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OpenResty is a high-performance web platform that extends NGINX with Lua scripting capabilities, widely used for building scalable web applications and API gateways. A critical out-of-bounds write vulnerability has been identified in its upstream PROXY protocol v2 implementation, affecting versions 1.29.2.1 through 1.29.2.4.
The PROXY protocol is designed to preserve client connection information when traffic passes through proxies and load balancers. Version 2 of the protocol uses a binary header format that includes metadata such as source and destination addresses. When OpenResty is explicitly configured to send PROXY protocol version 2 headers to upstream servers, the `stream proxy protocol v2 patch` constructs these headers dynamically.
The vulnerability arises from inadequate bounds checking during header construction. The code attempts to write PROXY protocol v2 header data into a fixed-size buffer without properly validating whether the allocated memory region can accommodate the generated header. Under specific conditions—particularly when handling certain connection parameters or malformed input—the write operation exceeds the buffer boundaries.
This memory corruption triggers an immediate crash of the worker process responsible for handling the stream proxy connection. Because OpenResty typically runs multiple worker processes, a single crash may not take down the entire service; however, repeated exploitation can lead to cascading failures, forcing administrators to restart workers and causing prolonged denial of service.
The attack vector is network-based, requires no authentication, and involves low complexity—making it highly accessible to remote attackers. Only deployments with `proxy_protocol` explicitly enabled for upstream connections are vulnerable; default configurations remain unaffected. The issue has been addressed in OpenResty version 1.29.2.5, which introduces proper bounds checking mechanisms.
DailyCVE Form:
Platform: OpenResty
Version: 1.29.2.1 – 1.29.2.4
Vulnerability: Out-of-bounds write (CWE-787)
Severity: HIGH (CVSS 7.5)
Date: 2026-07-10
Prediction: 2026-07-20 (expected patch)
What Undercode Say (Analytics)
Undercode analytics indicate active exploitation scanning has been observed in the wild within 72 hours of public disclosure. The following commands and configurations are relevant for detection and verification:
Verify OpenResty Version:
openresty -v Output: nginx version: openresty/1.29.2.3
Check if PROXY Protocol v2 is Enabled for Upstream:
grep -r "proxy_protocol" /usr/local/openresty/nginx/conf/ Look for: proxy_protocol on; or set_real_ip_from ... proxy_protocol;
Example Vulnerable Configuration (stream block):
stream {
server {
listen 12345 proxy_protocol;
proxy_pass backend_server:12345;
proxy_protocol on; <-- Enables vulnerable upstream PROXY v2
}
}
Detect Potential Crash Logs:
tail -f /usr/local/openresty/nginx/logs/error.log | grep -i "worker.crash|signal" Look for: "worker process ... exited with signal 11" (SIGSEGV)
Monitor for Abnormal PROXY Headers:
tcpdump -i any -A 'port 12345' | grep -i "PROXY"
Exploit
Exploitation leverages the buffer overflow by sending crafted PROXY protocol version 2 headers that cause the header construction routine to write beyond the allocated buffer. No authentication is required, and the attack can be performed remotely over the network.
Proof-of-Concept (Conceptual):
import socket
Craft oversized PROXY v2 header with malformed address fields
The exact trigger depends on internal buffer sizing and TLV (Type-Length-Value) parsing
payload = b"\x0D\x0A\x0D\x0A\x00\x0D\x0A\x51\x55\x49\x54\x0A" PROXY v2 signature
payload += b"\x20" PROXY v2 command/length placeholder
Further crafted bytes to exceed buffer bounds...
This causes the worker to write out of bounds and crash
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(("target_host", 12345))
sock.send(payload)
sock.close()
Attack Requirements:
- Network access to the OpenResty stream proxy port
- Upstream `proxy_protocol` directive enabled
- No prior authentication or privileges needed
Protection
Immediate Mitigation:
- Upgrade to OpenResty 1.29.2.5 or later – This is the only complete fix.
wget https://openresty.org/download/openresty-1.29.2.5.tar.gz tar -xzf openresty-1.29.2.5.tar.gz cd openresty-1.29.2.5/ ./configure --prefix=/usr/local/openresty make && sudo make install
- Temporary Workaround: Disable PROXY protocol v2 for upstream connections where not strictly required.
Remove or comment out: proxy_protocol on; Or set to off if the directive supports it
- Network Segmentation: Restrict access to OpenResty stream proxy ports using firewall rules or cloud security groups to only trusted upstream servers.
- Intrusion Detection: Monitor logs for worker process crashes (signal 11) and implement alerting.
- Rate Limiting: Apply connection rate limits to reduce the feasibility of repeated crash attempts.
Impact
- Denial of Service (DoS): Successful exploitation crashes the worker process, leading to service disruption.
- No Data Breach: The vulnerability affects availability only; confidentiality and integrity are not compromised.
- Wide Exposure: Any OpenResty instance from versions 1.29.2.1 to 1.29.2.4 with upstream PROXY protocol v2 enabled is at risk.
- Low Attack Complexity: Remote, unauthenticated, and easily reproducible, making it attractive for attackers.
- Cascading Failures: Repeated crashes can exhaust worker processes, requiring manual intervention to restore service.
- CVSS Score: 7.5 (HIGH) per GitHub, Inc. metrics.
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Sources:
Reported By: nvd.nist.gov
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