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CVE-2026-78661: Avoid quadratic complexity when coalescing text nodes in html in golang.org/x/net

• Severity: unknown
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-27 · updated 2026-10-28

Description

When coalescing adjacent text nodes in addText and fosterParent, appending to an existing TextNode's Data via string concatenation (n.Data += text) reallocated and copied the accumulated string on every token. Inputs containing many short text runs separated by ignored end tags (such as "a</x>") caused O(n^2) allocations and byte copies. Accumulate coalesced text in per-node byte slices during parsing and materialize Data strings once at the end of parse.

Affected products

• Golang.Org/X/Net: before 0.62.0 — fixed in 0.62.0
• golang.org/x/net/html: before 0.62.0 — fixed in 0.62.0

How to fix

Upgrade to a fixed version: Golang.Org/X/Net 0.62.0; golang.org/x/net/html 0.62.0. Check the vendor advisory for mitigations.

References

• https://go.dev/cl/849365
• https://go.dev/issue/82224
• https://groups.google.com/g/golang-announce/c/sqrs2vTx_Xs
• https://pkg.go.dev/vuln/GO-2026-6860

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