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urllib3: HTTPS proxy TLS configuration may be ignored or overridden

High severity GitHub Reviewed Published Sep 15, 2026 in urllib3/urllib3 • Updated Sep 30, 2026

Package

pip urllib3 (pip)

Affected versions

>= 1.26.0, < 2.8.0

Patched versions

2.8.0

Description

Impact

urllib3 supports configuring TLS independently for an HTTPS proxy and the target server.

proxy_ssl_context, proxy_assert_hostname, and proxy_assert_fingerprint configure the TLS connection to the proxy. ssl_context and the other target-specific TLS parameters configure the connection to the target server.

In urllib3 versions 1.26.0 through 2.7.0, these configurations were not consistently separated. Depending on the proxy mode, urllib3 could:

  1. Ignore proxy_ssl_context and use the target server's SSL context for the TLS connection to an HTTPS forwarding proxy.
  2. Override the HTTPS proxy's certificate-verification policy with the target server's certificate-verification policy.
  3. Apply target-specific SNI, hostname assertions, certificate fingerprint assertions, or TLS client credentials to the TLS connection to an HTTPS forwarding proxy.

In particular, configuring cert_reqs="CERT_NONE" for a target server could overwrite the verify_mode of the SSL context configured for the HTTPS proxy. This modification occurred in place and persisted on the context object, potentially disabling proxy certificate verification for later connections that reused the same context.

An attacker able to intercept the connection to an HTTPS proxy may be able to impersonate the proxy when the effective proxy TLS configuration disables certificate verification or otherwise accepts the attacker's certificate. This may occur, for example, when the target server's trust or identity policy is incorrectly applied to the proxy connection.

When HTTPS forwarding is enabled, an impersonated proxy can observe or modify forwarded requests and responses, potentially exposing credentials, authentication tokens, request bodies, response data, and other sensitive information.

A TLS client certificate intended for the target server may also be presented to the proxy or to an attacker impersonating it. This can disclose the client's identity and provide proof of possession of the corresponding private key. The private key itself is not transmitted during the TLS handshake.

In CONNECT tunneling mode, impersonating the HTTPS proxy does not by itself defeat the separate end-to-end TLS connection between the client and the target server.

Affected Usages

Code using urllib3 versions 1.26.0 through 2.7.0 may be affected in any of the following cases.

1. The target SSL context is used for an HTTPS forwarding proxy

HTTPS requests are forwarded through an HTTPS proxy with use_forwarding_for_https=True, and proxy_ssl_context is configured for the proxy.

urllib3 may ignore proxy_ssl_context and use the target server's ssl_context for the proxy TLS handshake. The proxy may therefore be verified using the target server's trust and certificate policy instead of the policy explicitly configured for the proxy.

2. The target verification policy overrides the proxy policy

An HTTPS proxy is configured with proxy_ssl_context, while the target server uses a different certificate-verification policy.

urllib3 may apply the target server's cert_reqs value to the proxy SSL context. For example, setting cert_reqs="CERT_NONE" for the target server may also disable certificate verification for the HTTPS proxy, even when proxy_ssl_context was configured to require verification.

This issue can affect the TLS connection to an HTTPS proxy in both forwarding and CONNECT tunneling configurations.

3. A mutated proxy SSL context is reused

The same SSL context is reused as proxy_ssl_context across multiple connections, and certificate verification is disabled for one target server.

urllib3 may modify the proxy SSL context's verify_mode in place. Later connections that reuse the same context may therefore connect to the HTTPS proxy without certificate verification.

4. Target-specific TLS identity or credentials are applied to the proxy

HTTPS requests are forwarded through an HTTPS proxy with use_forwarding_for_https=True, and target-specific SNI, hostname assertions, certificate fingerprint assertions, or TLS client credentials are configured.

urllib3 may apply these target-specific settings to the proxy TLS handshake. This may cause urllib3 to:

  • send SNI intended for the target server to the proxy;
  • verify the proxy using a hostname or certificate fingerprint intended for the target server; or
  • present a TLS client certificate intended for the target server to the proxy.

Code connecting through a plain HTTP proxy does not establish a TLS connection to the proxy and is not affected by this issue.

Remediation

Upgrade to urllib3 2.8.0 or later.

urllib3 2.8.0 independently applies the explicitly configured proxy_ssl_context and proxy-specific certificate assertions to the HTTPS proxy connection. Target-specific SNI, certificate assertions, and TLS client certificate parameters are no longer applied to the HTTPS proxy handshake.

For backward compatibility, when an HTTPS proxy is used with use_forwarding_for_https=True and proxy_ssl_context is not provided, urllib3 2.8.0 continues to use ssl_context for the TLS connection to the proxy. This configuration emits a FutureWarning. In urllib3 3.0, passing ssl_context with use_forwarding_for_https=True for an HTTPS proxy will raise an error. Applications should use proxy_ssl_context to configure TLS for an HTTPS forwarding proxy.

The fixes were implemented in commits b6447295fff7b38fdffc67e0df9712d60cef3cc3 and 07408cec79d1856d81bb42c74a904a24fdb9e465.

References

@illia-v illia-v published to urllib3/urllib3 Sep 15, 2026
Published by the National Vulnerability Database Sep 29, 2026
Published to the GitHub Advisory Database Sep 30, 2026
Reviewed Sep 30, 2026
Last updated Sep 30, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(11th percentile)

Weaknesses

Improper Certificate Validation

The product does not validate, or incorrectly validates, a certificate. Learn more on MITRE.

Expected Behavior Violation

A feature, API, or function does not perform according to its specification. Learn more on MITRE.

CVE ID

CVE-2026-97687

GHSA ID

GHSA-8988-9cw3-xx77

Source code

Credits

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