CVE-2021-3737

Exp

A flaw was found in python. An improperly handled HTTP response in the HTTP client code of python may allow a remote attacker, who controls the HTTP server, to make the client script enter an infinite loop, consuming CPU time. The highest threat from this vulnerability is to system availability.

Published: 2022-03-04 Last update: 2026-06-17 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-3737 is rated High Exploit Risk (83.4/100): CVSS High severity, with high exploitation likelihood (EPSS 11.59%, 95th percentile). 1 public exploit reference(s) are indexed (Exploit-DB). EPSS rose +11.47% over the last day, indicating growing attacker interest. Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2021-3737

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2021-3737

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.12% 11.59% +11.47%
2 2026-02-07 0.16% 0.12% -0.04%
3 2026-02-04 — 0.16% —

Full EPSS history (35 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-3737

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 134c704f-9b21-4f2e-91b3-4a467353bcc0
7.1 2.0 HIGH
AV:N/AC:M/Au:N/C:N/I:N/A:C Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:M)
Exploitation needs some favorable conditions, but not exceptional ones.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:C)
Complete availability impact.
 8.6 6.9 [email protected]

Weakness enumeration for CVE-2021-3737

OS Trackers for CVE-2021-3737

vendor priority summary link
debian not yet assigned CVE-2021-3737 not yet assigned priority: Debian including 3 source packages (pypy3, python2.7, python3.9), 7 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 6, open 1. https://security-tracker.debian.org/tracker/CVE-2021-3737
redhat low — https://access.redhat.com/security/cve/CVE-2021-3737
suse medium CVE-2021-3737 severity moderate: SUSE including 637 source package names (0.0.17-1.1:libpython3_6m1_0-3.6.15-10.9.1, 0.0.17-1.1:python3-3.6.15-10.9.1, …), 2315 product×package rows across 432 product lines (Container bci/bci-base-fips, Container bci/bci-sle15-kernel-module-devel, … (432 product lines)): Fixed 2152, Known Affected 163. https://www.suse.com/security/cve/CVE-2021-3737/
ubuntu medium CVE-2021-3737 medium priority: Ubuntu including 7 source packages (python3.10, python3.4, …), 91 status rows across 13 suites (bionic, focal, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, trusty, upstream, xenial): DNE 64, released 9, ignored 7, needs-triage 7, not-affected 4. https://ubuntu.com/security/CVE-2021-3737

Affected software / configurations for CVE-2021-3737

Vendor Product Version Raw CPE
python python >= 3.6.0, < 3.6.14 cpe:2.3:a:python:python:*:*:*:*:*:*:*:*
python python >= 3.7.0, < 3.7.11 cpe:2.3:a:python:python:*:*:*:*:*:*:*:*
python python >= 3.8.0, < 3.8.11 cpe:2.3:a:python:python:*:*:*:*:*:*:*:*
python python >= 3.9.0, < 3.9.6 cpe:2.3:a:python:python:*:*:*:*:*:*:*:*
redhat codeready_linux_builder 8.0 cpe:2.3:a:redhat:codeready_linux_builder:8.0:*:*:*:*:*:*:*
redhat codeready_linux_builder_for_ibm_z_systems 8.0 cpe:2.3:a:redhat:codeready_linux_builder_for_ibm_z_systems:8.0:*:*:*:*:*:*:*
redhat codeready_linux_builder_for_power_little_endian 8.0 cpe:2.3:a:redhat:codeready_linux_builder_for_power_little_endian:8.0:*:*:*:*:*:*:*
redhat enterprise_linux 6.0 cpe:2.3:o:redhat:enterprise_linux:6.0:*:*:*:*:*:*:*
redhat enterprise_linux 7.0 cpe:2.3:o:redhat:enterprise_linux:7.0:*:*:*:*:*:*:*
redhat enterprise_linux 8.0 cpe:2.3:o:redhat:enterprise_linux:8.0:*:*:*:*:*:*:*
redhat enterprise_linux_for_ibm_z_systems 8.0 cpe:2.3:o:redhat:enterprise_linux_for_ibm_z_systems:8.0:*:*:*:*:*:*:*
redhat enterprise_linux_for_power_little_endian 8.0 cpe:2.3:o:redhat:enterprise_linux_for_power_little_endian:8.0:*:*:*:*:*:*:*
fedoraproject fedora 33 cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*
fedoraproject fedora 34 cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
canonical ubuntu_linux 14.04 cpe:2.3:o:canonical:ubuntu_linux:14.04:*:*:*:esm:*:*:*
canonical ubuntu_linux 16.04 cpe:2.3:o:canonical:ubuntu_linux:16.04:*:*:*:esm:*:*:*
canonical ubuntu_linux 18.04 cpe:2.3:o:canonical:ubuntu_linux:18.04:*:*:*:lts:*:*:*
canonical ubuntu_linux 20.04 cpe:2.3:o:canonical:ubuntu_linux:20.04:*:*:*:lts:*:*:*
canonical ubuntu_linux 21.04 cpe:2.3:o:canonical:ubuntu_linux:21.04:*:*:*:*:*:*:*
netapp hci — cpe:2.3:a:netapp:hci:-:*:*:*:*:*:*:*
netapp management_services_for_element_software — cpe:2.3:a:netapp:management_services_for_element_software:-:*:*:*:*:*:*:*
netapp netapp_xcp_smb — cpe:2.3:a:netapp:netapp_xcp_smb:-:*:*:*:*:*:*:*
netapp ontap_select_deploy_administration_utility — cpe:2.3:a:netapp:ontap_select_deploy_administration_utility:-:*:*:*:*:*:*:*
netapp xcp_nfs — cpe:2.3:a:netapp:xcp_nfs:-:*:*:*:*:*:*:*
oracle communications_cloud_native_core_binding_support_function 22.1.3 cpe:2.3:a:oracle:communications_cloud_native_core_binding_support_function:22.1.3:*:*:*:*:*:*:*
oracle communications_cloud_native_core_network_exposure_function 22.1.1 cpe:2.3:a:oracle:communications_cloud_native_core_network_exposure_function:22.1.1:*:*:*:*:*:*:*
oracle communications_cloud_native_core_policy 22.2.0 cpe:2.3:a:oracle:communications_cloud_native_core_policy:22.2.0:*:*:*:*:*:*:*

References for CVE-2021-3737

URL Tags
https://bugs.python.org/issue44022 Exploit Issue Tracking Vendor Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1995162 Issue Tracking Patch Third Party Advisory
https://github.com/python/cpython/pull/25916 Patch Third Party Advisory
https://github.com/python/cpython/pull/26503 Patch Third Party Advisory
https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html
https://python-security.readthedocs.io/vuln/urllib-100-continue-loop.html Patch Third Party Advisory
https://security.netapp.com/advisory/ntap-20220407-0009/ Third Party Advisory
https://ubuntu.com/security/CVE-2021-3737 Patch Third Party Advisory
https://www.oracle.com/security-alerts/cpujul2022.html Patch Third Party Advisory
https://lists.debian.org/debian-lts-announce/2024/11/msg00024.html
https://lists.debian.org/debian-lts-announce/2024/12/msg00000.html
cvelogic Threat Intelligence