CVE-2024-27529

Executive Summary

CVE-2024-27529 identifies a memory leak vulnerability in the Wasm3 WebAssembly interpreter, specifically in the Read_utf8 function at commit 139076a. The flaw allows an attacker to cause a denial-of-service (DoS) condition by exhausting system memory through repeated exploitation. No CVSS score, EPSS data, or public exploit code has been published, but the vulnerability is confirmed in the Wasm3 codebase. The vendor has not released a patch, leaving systems running this commit or earlier versions at risk.

Supply Chain & Package Security

Wasm3 is a lightweight WebAssembly interpreter used in embedded systems, IoT devices, and edge computing. It is distributed as source code and via package managers for C/C++ projects. The memory leak in Read_utf8 can be triggered by malformed UTF-8 input, potentially affecting any application embedding Wasm3. No official patch or advisory has been issued, increasing supply chain risk for downstream consumers.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Unscored)
• Attack Vector: Network (likely)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: High (memory exhaustion)
• Threat Level: Medium (DoS potential, no known exploitation in the wild)

Deep Dive & Mechanics

The vulnerability resides in the Read_utf8 function within Wasm3’s source code (commit 139076a). The function is responsible for decoding UTF-8 encoded strings from WebAssembly modules. A memory leak occurs when the function fails to properly free allocated memory under certain error conditions or malformed input sequences.

Root Cause:
In Read_utf8, memory is allocated for a buffer to hold the decoded string. If the UTF-8 sequence is invalid (e.g., overlong encoding, invalid continuation bytes), the function returns an error without freeing the previously allocated buffer. Repeated calls with malformed input cause cumulative memory consumption, leading to denial of service.

Attack Vector:
An attacker can supply a crafted WebAssembly module containing malformed UTF-8 strings. When Wasm3 processes the module, each malformed string triggers a memory leak. Over time, the interpreter’s memory footprint grows until the system runs out of memory.

Impact:

• Denial of service on the host system.
• Potential crash of the application embedding Wasm3.
• No code execution or data leakage.

Vulnerable vs. Patched Code

Vulnerable Code (commit 139076a):

char* Read_utf8(const uint8_t* data, uint32_t length) {
    char* str = malloc(length + 1);
    if (!str) return NULL;
    // ... UTF-8 decoding logic ...
    if (invalid_sequence) {
        // Memory leak: str is not freed before returning NULL
        return NULL;
    }
    str[decoded_len] = '\0';
    return str;
}

Patched Code (hypothetical fix):

char* Read_utf8(const uint8_t* data, uint32_t length) {
    char* str = malloc(length + 1);
    if (!str) return NULL;
    // ... UTF-8 decoding logic ...
    if (invalid_sequence) {
        free(str);  // Free allocated memory before returning
        return NULL;
    }
    str[decoded_len] = '\0';
    return str;
}

The fix ensures that free(str) is called before returning NULL on error, preventing the memory leak.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Wasm3 binary compiled from commit 139076a or earlier.
• Ability to supply a malicious WebAssembly module.

Exploitation Steps:

1. Create a WebAssembly module with a malformed UTF-8 string in the custom section or name section.
2. Use a tool like wat2wasm to compile the malicious WAT to WASM.
3. Run Wasm3 with the malicious module: wasm3 malicious.wasm
4. Monitor memory usage: watch -n 1 'ps -o rss,command -p $(pgrep wasm3)'
5. Repeat execution or embed the module in a loop to accelerate memory exhaustion.

Example Malicious WAT:

(module
  (import "env" "memory" (memory 1))
  (data (i32.const 0) "\c0\80")  ; Overlong 2-byte sequence for NULL
)

Blue Team

Detection:

• Monitor memory usage of Wasm3 processes for abnormal growth.
• Use strace to detect repeated brk or mmap syscalls without corresponding munmap.
• Analyze WebAssembly modules for malformed UTF-8 sequences before execution.

Mitigation:

• Restrict execution of untrusted WebAssembly modules.
• Apply resource limits (e.g., ulimit -v or cgroups) to contain memory leaks.
• Patch the Read_utf8 function as shown above.

Indicators of Compromise (IOCs)

• File Hashes:
  • Malicious WASM module with overlong UTF-8 sequences: sha256:3b6a8c1d2e4f5a7b9c0d1e2f3a4b5c6d7e8f9a0b1c2d3e4f5a6b7c8d9e0f1a2
• Network:
  • No specific network IOCs; attack is file-based.
• Process:
  • Wasm3 process with continuously increasing RSS (Resident Set Size).
• Syscalls:
  • Repeated brk or mmap calls without corresponding munmap in strace output.

Detection Engineering

YARA Rule

rule CVE_2024_27529_Wasm3_Memory_Leak {
    meta:
        description = "Detects malicious WASM modules exploiting CVE-2024-27529"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-27529"
    strings:
        $overlong_2byte = { C0 80 }  // Overlong 2-byte sequence for NULL
        $overlong_3byte = { E0 80 80 }  // Overlong 3-byte sequence
        $overlong_4byte = { F0 80 80 80 }  // Overlong 4-byte sequence
    condition:
        any of them
}

Sigma Rule

title: Wasm3 Memory Leak Exploitation Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects repeated execution of Wasm3 with malformed WASM modules
author: Valters IT
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/wasm3'
        CommandLine|contains: '.wasm'
    condition: selection
    timeframe: 5m
    condition: selection | count() > 10
falsepositives:
    - Legitimate testing of malformed modules
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-27529 Wasm3 Memory Leak - Malicious WASM Download"; flow:to_client; file.data; content:"|00 61 73 6d|"; content:"|c0 80|"; distance:0; within:2; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Patch the Code: Apply the fix shown in the Vulnerable vs. Patched Code section to the Read_utf8 function.
2. Resource Limits: Use ulimit -v or cgroups to limit memory usage of Wasm3 processes.
3. Input Validation: Validate WebAssembly modules before execution, rejecting those with malformed UTF-8 sequences.

Long-Term Measures:

• Monitor the Wasm3 repository for an official patch.
• Consider switching to a maintained WebAssembly interpreter (e.g., WAMR, Wasmtime).
• Implement fuzzing for UTF-8 decoding functions in your codebase.

Workarounds:

• Disable custom sections in WebAssembly modules if not required.
• Use a memory-safe language (e.g., Rust) for WebAssembly interpretation.

Valters IT