CVE-2026-25680: HTML Parsing DoS Vulnerability

CVE-2026-25680 Overview

CVE-2026-25680 affects the Go programming language's HTML parsing functionality. Parsing arbitrary HTML can consume excessive CPU time, leading to a denial of service condition. The vulnerability falls into the algorithmic complexity attack category, where crafted input forces the parser into expensive processing paths.

Attackers can exploit this issue remotely by submitting malicious HTML to any Go service that parses untrusted input. Successful exploitation degrades availability of the affected service without requiring authentication. The flaw is tracked under Go vulnerability advisory GO-2026-5028.

Critical Impact

Remote attackers can trigger CPU exhaustion in Go applications that parse untrusted HTML, resulting in service-level denial of service.

Affected Products

• Go programming language standard library HTML parser
• Applications and services built on Go that process untrusted HTML input
• Go modules referenced in advisory GO-2026-5028

Discovery Timeline

• 2026-05-22 - CVE-2026-25680 published to the National Vulnerability Database (NVD)
• 2026-05-22 - Last updated in NVD database

Technical Details for CVE-2026-25680

Vulnerability Analysis

The vulnerability is an algorithmic complexity issue in Go's HTML parsing logic. Specifically crafted HTML content forces the parser to perform a disproportionate amount of work relative to input size. The result is sustained CPU consumption that blocks the goroutine and starves other workloads on the host.

The issue does not affect data confidentiality or integrity. The CVSS vector indicates a high availability impact with no confidentiality or integrity exposure. User interaction is required, which typically maps to a user or upstream system supplying the malicious HTML payload to a parsing routine.

Root Cause

The root cause is inefficient handling of certain HTML constructs during tokenization or tree construction. Specific input patterns produce nonlinear time complexity in the parser's state transitions. Technical details are documented in the Go issue tracker entry referenced below.

Attack Vector

An attacker delivers crafted HTML to a Go application that invokes the standard library HTML parser. Typical delivery channels include HTTP request bodies, file uploads, scraped pages, or webhook payloads. Each parse operation consumes excessive CPU time, and repeated submissions amplify the denial of service.

For full technical context, refer to the Go.dev Issue Tracker Entry, the Go.dev Change Log Entry, and the Go Vulnerability Advisory GO-2026-5028. No verified public exploit code is currently available.

Detection Methods for CVE-2026-25680

Indicators of Compromise

• Sustained high CPU utilization on Go application processes correlated with HTML parsing workloads
• HTTP requests containing unusually large or deeply nested HTML payloads submitted to endpoints that consume HTML
• Goroutines blocked for extended periods inside HTML parser call stacks visible in pprof profiles

Detection Strategies

• Run govulncheck against Go source trees and binaries to identify code paths that invoke vulnerable HTML parsing functions
• Profile production services with net/http/pprof and inspect CPU samples for time spent in golang.org/x/net/html or html package call frames
• Correlate web application firewall logs with backend CPU spikes to identify request payloads that trigger parser slowdowns

Monitoring Recommendations

• Alert on per-request latency or CPU time exceeding baselines for endpoints that parse HTML
• Track goroutine count and CPU saturation as service-level indicators in observability dashboards
• Log request sizes and content types at ingress points to enable post-incident replay against patched parser versions

How to Mitigate CVE-2026-25680

Immediate Actions Required

• Upgrade Go and any vendored copies of golang.org/x/net/html to the fixed versions listed in advisory GO-2026-5028
• Rebuild and redeploy all Go services that parse untrusted HTML using the patched toolchain
• Apply request size limits and parse timeouts to endpoints that accept HTML from external sources

Patch Information

The upstream fix is tracked in the Go.dev Change Log Entry and announced via the Go Programming Announcement. Consult the Go Vulnerability Advisory GO-2026-5028 for the exact module versions containing the fix.

Workarounds

• Enforce strict size limits on HTML inputs before invoking the parser
• Wrap parser invocations with context.WithTimeout to cap CPU time per request
• Reject or sanitize HTML from untrusted sources at the network edge until patched builds are deployed

# Configuration example: identify vulnerable Go modules and upgrade
govulncheck ./...
go get golang.org/x/net@latest
go mod tidy
go build ./...