Avoid truncation when encoding Kerberos token #5435
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The "httpEncode64_2" function appends padding (0-3x "="). The buffer
size calculation in "_cupsSetNegotiateAuthString" did the calculation
wrongly and would have a buffer overflow for tokens of size (N * 4)
+ 1 and (N * 4) + 2. With this change the buffer size is computed
correctly.
Proof-of-concept in Python:
$ python <<'EOF'
import base64
def calc(c):
raw = c * "A"
enclen = len(base64.b64encode(raw))
origlen = len(raw) * 4 / 3 + 1
fixedlen = ((4 * len(raw) / 3) + 3) & ~3
print
print "input len = ", c
print "encoded len =", enclen
print "orig len = ", origlen, ("(bad)" if enclen > origlen else "")
print "fixed len = ", fixedlen, ("(bad)" if enclen > fixedlen else "")
print "waste = ", fixedlen - enclen
for i in range(7): calc(i)
EOF
Output:
---
input len = 0
encoded len = 0
orig len = 1
fixed len = 0
waste = 0
input len = 1
encoded len = 4
orig len = 2 (bad)
fixed len = 4
waste = 0
input len = 2
encoded len = 4
orig len = 3 (bad)
fixed len = 4
waste = 0
input len = 3
encoded len = 4
orig len = 5
fixed len = 4
waste = 0
input len = 4
encoded len = 8
orig len = 6 (bad)
fixed len = 8
waste = 0
input len = 5
encoded len = 8
orig len = 7 (bad)
fixed len = 8
waste = 0
input len = 6
encoded len = 8
orig len = 9
fixed len = 8
waste = 0
---
|
Changing the title to reflect the issue - httpEncode64_2 won't overflow, it will truncate. |
michaelrsweet
changed the title
michaelrsweet
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Nov 14, 2018
michaelrsweet
added a commit
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this pull request
Nov 14, 2018
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The "httpEncode64_2" function appends padding (0-3x "="). The buffer
size calculation in "_cupsSetNegotiateAuthString" did the calculation
wrongly and would have a buffer overflow for tokens of size (N * 4)
+ 1 and (N * 4) + 2. With this change the buffer size is computed
correctly.
See commit message for detailed calculation.