Overview
CVE-2026-25793 describes an ECDSA signature malleability issue in Nebula that allows an attacker to evading a blocklist entry that was created against the fingerprint of a certificate by manipulating the signature to produce a certificate with a different fingerprint. This is categorized under CWE-347: Insecure Certificate Validation/Networking and manifests as blocklists or pinning that rely solely on a certificate's fingerprint, which can be altered via malleable ECDSA signatures. In real-world deployments, this enables attackers to bypass access restrictions or tamper-evident controls that depend on fingerprint checks, undermining trust boundaries in TLS-based communications. The patch, introduced in Nebula 1.10.3, mitigates these bypass opportunities by avoiding brittle fingerprint-based checks and reinforcing certificate validation semantics. For Go (Gin) applications, this class of issue translates to similar bypass risks when developers implement ad-hoc fingerprint or pinning logic that can be undermined by cryptographic malleability rather than relying on proper TLS verification and robust pinning strategies.
Affected Versions
Nebula 1.7.0-1.10.2; patched in 1.10.3
Code Fix Example
Go (Gin) API Security Remediation
Vulnerable pattern (Go with Gin using a fingerprint blocklist in TLS/VerifyPeerCertificate):
package main
import (
"crypto/sha256"
"crypto/tls"
"crypto/x509"
"encoding/hex"
"errors"
"log"
"net/http"
"github.com/gin-gonic/gin"
)
var blockedFingerprints = map[string]bool{
// example fingerprint(s) in hex
"abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789": true,
}
func main() {
r := gin.Default()
r.GET("/", func(c *gin.Context) {
c.String(http.StatusOK, "Hello World")
})
// Vulnerable: fingerprint-based blocklisting via VerifyPeerCertificate
tlsConfigVuln := &tls.Config{
ClientAuth: tls.RequireAndVerifyClientCert,
MinVersion: tls.VersionTLS12,
VerifyPeerCertificate: func(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
if len(rawCerts) == 0 {
return errors.New("no client certificate presented")
}
cert, err := x509.ParseCertificate(rawCerts[0])
if err != nil {
return err
}
fp := sha256.Sum256(cert.Raw)
if blockedFingerprints[hex.EncodeToString(fp[:])] {
return errors.New("blocked certificate fingerprint detected")
}
return nil
},
}
srv := &http.Server{
Addr: ":8443",
Handler: r,
TLSConfig: tlsConfigVuln,
}
go func() {
log.Fatal(srv.ListenAndServeTLS("server.crt", "server.key"))
}()
select {}
}
// Fixed pattern: use proper TLS verification and SPKI pinning instead of fingerprint-based blocklists
package main
import (
"crypto/sha256"
"crypto/tls"
"crypto/x509"
"encoding/hex"
"errors"
"log"
"net/http"
"github.com/gin-gonic/gin"
)
// Allowed SPKI fingerprints (sha256 of cert.RawSubjectPublicKeyInfo)
var allowedSPKIFingerprints = map[string]bool{
// example SPKI fingerprints in hex; populate with real values for your infra
"0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef": true,
}
func main() {
r := gin.Default()
r.GET("/", func(c *gin.Context) { c.String(http.StatusOK, "Hello World") })
// Fixed: verify client certs using standard chain verification and SPKI pinning
tlsConfigFix := &tls.Config{
ClientAuth: tls.RequireAndVerifyClientCert,
MinVersion: tls.VersionTLS12,
VerifyPeerCertificate: func(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
if len(rawCerts) == 0 {
return errors.New("no client certificate presented")
}
cert, err := x509.ParseCertificate(rawCerts[0])
if err != nil {
return err
}
// Ensure the certificate chain is valid using system roots
// Note: we rely on Verify to perform standard chain checks; here we do a light inline verification
opts := x509.VerifyOptions{Roots: nil}
if _, err := cert.Verify(opts); err != nil {
return err
}
// SPKI pinning: enforce allowed fingerprints rather than fingerprinting the entire cert
spki := sha256.Sum256(cert.RawSubjectPublicKeyInfo)
if !allowedSPKIFingerprints[hex.EncodeToString(spki[:])] {
return errors.New("untrusted client SPKI fingerprint")
}
return nil
},
}
srv := &http.Server{Addr: ":8443", Handler: r, TLSConfig: tlsConfigFix}
go func() {
log.Fatal(srv.ListenAndServeTLS("server.crt", "server.key"))
}()
select {}
}