libssh2

from the submitter:
I believe temporary internet access can be provided if needed to test the
fix.
Thanks!

I figure one way to approach this (as I assume this behavior is considered
good and works with other ssh clients such as openssh's), as we then
clearly need to adapt to this data, is to check how openssh and/or putty
deal with it.

From the submitter-
The issue was observed when connecting to a juniper E-series router.
Unfortinately it is behind a firewall.
The issue can still be investigating by looking at the packet the server
sent. It is in the attachment and also bellow. It looks like the format
of the packet that the server is sending is different from what libssh2
expects. For example the server sends the DSS signature lenght at the end
of the packet - the DSS signature is the last 40 bytes of the packet. Its
length is contained as 32 bits uint (4 bytes) immediately preceding the DSS
signature lenght ( 00 00 00 28 -> this equals 40 decimal when
represented as a uint32_t ). Note that the packet lenght is 615 bytes.
On the other hand It looks like libssh2 is expecting the DSS signature to
be towards the begining of the packet hence the described calculation:
keyname length(4), key name(7) and sig length(4) 4+7+4= 15. 40 - 15 == 25.
Indeed the key name(ssh-dss) is near the beginning at the packet but the
DSS signature in this case is at the end so the calculation seems to not
work out for this packet format.

And bellow comes the packet sent by the server:
=> libssh2_transport_read() plain (615 bytes)
0000: 1f 00 00 01 b1 00 00 00 07 73 73 68 2d 64 73 73 .........ssh-dss
0010: 00 00 00 81 00 af 8d cf ea 6d 73 c1 2d aa b1 5b .........ms.-..[
0020: fd 58 e0 89 88 26 04 aa ea cb a6 b0 81 2a 1a a0 .X...&.......*..
0030: 56 b6 d1 3a c8 8a d9 95 94 cf 22 16 df 59 63 6a V..:......"..Ycj
0040: 19 44 2a a8 91 20 d8 ad 71 30 f4 8a b8 b0 75 59 .D*.. ..q0....uY
0050: 40 28 a9 ae 49 36 74 1a a8 d7 2d bd 3c ed e4 76 @(..I6t...-.<..v
0060: 98 36 50 50 0b c9 4a 2c 70 c5 ed fa 2e 19 1b 72 .6PP..J,p......r
0070: 2f 5e fb 6f 8a 62 36 0f 6f ba 96 1d ca a8 71 d9 /^.o.b6.o.....q.
0080: b8 87 f1 74 e5 c7 83 3c 39 66 36 10 18 9a 9d 8c ...t...<9f6.....
0090: d6 4c c6 59 a9 00 00 00 15 00 c3 7e 0d 3d 81 b2 .L.Y.......~.=..
00a0: 06 33 86 1a 85 eb 4e df fc d2 27 34 9b ab 00 00 .3....N...'4....
00b0: 00 80 24 fa 68 5c 33 1a e8 8c f8 55 2e 3f bb bc ..$.h\3....U.?..
00c0: 90 85 0a 56 11 87 b6 b5 e1 d1 0f 7d 28 0e 46 5e ...V.......}(.F^
00d0: bc 19 9f 99 36 82 a8 59 27 a8 1b a6 bd 75 fa 9e ....6..Y'....u..
00e0: da 47 ad 8d fc 17 6f f7 ac 58 3e 2d 7d fa 49 63 .G....o..X>-}.Ic
00f0: 55 37 c6 09 ad bd f8 b2 50 e4 e9 c8 1b a0 8e 08 U7......P.......
0100: 7e d7 04 2b 87 d0 e5 26 8e 84 2b d5 95 3a ad df ~..+...&..+..:..
0110: db d6 b8 f8 22 1b ba 5d 9b cc 30 87 c0 47 cf 53 ...."..]..0..G.S
0120: 8f 42 58 4c 8d 8f 23 d9 2c f1 65 72 fe e3 ee 8a .BXL..#.,.er....
0130: 53 b0 00 00 00 80 31 c2 98 57 f3 e2 0b 12 49 e7 S.....1..W....I.
0140: c5 e2 d3 48 66 53 52 d9 f3 f7 77 67 cb b7 b3 95 ...HfSR...wg....
0150: 7b e6 29 b0 af b0 42 9e 6a b3 b9 e7 4e f5 2c 40 {.)...B.j...N.,@
0160: 75 54 d8 8e 2b 38 91 f5 fc 74 2e 09 bd 0a 92 44 uT..+8...t.....D
0170: 22 cd af 4f 8e 8c f4 72 f3 8e 2d 6a 59 0e 86 fd "..O...r..-jY...
0180: 70 ec 52 1d 1f 2c d6 6d 3b 78 6b 48 76 e2 ec 1a p.R..,.m;xkHv...
0190: 66 72 13 61 83 64 2f 82 be be 3e 40 84 e4 10 7c fr.a.d/...>@...|
01a0: 35 b0 07 e8 71 a0 0c 98 d4 72 26 d2 40 e1 2f 60 5...q....r&.@./`
01b0: e5 8a 69 4a d7 f5 00 00 00 81 00 e1 de 67 56 2c ..iJ.........gV,
01c0: ce d8 08 c3 9d 27 a1 1e 7c 64 be ee ca 17 68 a2 .....'..|d....h.
01d0: 64 61 3d ed 75 41 09 a9 64 2a 50 f1 de 71 62 48 da=.uA..d*P..qbH
01e0: e0 6e 95 b5 67 97 84 07 69 1d 78 3a 42 4e 28 b0 .n..g...i.x:BN(.
01f0: 25 5c 3e 2c 45 53 43 fb 0b 20 4b 41 5a 8f 39 76 %\>,ESC.. KAZ.9v
0200: 69 c0 a0 16 6a de 2c bb 71 93 28 97 83 bf d9 e3 i...j.,.q.(.....
0210: 7d bf 24 43 79 8d a1 fd 90 e2 91 87 5d 52 0e 9e }.$Cy.......]R..
0220: 48 39 12 02 d0 3f 2a e7 a8 c1 53 96 a3 72 f3 3d H9...?*...S..r.=
0230: 5c a6 34 79 25 a9 ff 92 60 05 5c 00 00 00 28 70 \.4y%...`.\...(p
<-- 00 00 00 28 is DSS sig len
0240: 9a af 73 58 d2 d0 b8 de 23 e6 ce 4e 66 34 8d bd ..sX....#..Nf4..
0250: 03 e2 82 36 de ce dd c2 22 ff a6 07 95 a9 06 5d ...6...."......]
0260: c0 91 ec ed 79 a9 c2 ....y..

Thanks for your assistance guys!

src/hostkey.c:hostkey_method_ssh_dss_sig_verify() checks that the sig
length is 40 bytes. The additional 15 is the the field lengths: keyname
length(4), key name(7) and sig length(4) 4+7+4= 15. 40 - 15 == 25.

So the entire data chunk should rather be 55 bytes to please libssh2.

do you have a decent way to repeat this problem?

from the submitter:
I am using libssh2-1.2 and trying to connect to junose router which uses
dss key. tcpdump trace can be provided upon request.

Artifact 2529398 might be the same problem if the server sends DSS length
of 40 and libssh2 thinks it is 25.