Attached is my patch for Stretch, based on the upstream patch.
I am a bit uneasy about applying this and marking CVE-2019-11841 as
fixed, because contrary to what upstream say I don't think
CVE-2019-11841 is actually fixed. From the CVE description:
[...] However, the Go clearsign package ignores the value of this
header, which allows an attacker to spoof it. Consequently, an
attacker can lead a victim to believe the signature was generated
using a different message digest algorithm than what was actually
used. [...]
The upstream patch has done nothing to address this.
--
Brian May <b...@debian.org>
diff -Nru golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/changelog golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/changelog
--- golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/changelog 2017-04-26 17:42:23.000000000 +1000
+++ golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/changelog 2020-09-07 08:29:03.000000000 +1000
@@ -1,3 +1,10 @@
+golang-go.crypto (1:0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782-1+deb8u1) jessie-security; urgency=medium
+
+ * Non-maintainer upload by the LTS Security Team.
+ * Fix CVE-2019-11841: reject potentially misleading headers and messages.
+
+ -- Brian May <b...@debian.org> Mon, 07 Sep 2020 08:29:03 +1000
+
golang-go.crypto (1:0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782-1) unstable; urgency=medium
* Reverts previous upload to permit freeze exception.
diff -Nru golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/patches/CVE-2019-11841.patch golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/patches/CVE-2019-11841.patch
--- golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/patches/CVE-2019-11841.patch 1970-01-01 10:00:00.000000000 +1000
+++ golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/patches/CVE-2019-11841.patch 2020-09-07 08:27:22.000000000 +1000
@@ -0,0 +1,261 @@
+commit 6c9f9a831536511295256d4b4960fc4024ed967c
+Author: Filippo Valsorda <fili...@golang.org>
+Date: Tue Apr 23 15:32:34 2019 -0400
+
+ openpgp/clearsign: reject potentially misleading headers and messages
+
+ Aida Mynzhasova of SEC Consult Vulnerability Lab reported that the
+ clearsign package accepts some malformed messages, which can make it
+ possible for an attacker to trick a human user (but not a Go program)
+ into thinking unverified text is part of the message.
+
+ For example, if in the following message the vertical tab character is
+ printed as a new line, a human observer could believe that the
+ unverified header text is part of the signed message.
+
+ -----BEGIN PGP SIGNED MESSAGE-----
+ Hash: SHA1\x0b\x0bThis text is part of the header.
+
+ Hello world!
+ -----BEGIN PGP SIGNATURE-----
+ Version: GnuPG v1.4.10 (GNU/Linux)
+
+ iJwEAQECAAYFAk8kMuEACgkQO9o98PRieSpMsAQAhmY/vwmNpflrPgmfWsYhk5O8
+ [...]
+ MyTpno24AjIAGb+mH1U=
+ =hIJ6
+ -----END PGP SIGNATURE-----
+
+ The OpenPGP specs are delightfully vague about purpose and validation of
+ these headers. RFC 4880, Section 7 says
+
+ The cleartext signed message consists of:
+
+ - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
+ single line,
+
+ - One or more "Hash" Armor Headers,
+
+ - Exactly one empty line not included into the message digest,
+ [...]
+
+ but also
+
+ If MD5 is the only hash used, then an
+ implementation MAY omit this header for improved V2.x compatibility.
+
+ and
+
+ If more than one message digest is used in the signature, the "Hash"
+ armor header contains a comma-delimited list of used message digests.
+
+ which seems to suggest that there can be zero or more Hash headers, each
+ with one or more algorithms, and no other header types.
+
+ Anyway, it's entirely unclear what security purpose, if any, the Hash
+ header accomplishes. If the hash is too weak to be secure or
+ unsupported, the verification will fail. Otherwise, the user shouldn't
+ care. Given its dubious function, avoid breaking abstractions to check
+ that it matches the signature, and just document it as unverified.
+
+ As for valid characters, RFC 4880 is silent, except reluctantly
+ mentioning that the Comment header can be UTF-8, but I am going to
+ assume that all hash algorithms will have ASCII names, because come on.
+
+ Even more importantly, reject non-Hash SIGNED MESSAGE headers (as opposed
+ to the SIGNATURE headers), to prevent a "Thank you!" message turning into
+
+ -----BEGIN PGP SIGNED MESSAGE-----
+ Reminder: I need you to wire $100k to 12345566 as soon as possible.
+
+ Thank you!
+ -----BEGIN PGP SIGNATURE-----
+ [...]
+
+ While at it, also check for trailing characters after the signed message
+ delimiter, as they are invalid and can be similarly used to confuse humans.
+
+ The Decode API is also unfortunate in that it doesn't return an error,
+ so we can't tell the user what's wrong with the message, but that's what
+ we've got.
+
+ Change-Id: I8a72c4851075337443d7a27e0b49a6b6e39f5a41
+ Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/453011
+ Reviewed-by: Adam Langley <a...@google.com>
+ Reviewed-on: https://go-review.googlesource.com/c/crypto/+/173778
+ Run-TryBot: Filippo Valsorda <fili...@golang.org>
+ Reviewed-by: Adam Langley <a...@golang.org>
+ TryBot-Result: Gobot Gobot <go...@golang.org>
+
+diff --git a/openpgp/clearsign/clearsign.go b/openpgp/clearsign/clearsign.go
+index def4cab..4fd02a1 100644
+--- a/openpgp/clearsign/clearsign.go
++++ b/openpgp/clearsign/clearsign.go
+@@ -17,6 +17,7 @@ import (
+ "io"
+ "net/textproto"
+ "strconv"
++ "strings"
+
+ "golang.org/x/crypto/openpgp/armor"
+ "golang.org/x/crypto/openpgp/errors"
+@@ -26,7 +27,7 @@ import (
+ // A Block represents a clearsigned message. A signature on a Block can
+ // be checked by passing Bytes into openpgp.CheckDetachedSignature.
+ type Block struct {
+- Headers textproto.MIMEHeader // Optional message headers
++ Headers textproto.MIMEHeader // Optional unverified Hash headers
+ Plaintext []byte // The original message text
+ Bytes []byte // The signed message
+ ArmoredSignature *armor.Block // The signature block
+@@ -68,8 +69,13 @@ func getLine(data []byte) (line, rest []byte) {
+ return data[0:i], data[j:]
+ }
+
+-// Decode finds the first clearsigned message in data and returns it, as well
+-// as the suffix of data which remains after the message.
++// Decode finds the first clearsigned message in data and returns it, as well as
++// the suffix of data which remains after the message. Any prefix data is
++// discarded.
++//
++// If no message is found, or if the message is invalid, Decode returns nil and
++// the whole data slice. The only allowed header type is Hash, and it is not
++// verified against the signature hash.
+ func Decode(data []byte) (b *Block, rest []byte) {
+ // start begins with a newline. However, at the very beginning of
+ // the byte array, we'll accept the start string without it.
+@@ -82,8 +88,11 @@ func Decode(data []byte) (b *Block, rest []byte) {
+ return nil, data
+ }
+
+- // Consume the start line.
+- _, rest = getLine(rest)
++ // Consume the start line and check it does not have a suffix.
++ suffix, rest := getLine(rest)
++ if len(suffix) != 0 {
++ return nil, data
++ }
+
+ var line []byte
+ b = &Block{
+@@ -102,15 +111,25 @@ func Decode(data []byte) (b *Block, rest []byte) {
+ break
+ }
+
++ // Reject headers with control or Unicode characters.
++ if i := bytes.IndexFunc(line, func(r rune) bool {
++ return r < 0x20 || r > 0x7e
++ }); i != -1 {
++ return nil, data
++ }
++
+ i := bytes.Index(line, []byte{':'})
+ if i == -1 {
+ return nil, data
+ }
+
+- key, val := line[0:i], line[i+1:]
+- key = bytes.TrimSpace(key)
+- val = bytes.TrimSpace(val)
+- b.Headers.Add(string(key), string(val))
++ key, val := string(line[0:i]), string(line[i+1:])
++ key = strings.TrimSpace(key)
++ if key != "Hash" {
++ return nil, data
++ }
++ val = strings.TrimSpace(val)
++ b.Headers.Add(key, val)
+ }
+
+ firstLine := true
+diff --git a/openpgp/clearsign/clearsign_test.go b/openpgp/clearsign/clearsign_test.go
+index 2c09480..1f52cd9 100644
+--- a/openpgp/clearsign/clearsign_test.go
++++ b/openpgp/clearsign/clearsign_test.go
+@@ -44,12 +44,6 @@ func TestParse(t *testing.T) {
+ testParse(t, clearsignInput2, "\r\n\r\n(This message has a couple of blank lines at the start and end.)\r\n\r\n", "\n\n(This message has a couple of blank lines at the start and end.)\n\n\n")
+ }
+
+-func TestParseInvalid(t *testing.T) {
+- if b, _ := Decode(clearsignInput3); b != nil {
+- t.Fatal("decoded a bad clearsigned message without any error")
+- }
+-}
+-
+ func TestParseWithNoNewlineAtEnd(t *testing.T) {
+ input := clearsignInput
+ input = input[:len(input)-len("trailing")-1]
+@@ -125,6 +119,59 @@ func TestSigning(t *testing.T) {
+ }
+ }
+
++const signatureBlock = `
++-----BEGIN PGP SIGNATURE-----
++Version: OpenPrivacy 0.99
++
++yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
++vBSFjNSiVHsuAA==
++=njUN
++-----END PGP SIGNATURE-----
++`
++
++var invalidInputs = []string{
++ `
++-----BEGIN PGP SIGNED MESSAGE-----
++Hash: SHA256
++
++(This message was truncated.)
++`,
++ `
++-----BEGIN PGP SIGNED MESSAGE-----garbage
++Hash: SHA256
++
++_o/
++` + signatureBlock,
++ `
++garbage-----BEGIN PGP SIGNED MESSAGE-----
++Hash: SHA256
++
++_o/
++` + signatureBlock,
++ `
++-----BEGIN PGP SIGNED MESSAGE-----
++Hash: SHA` + "\x0b\x0b" + `256
++
++_o/
++` + signatureBlock,
++ `
++-----BEGIN PGP SIGNED MESSAGE-----
++NotHash: SHA256
++
++_o/
++` + signatureBlock,
++}
++
++func TestParseInvalid(t *testing.T) {
++ for i, input := range invalidInputs {
++ if b, rest := Decode([]byte(input)); b != nil {
++ t.Errorf("#%d: decoded a bad clearsigned message without any error", i)
++ } else if string(rest) != input {
++ t.Errorf("#%d: did not return all data with a bad message", i)
++ }
++ }
++}
++
+ var clearsignInput = []byte(`
+ ;lasjlkfdsa
+
+@@ -167,13 +214,6 @@ qZg6BaTvOxepqOxnhVU=
+
+ trailing`)
+
+-var clearsignInput3 = []byte(`
+------BEGIN PGP SIGNED MESSAGE-----
+-Hash: SHA256
+-
+-(This message was truncated.)
+-`)
+-
+ var signingKey = `-----BEGIN PGP PRIVATE KEY BLOCK-----
+ Version: GnuPG v1.4.10 (GNU/Linux)
+
diff -Nru golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/patches/series golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/patches/series
--- golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/patches/series 2017-04-26 17:09:27.000000000 +1000
+++ golang-go.crypto-0.0~git20170407.0.55a552f+REALLY.0.0~git20161012.0.5f31782/debian/patches/series 2020-09-07 08:29:03.000000000 +1000
@@ -1 +1,2 @@
0001-ssh-require-host-key-checking_CVE-2017-3204.patch
+CVE-2019-11841.patch
