Stack Buffer Overflow in Pico HTTP Server URI Parsing Enables RCE

Overview

CVE-2024-22087 represents a critical vulnerability in Pico HTTP Server, a lightweight C-based HTTP server implementation. The vulnerability exists in the URI routing logic within main.c, where improper bounds checking on user-supplied HTTP request paths allows attackers to trigger a stack-based buffer overflow. By crafting an HTTP request with an abnormally long URI, an unauthenticated remote attacker can overwrite stack memory and achieve arbitrary code execution on the target system.

The vulnerability was introduced through insecure use of the sprintf() function without proper length validation when processing incoming HTTP requests. This represents a classic memory safety issue in C applications where format string functions lack inherent bounds checking. The attack surface is particularly dangerous because HTTP servers are frequently exposed to untrusted networks, and the vulnerability requires no authentication or special privileges to trigger.

Technical Analysis

The root cause of CVE-2024-22087 lies in the unsafe use of sprintf() when constructing route paths from user-supplied URI data. The vulnerable code pattern appears in the request handler within main.c:

char route_buffer[256];
// ... request parsing code ...
sprintf(route_buffer, "%s", uri_from_request);
// Subsequent processing without bounds validation

The fundamental issue is that sprintf() performs no bounds checking on the destination buffer. When an attacker sends an HTTP request with a URI significantly exceeding 256 bytes, the function blindly writes beyond the allocated buffer boundary, overflowing adjacent stack memory.

In a typical attack scenario, an attacker constructs an HTTP GET request such as:

GET /AAAAAAAAAA...AAAAAAAAAA[ROP_GADGETS][SHELLCODE] HTTP/1.1
Host: target-server.com

Where the URI length dramatically exceeds the buffer size. The overflow overwrites:

• Saved return addresses on the stack
• Function pointers stored in stack frames
• Other critical control flow data structures

By carefully crafting the overflow payload with address-to-gadget mappings and shellcode, an attacker achieves code execution within the HTTP server process context. On modern systems with ASLR enabled, attackers must resort to Return-Oriented Programming (ROP) techniques to bypass address space layout randomization.

The vulnerable code path is triggered immediately upon receiving the HTTP request, before any authentication or authorization checks, making this a pre-authentication remote code execution vulnerability with a CVSS base score of 9.8 (Critical).

Impact

The consequences of CVE-2024-22087 are severe and immediate:

Remote Code Execution: Attackers achieve arbitrary code execution with the privileges of the HTTP server process. On systems where the server runs as root or with elevated capabilities, the impact extends to full system compromise.

Data Breach: Compromise of the HTTP server process allows attackers to access sensitive data processed or stored by applications relying on the server, including credentials, configuration files, and application state.

Service Disruption: Beyond code execution, attackers can exploit the buffer overflow to crash the service, creating a denial-of-service condition.

Lateral Movement: A compromised HTTP server can serve as a pivot point for attacking other systems on the same network, particularly if the server has access to internal resources or authentication mechanisms.

Supply Chain Risk: Organizations deploying Pico HTTP Server in embedded systems, IoT devices, or containerized environments face widespread exposure, especially if affected versions are baked into production images.

How to Fix It

Immediate Mitigation: Discontinue use of vulnerable versions and upgrade to the patched release. The fix was applied in commit 7a5e4e242121c839cb77f5b9003e735a852f4e58 and subsequent security releases.

The remediation involves replacing sprintf() with length-safe alternatives:

// VULNERABLE (DO NOT USE)
char route_buffer[256];
sprintf(route_buffer, "%s", uri_from_request);

// SECURE FIX
char route_buffer[256];
int max_len = sizeof(route_buffer) - 1;
snprintf(route_buffer, max_len, "%s", uri_from_request);

// OR: Implement proper length validation before processing
if (strlen(uri_from_request) > MAX_SAFE_URI_LENGTH) {
    return HTTP_400_BAD_REQUEST;
}

Upgrade Instructions:

1. Clone or update your Pico HTTP Server repository
2. Checkout commit 7a5e4e242121c839cb77f5b9003e735a852f4e58 or later
3. Rebuild the application: make clean && make
4. Restart the HTTP server with the patched binary
5. Verify the new version via the application’s version endpoint or log output

Defense in Depth:

• Deploy the HTTP server in a restricted container with minimal capabilities
• Run the server process with the lowest required privileges (non-root where possible)
• Implement network-level request filtering to reject oversized URIs before they reach the application
• Deploy Web Application Firewalls (WAF) configured to block abnormally large request paths

Our Take

CVE-2024-22087 exemplifies why memory-unsafe languages like C demand rigorous security practices and tooling. The vulnerability could have been prevented through:

1. Consistent Use of Safe APIs: Modern C libraries provide snprintf(), strlcpy(), and similar bounded functions specifically to prevent this class of vulnerability.

2. Input Validation: Whitelisting and length validation of HTTP request URIs before processing represents a fundamental security control.

3. Automated Detection: Static analysis tools can identify sprintf() usage patterns and flag them as high-risk, particularly when operating on untrusted input.

For enterprises relying on Pico HTTP Server or similar C-based network services, this vulnerability underscores the importance of:

• Maintaining a current software bill of materials (SBOM) with precise version tracking
• Implementing automated vulnerability scanning in your deployment pipeline
• Establishing rapid patching procedures for critical network-exposed services
• Deploying defense-in-depth controls (WAF, network segmentation, privilege minimization)

Detection with SAST

Static analysis tools can effectively identify this vulnerability class through multiple detection patterns:

Pattern 1 - Unsafe String Functions (CWE-120):

Detects direct usage of sprintf(), strcpy(), gets(), strcat() without bounds checking
Risk Level: HIGH when operating on external input

Pattern 2 - Unbounded Buffer Operations (CWE-119):

Flags stack-allocated buffers < 512 bytes receiving formatted input from:
- Network sockets
- HTTP request parsing functions
- External file data

Pattern 3 - Missing Input Validation (CWE-20):

Identifies code paths where user-controlled data (URI, headers, body)
flows directly to buffer operations without length verification

Pattern 4 - Format String Vulnerabilities (CWE-134):

Detects user input used as format string argument or in dynamic size calculations

Offensive360’s SAST platform specifically flags the sprintf(buffer, "%s", untrusted_input) anti-pattern and recommends automatic remediation to snprintf(buffer, sizeof(buffer), "%s", untrusted_input), significantly reducing the mean time to remediation (MTTR) for similar issues across your codebase.

References

• CVE-2024-22087 - GitHub Issue #31
• Security Fix Commit 7a5e4e2
• Follow-up Security Commit e2d172f
• Pico HTTP Server Repository
• CWE-120: Buffer Copy without Checking Size of Input
• CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer