Stack Buffer Overflow in Cherry HTTP Server URI Parsing via sscanf

Overview

CVE-2024-22086 represents a critical vulnerability in Cherry, a lightweight HTTP server written in C. The flaw exists in the handle_request function within http.c, where improper use of sscanf to parse HTTP request URIs fails to enforce buffer boundaries. An attacker can craft an HTTP request with an excessively long URI to trigger a stack-based buffer overflow, ultimately achieving unauthenticated remote code execution on affected systems.

Cherry is a minimalist HTTP server often deployed in embedded systems, IoT devices, and resource-constrained environments where its small footprint is attractive. The vulnerability’s CVSS 9.8 score reflects the complete absence of authentication requirements and the trivial nature of exploitation—any network-accessible Cherry instance becomes instantly compromisable through a single malformed HTTP request. This makes it one of the most dangerous vulnerability classes: an out-of-bounds write primitive reachable from untrusted network input without prerequisites.

Technical Analysis

The vulnerability stems from unsafe string parsing in the HTTP request handler. The vulnerable code path looks similar to this:

void handle_request(int client_socket) {
    char request[256];
    char uri[128];
    char method[16];
    
    // Receive request from socket
    recv(client_socket, request, sizeof(request), 0);
    
    // VULNERABLE: sscanf with no bounds checking on uri
    sscanf(request, "%s %s HTTP/1.1", method, uri);
    
    // Process request...
}

The critical flaw is that sscanf(request, "%s %s HTTP/1.1", method, uri) provides no mechanism to limit the number of bytes written to the uri buffer. If the HTTP request contains a URI longer than 128 bytes, the %s format specifier will write beyond the buffer’s allocated stack memory.

The root cause involves two compounding issues:

1. Unbounded format specifier: The %s specifier in sscanf copies characters until whitespace is encountered, with no width limitation specified (e.g., %127s would be the safe variant).

2. Stack-based allocation: The uri buffer is allocated on the stack, making it immediately adjacent to function return addresses and saved registers in typical calling conventions.

An attacker-controlled payload crafted as:

GET /AAAABBBBCCCCDDDD....[1000+ bytes]... HTTP/1.1

will overflow the stack buffer, overwriting the return address. By carefully constructing the payload with shellcode and a calculated return address, arbitrary code execution is achieved.

The vulnerability is particularly severe because:

• No authentication is required
• The input is directly reachable via raw socket input
• Stack canaries may not be enabled in embedded builds
• ASLR is frequently disabled in constrained environments

Impact

Exploitation of CVE-2024-22086 grants complete system compromise on affected Cherry deployments:

Immediate consequences:

• Unauthenticated remote code execution with the privileges of the Cherry process
• Full read/write access to the filesystem if Cherry runs as root (common in embedded deployments)
• Potential lateral movement within IoT networks where Cherry instances aggregate data
• Botnet recruitment—infected devices become command-and-control nodes

Enterprise risk scenarios:

• IoT gateway devices running Cherry become entry points for network infiltration
• Embedded monitoring systems lose integrity, producing false telemetry
• Supply chain compromise if Cherry is bundled in commercial appliances
• Operational technology (OT) networks face direct compromise without segmentation

The practical exploitability is exceptionally high—a proof-of-concept requires only standard HTTP tooling and basic ROP knowledge.

How to Fix It

Immediate action: Upgrade Cherry to a patched version beyond commit 4b877df or apply the following code fix:

void handle_request(int client_socket) {
    char request[256];
    char uri[128];
    char method[16];
    
    recv(client_socket, request, sizeof(request), 0);
    
    // FIXED: Specify maximum width to prevent overflow
    // "15" = sizeof(method) - 1, "127" = sizeof(uri) - 1
    sscanf(request, "%15s %127s HTTP/1.1", method, uri);
    
    // Process request...
}

Alternatively, replace sscanf with safer parsing:

void handle_request(int client_socket) {
    char request[256];
    char uri[128];
    char method[16];
    
    recv(client_socket, request, sizeof(request), 0);
    
    // Use snprintf or strtok with explicit length validation
    if (sscanf(request, "%15s %127s HTTP/1.1", method, uri) != 2) {
        // Handle parsing error
        return;
    }
    
    // Validate uri length before processing
    if (strlen(uri) >= sizeof(uri) - 1) {
        send_error(client_socket, 414);  // URI Too Long
        return;
    }
}

Patching strategy:

1. Identify all systems running Cherry via network scanning or asset inventory
2. Rebuild Cherry from patched source or obtain vendor security updates
3. Redeploy to production with restart coordination
4. Implement network segmentation to isolate remaining unpatched instances during transition

Our Take

CVE-2024-22086 exemplifies why C-based HTTP servers require exceptional scrutiny. The sscanf function family is a notorious source of buffer overflows—its variable-width parsing makes it fundamentally incompatible with fixed-size stack buffers. Security teams should treat any use of sscanf, sprintf, or gets in HTTP parsing code as a red flag during code review.

For enterprises deploying Cherry or similar lightweight HTTP servers, this vulnerability underscores three critical practices:

1. Input validation at boundaries: Network-exposed functions must enforce strict size limits on all string operations, regardless of apparent buffer capacity
2. Safer alternatives: Modern C libraries offer snprintf, bounded string functions, and parser combinators that eliminate this class of defect
3. Embedded security hygiene: IoT and embedded systems often skip security hardening (ASLR, canaries, DEP) that would mitigate exploitation—compensate with stricter code review

Detection with SAST

Static analysis tools detect this vulnerability through multiple mechanisms:

Direct pattern matching: SAST engines flag unsafe format specifiers:

• sscanf with unbounded %s on stack-allocated buffers
• CWE-120 (Buffer Copy without Checking Size of Input) and CWE-121 (Stack-based Buffer Overflow)

Taint analysis: Tracking untrusted input (socket data) flowing to unsafe functions:

• Source: recv(), read(), fgets() from network sockets
• Sink: sscanf() with width-unspecified format strings
• Path validation: Confirming no validation occurs between source and sink

Offensive360’s SAST approach flags:

CWE-120: Unbounded use of scanf-family on socket input
Severity: CRITICAL
Pattern: sscanf/fscanf with %s on stack buffer < 256 bytes
Recommendation: Use bounded format specifiers (%Ns) or safer alternatives

Developers should enable strictness flags in compilers (-Wformat-overflow, -Wstack-protector) and integrate SAST scanning into CI/CD pipelines to catch these patterns before deployment.

References

• National Vulnerability Database: CVE-2024-22086
• Cherry HTTP Server GitHub Issue #1
• CWE-120: Buffer Copy without Checking Size of Input
• CWE-121: Stack-based Buffer Overflow
• Cherry HTTP Server Repository