GHSA-q2q7-5pp4-w6pg CVE-2021-33503 PYSEC-2021-108
Catastrophic backtracking in URL authority parser when passed URL containing many @ characters
High
PyPI
- Severity
- 8.7 / 10
- High · CVSS 4.0
- Exploit chance (EPSS)
- 3.27%
- Chance of exploitation in the next 30 days. Higher than 88.42% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-400 Uncontrolled Resource Consumption
- Fix
1.26.5- Upgrade urllib3 to this version or later
Description
Impact
When provided with a URL containing many @ characters in the authority component the authority regular expression exhibits catastrophic backtracking causing a denial of service if a URL were passed as a parameter or redirected to via an HTTP redirect.
Patches
The issue has been fixed in urllib3 v1.26.5.
References
For more information
If you have any questions or comments about this advisory:
- Ask in our community Discord
- Email sethmichaellarson@gmail.com
Affected versions
PyPI
| Package | Vulnerable | Fixed in |
|---|---|---|
| urllib3 | >= 1.25.4, < 1.26.5 |
1.26.5 |
| urllib3 | >= 1.25.4, < 1.26.5 |
1.26.5 |
| urllib3 | >= 1.25.4, < 1.26.5 |
1.26.5 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
urllib3 PyPI → 1.26.5
- pip
pip install --upgrade "urllib3>=1.26.5"- poetry
poetry add "urllib3>=1.26.5"- uv
uv add "urllib3>=1.26.5"
Severity breakdown
CVSS 4.0 8.7
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- Attack vector
- Network
- Attack complexity
- Low
- Attack requirements
- None
- Privileges required
- None
- User interaction
- None
- Confidentiality (vulnerable system)
- None
- Integrity (vulnerable system)
- None
- Availability (vulnerable system)
- High
- Confidentiality (subsequent systems)
- None
- Integrity (subsequent systems)
- None
- Availability (subsequent systems)
- None
CVSS 3.1 7.5
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
Weakness
CWE-400: Uncontrolled Resource Consumption
The product does not properly control the allocation and maintenance of a limited resource.
How to prevent it
- Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
- Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution is simply difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
- Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
- Implementation: Ensure that all failures in resource allocation place the system into a safe posture.
References
Advisories
Fixes and patches
Packages
Other links
- security.gentoo.org/glsa/202107-36
- oracle.com/security-alerts/cpuoct2021.html
- github.com/pypa/advisory-database/tree/main/vulns/urllib3/PYSEC-2021-108.yaml
- lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/6SCV7…
- lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/FMUGW…
- lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/6SC…
- lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/FMU…
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-vxq7-64xx-v4gw urllib3: HTTPResponse.stream()/read_chunked() buffers an unbounded chunk-size line into memory | High | Also affects urllib3 |
| GHSA-gh4c-6fx4-qh6g urllib3: Chunked Deflate streaming can enter an infinite loop | Medium | Also affects urllib3 |
| GHSA-8988-9cw3-xx77 urllib3: HTTPS proxy TLS configuration may be ignored or overridden | High | Also affects urllib3 |
| GHSA-mf9v-mfxr-j63j urllib3: Decompression-bomb safeguards bypassed in parts of the streaming API | High | Also affects urllib3 |
| GHSA-qccp-gfcp-xxvc urllib3: Sensitive headers forwarded across origins in proxied low-level redirects | High | Also affects urllib3 |
| GHSA-38jv-5279-wg99 Decompression-bomb safeguards bypassed when following HTTP redirects (streaming API) | High | Also affects urllib3 |
| GHSA-2xpw-w6gg-jr37 urllib3 streaming API improperly handles highly compressed data | High | Also affects urllib3 |
| GHSA-gm62-xv2j-4w53 urllib3 allows an unbounded number of links in the decompression chain | High | Also affects urllib3 |
Credits: NariyoshiChida, ap-wtioit
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