Use-After-Free in Lotos WebServer Buffer Handling via Long URI

Overview

Lotos WebServer, a lightweight HTTP server implementation written in C, contains a critical use-after-free vulnerability in its buffer management subsystem. The vulnerability resides in the buffer_avail() function located in buffer.h and is triggered when processing HTTP requests containing exceptionally long URIs. The root cause stems from improper handling of the realloc() function during dynamic buffer expansion, resulting in freed memory being dereferenced after reallocation failures or pointer invalidation.

This vulnerability affects Lotos WebServer through version 0.1.1 (commit 3eb36cc) and carries a CVSS v3.1 base score of 9.8, classifying it as critical severity. An unauthenticated remote attacker can exploit this flaw by sending a specially crafted HTTP request with an oversized URI to trigger the use-after-free condition, potentially achieving arbitrary code execution with the privileges of the web server process. The vulnerability was discovered through security research and disclosed via the project’s GitHub issue tracker, prompting immediate investigation into the buffer management implementation.

Technical Analysis

The vulnerability originates in the dynamic buffer allocation logic used to handle incoming HTTP request URIs. When a client sends a request with a URI exceeding the preallocated buffer size, the server attempts to resize the buffer using realloc() to accommodate the longer request. However, the implementation fails to correctly handle edge cases in memory reallocation.

The vulnerable code pattern in buffer.h resembles the following:

typedef struct {
    char *data;
    size_t capacity;
    size_t length;
} Buffer;

int buffer_avail(Buffer *buf, size_t required) {
    if (buf->capacity < buf->length + required) {
        size_t new_capacity = buf->capacity * 2;
        if (new_capacity < buf->length + required) {
            new_capacity = buf->length + required;
        }
        
        // VULNERABLE: No NULL check after realloc failure
        char *new_data = realloc(buf->data, new_capacity);
        buf->data = new_data;  // Use-after-free if realloc returns NULL
        buf->capacity = new_capacity;
    }
    return 0;
}

The critical flaw occurs because the code assigns the result of realloc() directly to buf->data without checking for allocation failure. When realloc() fails and returns NULL, this assignment causes buf->data to become a null pointer. Subsequent operations that dereference buf->data trigger undefined behavior. Additionally, if the original buffer pointer was previously freed through another code path before the assignment completes, a use-after-free condition manifests.

A long URI causes repeated buffer reallocation attempts, increasing the probability of triggering memory pressure conditions that cause realloc() to fail or that expose pointer invalidation bugs in the buffer management state machine.

Impact

An unauthenticated remote attacker positioned to send HTTP requests to a Lotos WebServer instance can trigger the use-after-free vulnerability with minimal effort. By crafting a request with a URI exceeding typical buffer boundaries—for example, a URI containing tens of thousands of characters—the attacker forces the buffer reallocation code path.

The memory corruption resulting from dereferencing invalid pointers can lead to:

• Information Disclosure: Reading adjacent memory regions containing sensitive data (credentials, tokens, application state)
• Denial of Service: Crashes resulting from invalid memory access, disrupting service availability
• Arbitrary Code Execution: In exploitation scenarios where attackers control freed memory content, heap grooming techniques enable code execution with server-level privileges

Lotos WebServer deployments exposed to untrusted networks face immediate exploitation risk. The vulnerability requires no authentication and can be exploited through a single malformed HTTP request, making it trivial for automated scanners and attackers to identify and compromise affected instances.

How to Fix It

Immediate Actions:

1. Upgrade to a patched version of Lotos WebServer once available from the upstream project
2. Implement a Web Application Firewall (WAF) rule to reject HTTP requests with excessively long URIs (>8192 bytes recommended)
3. Restrict network exposure of Lotos WebServer instances to trusted clients only

Code Remediation:

The vulnerability must be fixed by properly validating realloc() return values:

int buffer_avail(Buffer *buf, size_t required) {
    if (buf->capacity < buf->length + required) {
        size_t new_capacity = buf->capacity * 2;
        if (new_capacity < buf->length + required) {
            new_capacity = buf->length + required;
        }
        
        // FIXED: Check realloc return value before assignment
        char *new_data = realloc(buf->data, new_capacity);
        if (new_data == NULL) {
            // Handle allocation failure gracefully
            errno = ENOMEM;
            return -1;
        }
        
        buf->data = new_data;
        buf->capacity = new_capacity;
    }
    return 0;
}

Additionally, implement URI length validation at the request parsing stage:

#define MAX_URI_LENGTH 8192

if (uri_length > MAX_URI_LENGTH) {
    return HTTP_REQUEST_URI_TOO_LONG;  // Return 414 status
}

Our Take

This vulnerability exemplifies a class of memory safety defects that plague C-based network services. The improper handling of realloc() is a well-documented antipattern, yet continues to appear in production code. At Offensive360, we emphasize that defensive programming practices—specifically, always checking allocation function return values—are non-negotiable in memory-unsafe languages.

For enterprises deploying or developing C-based services, this CVE reinforces the critical importance of:

1. Static analysis integration into the build pipeline to catch unsafe realloc() patterns
2. Fuzzing campaigns targeting input parsing logic with varying buffer sizes
3. Memory sanitizers (ASan, MSan) in development and CI/CD environments to detect use-after-free conditions early

Projects with memory-critical attack surface areas should consider memory-safe language alternatives (Rust, Go) for new components and gradually migrate legacy services toward safer implementations.

Detection with SAST

Static analysis security testing tools should flag this vulnerability through the following patterns:

CWE Identifiers:

• CWE-416: Use After Free
• CWE-252: Unchecked Return Value

Detection Patterns:

• Assignment of realloc() return value without NULL check
• Subsequent dereference of pointers that may have been freed
• Buffer capacity modifications without validation
• Absence of error handling paths in memory allocation sequences

SAST engines configured to detect unsafe memory patterns in C codebases will identify direct assignments from realloc(), pointer arithmetic on unvalidated allocations, and missing NULL checks. Configure your SAST toolchain to enforce strict allocation safety rules and flag any reallocation without explicit error handling as a critical finding.

References

• https://github.com/chendotjs/lotos/issues/7
• https://github.com/chendotjs/lotos
• https://cwe.mitre.org/data/definitions/416.html
• https://cwe.mitre.org/data/definitions/252.html