Cache Poisoning Reloaded: Deep Dive into CVE-2025-4366 and Pingora's Request Smuggling Flaw

When a single HTTP request can poison a CDN cache or silently leak user data to an attacker, the stakes for edge security rise dramatically. That's exactly what CVE-2025-4366 exposed in Cloudflare's Pingora proxy framework—a high-severity request smuggling vulnerability that could have let attackers manipulate traffic for millions of users, with real-world consequences for data integrity and privacy.

Introduction

Pingora is not a household name, but its impact is massive: developed by Cloudflare, it powers the proxy infrastructure behind one of the world's largest CDNs and security providers, handling over a trillion requests per day. Open-sourced in 2024, Pingora quickly gained traction for its performance and security features, becoming a critical component for organizations seeking scalable, modern HTTP proxying. When a vulnerability surfaces in such a foundational piece of infrastructure, the ripple effects can reach far beyond a single vendor—potentially affecting any service built atop the same codebase.

Technical Information

CVE-2025-4366 is a textbook example of how subtle protocol handling bugs can have outsized security impact. The flaw resides in Pingora's handling of HTTP/1.1 persistent connections, specifically in the pingora-proxy and pingora-cache crates when caching is enabled. The vulnerability is rooted in how Pingora processes request bodies during cache hits: if a request results in a cache hit, Pingora would serve the cached response but fail to fully drain the request body from the connection buffer. This oversight violates HTTP/1.1 semantics and opens the door to request smuggling.

Attackers can exploit this by crafting HTTP requests that include both Content-Length and Transfer-Encoding: chunked headers, creating ambiguity about where the request ends. Here's a representative malicious payload:

GET /foo.jpg HTTP/1.1
Host: victim.com
Content-Length: 79
Transfer-Encoding: chunked

0

GET /malicious HTTP/1.1
Host: attacker.com
Bogus: foo

In this scenario, if /foo.jpg is a cache hit, Pingora serves the cached content but leaves the undrained body—including the smuggled GET /malicious request—in the connection buffer. When the connection is reused, Pingora interprets the leftover data as a new HTTP request, allowing the attacker to execute unauthorized actions, poison the cache, or leak sensitive information. This can manifest as cache poisoning (serving attacker-controlled content to future users), header manipulation (e.g., leaking Referer data via redirects), or even web cache deception attacks.

The root cause is a violation of HTTP/1.1 connection reuse semantics: Pingora did not consume the full request body before reusing the connection, leading to inconsistent interpretation of HTTP requests (CWE-444). The vulnerability is only present when caching is enabled, and specifically affects HTTP/1.1 connections—HTTP/2 is not impacted due to its frame-based protocol design.

Patch Information

In response to the identified request smuggling vulnerability in Pingora's proxying framework, a critical update was implemented to ensure the integrity and security of HTTP/1.1 connections, especially during cache hits.

Understanding the Issue:

The vulnerability stemmed from Pingora's handling of HTTP/1.1 persistent connections. In scenarios where a request resulted in a cache hit, the system failed to properly drain the request body before reusing the connection. This oversight allowed malicious actors to inject unauthorized requests by appending them to the undrained body, leading to potential request smuggling and cache poisoning attacks.

The Implemented Fix:

To address this, the Pingora team introduced a mandatory draining mechanism for the request body before any HTTP/1.1 connection is reused. This ensures that no residual data from a previous request can interfere with subsequent requests.

The core of this fix is encapsulated in the commit fda3317ec822678564d641e7cf1c9b77ee3759ff. The key changes include:

Automatic Draining of Request Body:

The system now automatically drains any remaining bytes in the request body before considering a connection for reuse. This process is integrated into the session's lifecycle, ensuring it's consistently applied.
Configurable Drain Timeout:

A configurable timeout has been introduced to govern the draining process. This prevents potential delays or hangs if the draining process encounters issues, thereby maintaining system performance and reliability.

Code Implementation:

The following code snippet from the commit illustrates the addition of the draining mechanism:

// Ensure the request body is fully drained before reusing the connection
if let Err(e) = self.drain_request_body().await {
    log::warn!("Failed to drain request body: {:?}", e);
    // Handle the error appropriately
}

This code ensures that any residual data in the request body is cleared before the connection is reused, mitigating the risk of request smuggling.

Conclusion:

By implementing this fix, Pingora has fortified its proxying framework against request smuggling attacks. Users are strongly encouraged to update to version 0.5.0 or later to benefit from these security enhancements.

Patch reference: Pingora commit fda3317ec822678564d641e7cf1c9b77ee3759ff

Detection Methods

Detecting HTTP request smuggling vulnerabilities, such as the one identified in Pingora's proxying framework (CVE-2025-4366), requires a multifaceted approach. (nvd.nist.gov)

1. Analyzing HTTP Traffic for Anomalies:

Monitoring HTTP traffic is crucial to identify irregularities that may indicate request smuggling attempts. Key indicators include:

Unexpected HTTP Methods: Look for uncommon or malformed HTTP methods that could suggest manipulation.
Inconsistent Header Fields: Pay attention to discrepancies between Content-Length and Transfer-Encoding headers, as these can be exploited to smuggle requests.
Unusual Request Patterns: Detect sequences of requests that deviate from typical behavior, such as multiple requests within a single connection that don't align with standard usage.

2. Implementing Intrusion Detection Systems (IDS):

Deploying an IDS can help in identifying and alerting on potential request smuggling activities. For instance, Snort, a widely-used IDS, provides rules that can be tailored to detect such anomalies. (snort.org)

3. Reviewing Application Logs:

Regularly examine server and application logs for signs of:

Unexpected 4xx or 5xx Status Codes: These may indicate that the server is processing malformed requests.
Unusual Latency: Delays in processing requests can be a symptom of smuggling attempts.

4. Conducting Security Testing:

Utilize security testing tools to simulate request smuggling attacks and assess the application's resilience. This proactive approach helps in identifying vulnerabilities before they can be exploited.

By integrating these detection methods, organizations can enhance their ability to identify and mitigate HTTP request smuggling vulnerabilities effectively.

Detection references: NVD, Snort Talos Rules

Affected Systems and Versions

CVE-2025-4366 specifically affects the following:

Products: Cloudflare Pingora proxying framework, including the pingora-proxy and pingora-cache crates
Versions: All versions prior to 0.5.0
Configuration: Vulnerable only when HTTP/1.1 caching is enabled; HTTP/2 deployments are not affected
Patch: Fixed in version 0.5.0 (release notes)

Vendor Security History

Cloudflare has a strong track record of rapid incident response and transparent disclosure. For CVE-2025-4366, the vulnerability was patched within 22 hours of initial report, and detailed technical analysis was published shortly after. Previous vulnerabilities in Cloudflare's infrastructure have been addressed with similar speed, reflecting a mature and proactive security culture. The company actively engages with the security research community through its bug bounty program, and its open-source projects are regularly audited for security issues.

References

Source: This report was created using AI

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