CVE-2025-61725: ParseAddress Function DoS Vulnerability

CVE-2025-61725 Overview

CVE-2025-61725 is a Denial of Service vulnerability affecting the Go programming language's ParseAddress function. The vulnerability exists in how domain-literal address components are constructed through repeated string concatenation. When processing large domain-literal components, this inefficient algorithm can lead to excessive CPU consumption, causing service degradation or complete denial of service for applications that parse email addresses.

Critical Impact

Applications using Go's ParseAddress function to process untrusted email addresses are vulnerable to CPU exhaustion attacks, potentially leading to service unavailability.

Affected Products

• Go standard library (net/mail package)
• Applications using Go's ParseAddress function for email address parsing
• Services that accept and parse user-supplied email addresses

Discovery Timeline

• 2025-10-29 - CVE CVE-2025-61725 published to NVD
• 2025-12-09 - Last updated in NVD database

Technical Details for CVE-2025-61725

Vulnerability Analysis

This vulnerability is classified as a CPU Exhaustion denial of service issue. The root cause lies in the algorithmic complexity of the ParseAddress function when handling domain-literal components in email addresses. Domain-literals are the bracketed portions of email addresses that can contain IP addresses or other domain specifications (e.g., user@[192.168.1.1]).

The function uses repeated string concatenation to build these domain-literal components. In Go, strings are immutable, meaning each concatenation operation creates a new string object and copies the existing content. When processing maliciously crafted input with large domain-literal sections, this results in O(n²) time complexity, where n is the size of the input.

An attacker can exploit this by sending specially crafted email addresses with extremely large domain-literal components. The parsing operation will consume excessive CPU cycles, potentially blocking other operations and degrading overall application performance.

Root Cause

The vulnerability stems from inefficient string handling in the ParseAddress function. Rather than using a more efficient method such as strings.Builder or pre-allocated byte slices for constructing domain-literal components, the implementation relies on naive string concatenation. This approach has quadratic time complexity for large inputs, making it susceptible to algorithmic complexity attacks.

Attack Vector

The attack can be executed remotely over the network. An attacker needs to send a specially crafted email address to any application endpoint that parses email addresses using the vulnerable ParseAddress function. This could include:

• User registration forms
• Email validation services
• Mail server components
• Contact form handlers
• API endpoints that accept email parameters

The attack requires no authentication or user interaction, making it straightforward to exploit. A single malicious request with an oversized domain-literal component can cause significant CPU consumption, and sustained attacks can lead to complete service denial.

The vulnerability mechanism involves constructing email addresses with abnormally large domain-literal sections that trigger the inefficient string concatenation behavior. For detailed technical information, see the Go Dev Vulnerability Report and the Go Dev Issue Report.

Detection Methods for CVE-2025-61725

Indicators of Compromise

• Unusually high CPU utilization on application servers without corresponding increase in legitimate traffic
• Slow response times or timeouts on endpoints that process email addresses
• Incoming requests containing abnormally long email address strings with large bracketed domain-literal sections
• Application logs showing parsing operations taking excessive time

Detection Strategies

• Monitor CPU utilization patterns on systems running Go applications that parse email addresses
• Implement request logging that captures the size of email address parameters for anomaly detection
• Set up alerting for parsing operations that exceed expected duration thresholds
• Review application dependencies to identify usage of the net/mail package's ParseAddress function

Monitoring Recommendations

• Configure resource monitoring to alert on sustained CPU spikes that correlate with email parsing operations
• Implement request rate limiting on endpoints that accept email addresses as input
• Use application performance monitoring (APM) tools to track parsing function execution times
• Establish baseline metrics for normal email address parsing operations to identify anomalies

How to Mitigate CVE-2025-61725

Immediate Actions Required

• Identify all Go applications in your environment that use the net/mail package for email address parsing
• Apply the official patch by updating to a patched version of Go as described in the Go Dev Issue Announcement
• Implement input validation to reject email addresses exceeding reasonable length limits before parsing
• Consider adding timeouts around parsing operations to prevent single requests from consuming excessive resources

Patch Information

The Go team has released a fix for this vulnerability. Administrators should update their Go installations to the patched version as soon as possible. Detailed patch information is available in the Go Dev Issue Announcement and the Golang Announce Mailing List.

Workarounds

• Implement strict input validation that limits the maximum length of email address strings before they reach the ParseAddress function
• Add timeout mechanisms around email parsing operations to prevent indefinite CPU consumption
• Use rate limiting on endpoints that accept email addresses to reduce the impact of sustained attacks
• Consider using alternative email validation approaches that don't rely on the vulnerable function until patching is complete

For applications that cannot be immediately patched, implementing a pre-validation step is recommended:

// Pre-validation workaround example
// Limit email address length before parsing
const maxEmailLength = 254 // RFC 5321 maximum

func validateEmailLength(email string) error {
    if len(email) > maxEmailLength {
        return errors.New("email address exceeds maximum length")
    }
    return nil
}