Go Toolchain CVE-2026-111943

Applies to applications built with affected Go versions (prior to 1.27.3 and 1.26.10) that rely on crypto/x509 Verify / crypto/tls to validate X.509 certificate chains. Concrete prerequisite for abuse: an attacker must possess a certificate that chains through one of the specific distrusted intermediate CA certificates and must hold a man-in-the-middle position to present it to the Go client. LIMITING FACTORS: exploitation is bounded to the narrow, specific set of intermediates in question (not all CAs); it requires both a mis-issued/attacker-held certificate from that CA lineage and network interception capability; applications using a custom verifier or pinning may be unaffected; the fix is purely a client-side trust filter, so only updating the Go toolchain and rebuilding mitigates it.

This is a defensive trust-store change rather than a code-execution flaw: Go 1.27.3 and 1.26.10 make crypto/x509 Verify filter out certificate chains that pass through a small set of intermediate CA certificates distrusted 'due to specific security concerns around their operation.' No CVSS vector, CWE, EPSS, KEV, or POC data is provided, so a quantitative severity signal is entirely absent. By SSVC-style reasoning, exploitation status is 'no public exploit identified at time of analysis' and the Go issue is labeled a Security release-blocker tracked on an URGENT internal track (b/575512169), which signals vendor-assessed importance but NOT confirmed in-the-wild abuse. The real-world risk is conditional: the danger is not in Go itself but in the possibility that the distrusted intermediates could issue or have issued certificates usable to impersonate TLS endpoints, which Go-based clients would previously have accepted. Because successful abuse requires both a certificate chaining to one of the specific intermediates AND a man-in-the-middle position, this is a targeted-prerequisite issue rather than a mass-exploitable one. CONFLICT/GAP TO NOTE: no CVSS was published, so any severity here is inferred; the specific CA and intermediates are not named in the provided data, so the precise exposure cannot be independently confirmed - verify against the Go release notes and go.dev/issue/82345.

An attacker who controls (or coerces) one of the distrusted intermediate CAs obtains a mis-issued certificate for a target domain, then positions themselves as a man-in-the-middle (e.g., on a shared or compromised network) against a Go application that uses crypto/tls/crypto/x509 to validate server certificates. On unpatched Go (prior to 1.27.3/1.26.10) the chain validates and the client trusts the forged endpoint, enabling interception or tampering of the TLS session. No public exploit code is identified at time of analysis.