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http.cookies.SimpleCookie.load() fails to consistently handle malformed cookies #127195
Description
Bug report
Bug description:
There are several issues with http.cookies.SimpleCookie.load() that deviate from current browser behavior:
- Malformed cookies are not processed at all
Consider the cookie a=b;c=d\x09d;e=f. The e value contains \x09, which is not allowed per RFC 6265, Section 4.1.1.
When this is sent to a browser (Chrome 130), the browser processes all valid cookies and filters out invalid ones:
HTTP/1.1 200 OK
Content-Type: text/html
Set-Cookie: a=b;
Set-Cookie: c=d d;
Set-Cookie: e=f
Resulting behavior:
> document.cookie
< 'a=b; e=f'
However, http.cookies.SimpleCookie.load() ignores the entire cookie string:
>>> from http import cookies
>>> C = cookies.SimpleCookie()
>>> C.load("a=b;c=d\x09d;e=f")
>>> C.output()
''
- Malformed cookies are inconsistently processed
Consider the cookie a=b;c={"d":"e"};f=g. The c value is invalid per RFC 6265, Section 4.1.1.
Browsers process this cookie without an issue:
HTTP/1.1 200 OK
Content-Type: text/html
Set-Cookie: a=b;
Set-Cookie: c={"d":"e"};
Set-Cookie: f=g
Resulting behavior:
> document.cookie
< 'a=b; c={"d":"e"}; f=g'
However, http.cookies.SimpleCookie.load() processes only the valid portion before the malformed cookie and stops entirely:
>>> from http import cookies
>>> C = cookies.SimpleCookie()
>>> C.load('a=b; c={"d":"e"}; f=g')
>>> C.output()
'Set-Cookie: a=b'
It seems we should ensure consistent handling by (a) processing all valid cookies and discarding only invalid ones, or
(b) rejecting the entire cookie string if any invalid cookie is present.
CPython versions tested on:
CPython main branch
Operating systems tested on:
No response
Activity
moonsikpark commentedon Nov 24, 2024
After further investigation, it appears the module exhibits inconsistent behavior when encountering unexpected inputs and it is likely due to its regex-based parsing, which assumes very clean input.
cpython/Lib/http/cookies.py
Lines 420 to 437 in a4d4c1e
If the maintainers are in agreement, I’d like to propose a patch to improve cookie parsing by following these steps:
(a) Validate the key. If invalid, issue a warning and skip the part.
(b) Validate the value. If invalid, issue a warning and skip the part.
Additionally, I’d like to gather opinions on whether we should allow invalid—but commonly occurring—characters in cookie values, such as those used in JSON. While the RFC advises against accepting these characters, they are widely accepted by major browsers and have become a common practice among web developers.
@picnixz, it seems you triaged my issue. If possible, could you notify the appropriate core maintainers?
picnixz commentedon Nov 27, 2024
cc @serhiy-storchaka
vadmium commentedon Dec 12, 2024
The first case with the tab character looks the same as Issue #75637. For the second case with double quotes, there are a few related reports:
nburns commentedon Aug 6, 2025
Was wondering if someone would be willing to take a look at my issue/PR (and provide feedback) related to a similar issue with the cookies parser:
Browsers tolerate json values well, and for example Zoom sets cookies with json in them:
#92936
so it would be nice if the cookie parser could handle them (webob for example is able to do so): #113663