bytes if filled > 0, otherwise branched to atEOF() without releasing buffer
                if (filled > 0)
                {
                    bytesIn.add(filled);
                }
                else
                {
                    if (filled < 0)
                        _parser.atEOF();
                    // FIX: Explicitly release the request buffer back to the pool if no bytes were read
                    releaseRequestBuffer();
                }
 
                if (LOG.isDebugEnabled())
                    LOG.debug("{} filled {} {}", this, filled, _retainableByteBuffer);
 
                return filled;
            }
            // FIX: Broadened catch block from IOException to Throwable to intercept all runtime exceptions
            catch (Throwable x)
            {
                if (LOG.isDebugEnabled())
                    LOG.debug("Unable to fill from endpoint {}", getEndPoint(), x);
                _parser.atEOF();
                // FIX: Ensure cleanup occurs and resource is released on any thrown exception
                if (_retainableByteBuffer != null)
                {
                    _retainableByteBuffer.clear();
                    releaseRequestBuffer();
                }
                return -1;
            }

The original implementation failed to address the scenario where filled == 0. In that condition, the connection did not call releaseRequestBuffer(), leading to an accumulation of retained buffers in memory. The corrected code ensures that any non-positive read outcome triggers an immediate release of the buffer. By broadening the exception handler to catch Throwable instead of just IOException, the patch prevents memory leaks stemming from unexpected application-level runtime failures.

Exploitation of CVE-2024-7708 requires an attacker to initiate HTTP transactions that result in empty read operations or early connection termination before the request body is transmitted. The most reliable method to achieve this condition involves using the Expect: 100-Continue HTTP header.

In a standard scenario, a client sends an HTTP request header containing Expect: 100-Continue to verify if the server will accept the payload. If the requested URL does not exist or triggers an immediate validation failure, Jetty generates a 404 Not Found or 400 Bad Request response prior to reading the request body. If the connection is then terminated, the server attempts to parse the remaining request envelope, resulting in zero bytes read and triggering the leak condition.

An attacker can automate this interaction by establishing multiple parallel TCP connections, sending headers containing Expect: 100-Continue to an invalid endpoint, and closing the write end of the socket. Because each transaction permanently leaks one RetainableByteBuffer in the shared RetainableByteBufferPool, executing this pattern at scale will steadily exhaust the server heap, leading to total service degradation without requiring high bandwidth.

The impact of this vulnerability is categorized as a high-severity Denial of Service (DoS). Because the leak targets the heap-allocated direct byte buffers used for network transport, memory exhaustion affects the entire Java Virtual Machine (JVM) hosting the Jetty container.

Once the RetainableByteBufferPool is fully depleted, the application server is unable to process any new incoming network requests. This results in standard socket connection timeouts for legitimate clients. If the leak continues, the JVM eventually throws an OutOfMemoryError (OOM), leading to abnormal termination of the server process.

This vulnerability has no direct impact on data confidentiality or integrity. It cannot be leveraged to execute arbitrary code or bypass security authorization controls. However, due to the ease of triggering the leak and the lack of authentication requirements, the potential for service disruption is significant.

The definitive resolution for this vulnerability is to upgrade the Eclipse Jetty deployment to a patched release. All deployments using Jetty 10 or Jetty 11 should be transitioned to the corrected versions.

If an immediate upgrade is not feasible, operators can apply temporary configuration mitigations to reduce exposure. Implementing reverse proxies or Web Application Firewalls (WAFs) to filter or drop requests containing Expect: 100-Continue headers can mitigate the primary vector. Additionally, reducing the maximum idle timeouts on Jetty connectors limit the lifetime of stalled connections, reducing the rate of buffer accumulation.

Administrators should also configure JVM monitoring tools to track the utilization of Direct ByteBuffers and the JVM heap. A continuous upward trend in buffer allocation that does not return to baseline during idle periods is a strong indicator of active exploitation or unchecked memory leaks.