CVE-2023-39326: Golang Go DoS Vulnerability

CVE-2023-39326 Overview

A vulnerability exists in Go's net/http package that allows a malicious HTTP sender to exploit chunk extensions in the chunked transfer encoding to cause receivers to read significantly more bytes from the network than are present in the actual body content. This can lead to resource exhaustion scenarios where servers automatically read up to approximately 1GiB of data when handlers fail to read the entire body of a request.

Critical Impact

Malicious HTTP clients can exploit this vulnerability to force servers into reading excessive amounts of data, potentially causing denial of service through resource exhaustion.

Affected Products

• Golang Go (versions prior to patched releases)

Discovery Timeline

• 2023-12-06 - CVE-2023-39326 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-39326

Vulnerability Analysis

This vulnerability exploits a feature of HTTP's chunked transfer encoding called chunk extensions. Chunk extensions are a little-used HTTP feature that permit including additional metadata in a request or response body sent using the chunked encoding format. Under normal circumstances, the net/http chunked encoding reader discards this metadata without issue.

The vulnerability arises because a malicious sender can insert large metadata segments with each byte of actual data transferred. Since the chunk reader discards this metadata but still processes it from the network, there is a significant disparity between the bytes read from the network and the actual body content delivered to the application. This amplification effect can force a server to consume substantial network bandwidth and memory resources.

The attack is particularly effective when HTTP handlers fail to read the entire body of a request, as the server will automatically attempt to drain the remaining body content, potentially reading up to 1GiB of crafted malicious data.

Root Cause

The root cause lies in the net/http chunked encoding reader's handling of chunk extensions. The implementation did not enforce any ratio limit between the encoded bytes (including chunk extensions) and the actual body content. This allowed attackers to craft requests where the vast majority of transmitted data consisted of discardable chunk extension metadata rather than actual body content, leading to resource amplification attacks.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker constructs malicious HTTP requests or responses using chunked transfer encoding with excessively large chunk extensions attached to each data chunk. When the target server processes these requests, it must read all the network data even though most of it is discarded metadata.

The attack flow involves:

1. Attacker sends HTTP request/response with chunked encoding
2. Each chunk contains minimal actual data but massive chunk extension metadata
3. Server's net/http reader processes entire stream from network
4. Metadata is discarded but network/memory resources are consumed
5. If handler doesn't fully read body, server attempts to drain remaining content automatically

Detection Methods for CVE-2023-39326

Indicators of Compromise

• Unusual network bandwidth consumption on HTTP endpoints without corresponding application data processing
• High memory usage in Go applications handling HTTP requests
• Slow HTTP request processing times with minimal actual data transfer
• Chunked transfer encoded requests with abnormally large chunk sizes relative to actual content

Detection Strategies

• Monitor for HTTP requests using chunked transfer encoding with disproportionate network traffic to body content ratios
• Implement network traffic analysis to detect requests with excessive chunk extension metadata
• Set up application performance monitoring to detect unusual resource consumption patterns in Go HTTP handlers
• Review HTTP access logs for requests with abnormally long processing times relative to content length

Monitoring Recommendations

• Deploy network-level monitoring for chunked transfer encoding abuse patterns
• Configure resource usage alerts for Go applications serving HTTP traffic
• Implement request timeout thresholds to limit exposure to slow-drip attacks
• Monitor connection duration and bytes transferred metrics per request

How to Mitigate CVE-2023-39326

Immediate Actions Required

• Update Go installations to patched versions that include the fix from Go CL #547335
• Rebuild and redeploy all Go applications using the updated runtime
• Review HTTP handler implementations to ensure they properly read or explicitly discard request bodies
• Consider implementing request body size limits at the reverse proxy or load balancer level

Patch Information

The Go development team has addressed this vulnerability by modifying the chunk reader to produce an error when the ratio of real body content to encoded bytes becomes too small. This prevents attackers from exploiting the amplification effect.

For detailed patch information, refer to:

• Go.dev CL #547335 - The code change addressing this vulnerability
• Go.dev Issue #64433 - Original issue tracking
• Go.dev Vulnerability Advisory - Official vulnerability advisory

Linux distribution users should check for updates:

• Fedora Package Announcement

Workarounds

• Implement request body size limits at the reverse proxy or load balancer level to restrict maximum request sizes
• Configure HTTP handlers to explicitly read and discard request bodies rather than relying on automatic draining
• Deploy web application firewalls (WAF) with rules to detect and block requests with suspicious chunked encoding patterns
• Set aggressive timeouts for HTTP request processing to limit resource exhaustion window

# Example: Configure nginx as reverse proxy with body size limits
# Add to nginx.conf or server block
client_max_body_size 10m;
client_body_timeout 30s;
proxy_read_timeout 60s;