CVE-2020-14040: Golang Text UTF-16 DOS Vulnerability

CVE-2020-14040 Overview

CVE-2020-14040 is a denial of service vulnerability in the Go x/text package affecting versions prior to 0.3.3. The vulnerability exists in the encoding/unicode module where the UTF-16 decoder can enter an infinite loop when processing maliciously crafted input, leading to program crashes or memory exhaustion.

An attacker can exploit this vulnerability by providing a single byte to a UTF-16 decoder instantiated with UseBOM or ExpectBOM. When the String function on the Decoder is called, or when the Decoder is passed to golang.org/x/text/transform.String, the infinite loop condition is triggered.

Critical Impact

This vulnerability allows remote attackers to cause denial of service conditions by triggering an infinite loop in Go applications using the vulnerable x/text package, potentially leading to application crashes or resource exhaustion.

Affected Products

• Golang Text (versions before 0.3.3)
• Fedora 32

Discovery Timeline

• 2020-06-17 - CVE-2020-14040 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-14040

Vulnerability Analysis

This vulnerability (CWE-835: Loop with Unreachable Exit Condition) affects the UTF-16 decoder implementation in the Go x/text package. The issue manifests when the decoder is configured with byte order mark (BOM) handling modes (UseBOM or ExpectBOM) and receives insufficient input data.

When a single byte is passed to the decoder under these configurations, the internal state machine fails to properly handle the incomplete input, resulting in the decoder continuously attempting to process the same byte without advancing or terminating. This creates an infinite loop condition that consumes CPU resources indefinitely.

The vulnerability is particularly dangerous because it can be triggered remotely through any network-accessible application that processes user-supplied text through the vulnerable decoder. Applications performing text encoding transformations on untrusted input are at risk.

Root Cause

The root cause lies in improper boundary checking within the UTF-16 decoder's BOM handling logic. When the decoder expects a byte order mark but receives only a single byte of input, it enters a state where it cannot proceed forward (insufficient data to determine BOM) but also cannot exit gracefully. The decoder's loop condition fails to account for this edge case, causing an unreachable exit condition.

The transform.String function and the Decoder.String method both rely on the underlying decoder's ability to properly signal completion or error states. With the vulnerable code, neither condition is signaled, resulting in infinite iteration.

Attack Vector

The attack vector for CVE-2020-14040 is network-based, requiring no authentication or user interaction. An attacker can craft a malicious payload containing a single byte and send it to any application endpoint that processes text through the vulnerable UTF-16 decoder.

The vulnerability can be exploited by sending specially crafted input to a target application that uses the golang.org/x/text/encoding/unicode package with UseBOM or ExpectBOM configuration. When the application attempts to decode this input using the String function or passes the decoder to transform.String, the infinite loop is triggered.

Applications that accept user-controlled text input for encoding transformation, internationalization processing, or character set conversion are potential targets. The attack requires minimal bandwidth and can be launched with a single malicious request.

Detection Methods for CVE-2020-14040

Indicators of Compromise

• Go applications experiencing sudden CPU spikes at 100% utilization on affected processes
• Application threads or goroutines stuck in encoding/decoding operations without completing
• Memory consumption steadily increasing in processes handling text transformation
• Application unresponsiveness or timeout errors during text processing operations

Detection Strategies

• Monitor application dependencies for golang.org/x/text package versions below 0.3.3
• Implement runtime monitoring for goroutines with abnormally long execution times in text processing functions
• Use static code analysis tools to identify usage of UseBOM or ExpectBOM decoder configurations
• Deploy application performance monitoring (APM) to detect infinite loop patterns in production environments

Monitoring Recommendations

• Set up alerting for sustained high CPU usage in Go applications handling text encoding
• Monitor for process restarts or OOM (out of memory) kills related to text processing services
• Implement request timeout mechanisms to detect and terminate stuck encoding operations
• Log and analyze text processing failures that may indicate exploitation attempts

How to Mitigate CVE-2020-14040

Immediate Actions Required

• Upgrade golang.org/x/text package to version 0.3.3 or later immediately
• Audit all Go applications for usage of the vulnerable encoding/unicode decoder
• Implement input validation to reject suspiciously small payloads destined for UTF-16 decoding
• Deploy request timeouts around text transformation operations as a defense-in-depth measure

Patch Information

The Go team has addressed this vulnerability in golang.org/x/text version 0.3.3. Organizations should update their Go module dependencies to include the patched version. For additional information, refer to the Google Group Announcement and Fedora Package Announcement.

Workarounds

• Avoid using UseBOM or ExpectBOM configurations with the UTF-16 decoder until patched
• Validate that input data is at least 2 bytes before passing to UTF-16 decoders
• Implement timeout wrappers around text transformation operations to prevent infinite loops from causing prolonged denial of service
• Consider using alternative text encoding libraries if immediate patching is not feasible

# Update Go module to patched version
go get golang.org/x/text@v0.3.3
go mod tidy