infinite loop with a malformed message

[James Bardin]

Found an infinite loop, triggered by a malformed ssh message.
I found it by trying to load a publickey which had been truncated, so
the actual length of the final mpint didn't match.

The first part is in Message.get_bytes.
If the length of the bytes read isn't as long as what was called for,
you get a string of '\x00'*n.
Shouldn't this simply pad the string with 0s, or is this required? (I
haven't read the entire rfc for this yet)
If I'm correct, the patch is easy:
- return '\x00'*n
+ return b + '\x00' * (n - len(b))

The infinite loop part is in in util.bit_length.
If n deflates to '\x00', hbyte will never AND with 0x80, so we loop forever.

Patch:
diff --git a/paramiko/util.py b/paramiko/util.py
index 8abdc0c..fab2b7e 100644
--- a/paramiko/util.py
+++ b/paramiko/util.py
@@ -135,6 +135,8 @@ def safe_string(s):
 def bit_length(n):
    norm = deflate_long(n, 0)
    hbyte = ord(norm[0])
+ if not hbyte:
+ return 1
    bitlen = len(norm) * 8
    while not (hbyte & 0x80):
        hbyte <<= 1

[Robey Pointer (robey)]

already applied via email