Impact
urllib3 supports configuring TLS independently for an HTTPS proxy and the target server.
proxysslcontext, proxyasserthostname, and proxyassertfingerprint configure the TLS connection to the proxy. sslcontext 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 proxysslcontext 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 certreqs="CERTNONE" for a target server could overwrite the verifymode 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 useforwardingforhttps=True, and proxysslcontext is configured for the proxy.
urllib3 may ignore proxysslcontext and use the target server's sslcontext 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 proxysslcontext, while the target server uses a different certificate-verification policy.
urllib3 may apply the target server's certreqs value to the proxy SSL context. For example, setting certreqs="CERTNONE" for the target server may also disable certificate verification for the HTTPS proxy, even when proxysslcontext 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 proxysslcontext across multiple connections, and certificate verification is disabled for one target server.
urllib3 may modify the proxy SSL context's verifymode 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 useforwardingforhttps=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 proxysslcontext 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 useforwardingforhttps=True and proxysslcontext is not provided, urllib3 2.8.0 continues to use sslcontext for the TLS connection to the proxy. This configuration emits a FutureWarning. In urllib3 3.0, passing sslcontext with useforwardingforhttps=True for an HTTPS proxy will raise an error. Applications should use proxysslcontext to configure TLS for an HTTPS forwarding proxy.
The fixes were implemented in commits b6447295fff7b38fdffc67e0df9712d60cef3cc3 and 07408cec79d1856d81bb42c74a904a24fdb9e465.
Impact
urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading content in chunks instead of loading the entire response body into memory at once.
urllib3 can decompress response bodies according to the HTTP Content-Encoding header. When streaming a compressed, chunked response, urllib3 first consumes data already buffered by the decoder before reading the next HTTP chunk.
However, urllib3 versions from 2.6.2 through 2.7.0 could enter an infinite loop when a chunked response contained bytes after the end of the Deflate stream. If the decompressed body exceeded the requested streaming chunk size, Python's zlib implementation could retain the trailing bytes as unconsumed input after reaching the end of the compressed stream. urllib3 would repeatedly attempt to decode those same bytes without making progress or reading more data from the network.
A malicious server could exploit this behavior to cause excessive CPU usage and prevent the affected request from completing on the client. Network read timeouts would not interrupt the loop because no further socket operation was required.
Affected usages
Applications and libraries using urllib3 versions 2.6.2 through 2.7.0 may be affected when all of the following conditions are met:
1. Compressed responses from an untrusted source are streamed using HTTPResponse.stream(amt=N) or HTTPResponse.readchunked(amt=N) with a positive, finite chunk size. 2. Content decoding is enabled. 3. The response uses both Transfer-Encoding: chunked and Content-Encoding: deflate. 4. The decoded body exceeds the requested chunk size and the encoded body contains bytes after the end of the Deflate stream.
HTTPResponse.stream() uses a finite chunk size by default and is therefore affected when the other conditions are met.
Remediation
Upgrade to urllib3 2.8.0, in which the Deflate decoder stops accepting input after reaching the end of the compressed stream and no longer reports trailing bytes as data that can produce more decoded output.
If upgrading is not immediately possible, disable automatic content decoding for responses from untrusted sources by setting decodecontent=False, or reject streamed responses using the Deflate content encoding. Applications that disable automatic decoding must handle the compressed response safely at another layer.
Impact
urllib3's streaming API is designed for efficiently handling large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. When decoding a chunked-transfer-encoded response, this API reads each chunk's size field by buffering until it sees \n or EOF.
A malicious HTTP server can return Transfer-Encoding: chunked and then send a very long run of bytes without any newline, causing the streaming client to buffer that entire run before urllib3 can reject the chunk size as invalid and allocate more memory than intended.
To fix the issue, we'll reject chunk-size fields larger than 65536 bytes, as already done in the non-streaming case, which is currently handled by the Python standard library. Note that this only applies to reading the chunk-size field, not the chunk data, which is already handled correctly. Thus, there shouldn't be any impact for non-malicious servers.
Affected usages
Applications and libraries using urllib3 versions earlier than 2.8.0 may be affected when streaming a chunked response from untrusted sources. Specifically, this affects the readchunked() and stream() methods of the HTTPResponse object.
This also affects requests streaming API, which uses urllib3 under the hood.
Remediation
Upgrade to urllib3 version 2.8.0 or later, where chunk-size fields larger than 65536 will be rejected. If upgrading is not immediately possible, consider reading the response at once using the read() method.
urllib3 is an HTTP client library for Python. urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). When using the streaming API, the library decompresses only the necessary bytes, enabling partial content consumption. Starting in version 1.22 and prior to version 2.6.3, for HTTP redirect responses, the library would read the entire response body to drain the connection and decompress the content unnecessarily. This decompression occurred even before any read methods were called, and configured read limits did not restrict the amount of decompressed data. As a result, there was no safeguard against decompression bombs. A malicious server could exploit this to trigger excessive resource consumption on the client. Applications and libraries are affected when they stream content from untrusted sources by setting preloadcontent=False when they do not disable redirects. Users should upgrade to at least urllib3 v2.6.3, in which the library does not decode content of redirect responses when preloadcontent=False. If upgrading is not immediately possible, disable redirects by setting redirect=False for requests to untrusted source.
Impact
urllib3 supports chained HTTP encoding algorithms for response content according to RFC 9110 (e.g., Content-Encoding: gzip, zstd).
However, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data.
Affected usages
Applications and libraries using urllib3 version 2.5.0 and earlier for HTTP requests to untrusted sources unless they disable content decoding explicitly.
Remediation
Upgrade to at least urllib3 v2.6.0 in which the library limits the number of links to 5.
If upgrading is not immediately possible, use preloadcontent=False and ensure that resp.headers["content-encoding"] contains a safe number of encodings before reading the response content.
Impact
urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once.
When streaming a compressed response, urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). The library must read compressed data from the network and decompress it until the requested chunk size is met. Any resulting decompressed data that exceeds the requested amount is held in an internal buffer for the next read operation.
The decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This can result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data; CWE-409) on the client side, even if the application only requested a small chunk of data.
Affected usages
Applications and libraries using urllib3 version 2.5.0 and earlier to stream large compressed responses or content from untrusted sources.
stream(), read(amt=256), read1(amt=256), readchunked(amt=256), readinto(b) are examples of urllib3.HTTPResponse method calls using the affected logic unless decoding is disabled explicitly.
Remediation
Upgrade to at least urllib3 v2.6.0 in which the library avoids decompressing data that exceeds the requested amount.
If your environment contains a package facilitating the Brotli encoding, upgrade to at least Brotli 1.2.0 or brotlicffi 1.2.0.0 too. These versions are enforced by the urllib3[brotli] extra in the patched versions of urllib3.
Credits
The issue was reported by @Cycloctane. Supplemental information was provided by @stamparm during a security audit performed by 7ASecurity and facilitated by OSTIF.
urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.0 and prior to 2.6.0, the Streaming API improperly handles highly compressed data. urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. When streaming a compressed response, urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). The library must read compressed data from the network and decompress it until the requested chunk size is met. Any resulting decompressed data that exceeds the requested amount is held in an internal buffer for the next read operation. The decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This can result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data.
urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.
urllib3 is an HTTP client library for Python. From 2.6.0 to before 2.7.0, urllib3 could decompress the whole response instead of the requested portion (1) during the second HTTPResponse.read(amt=N) call when the response was decompressed using the official Brotli library or (2) when HTTPResponse.drainconn() was called after the response had been read and decompressed partially (compression algorithm did not matter here). These issues could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This could result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data) on the client side. This vulnerability is fixed in 2.7.0.
urllib3 is an HTTP client library for Python. From 1.23 to before 2.7.0, cross-origin redirects followed from the low-level API via ProxyManager.connectionfromurl().urlopen(..., assertsamehost=False) still forward these sensitive headers. This vulnerability is fixed in 2.7.0.
https://github.com/urllib3/urllib3/security/advisories/GHSA-gm62-xv2j-4w53 discloses: Unbounded number of links in the decompression chain ====================================================
illia-v published GHSA-gm62-xv2j-4w53 Dec 5, 2025
Package: urllib3 (pip) Affected versions: >=1.24,<2.6.0 Patched versions: 2.6.0
Description ===========
Impact ------
urllib3 supports chained HTTP encoding algorithms for response content according to RFC 9110 (e.g., Content-Encoding: gzip, zstd).
However, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data.
Affected usages ---------------
Applications and libraries using urllib3 version 2.5.0 and earlier for HTTP requests to untrusted sources unless they disable content decoding explicitly.
Remediation -----------
Upgrade to at least urllib3 v2.6.0 in which the library limits the number of links to 5.
If upgrading is not immediately possible, use preloadcontent=False and ensure that resp.headers["content-encoding"] contains a safe number of encodings before reading the response content.
Severity: High 8.9 / 10 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H CVE ID: CVE-2025-66418 Weaknesses: Weakness CWE-770
Credits ------- @illia-v illia-v Reporter @sethmlarson sethmlarson Remediation reviewer @pquentin pquentin Remediation reviewer https://github.com/urllib3/urllib3/security/advisories/GHSA-2xpw-w6gg-jr37 advises: Streaming API improperly handles highly compressed data =======================================================
illia-v published GHSA-2xpw-w6gg-jr37 Dec 5, 2025
Package: urllib3 (pip) Affected versions: >=1.0,<2.6.0 Patched versions: 2.6.0
Description ===========
Impact ------
urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once.
When streaming a compressed response, urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). The library must read compressed data from the network and decompress it until the requested chunk size is met. Any resulting decompressed data that exceeds the requested amount is held in an internal buffer for the next read operation.
The decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This can result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data; CWE-409) on the client side, even if the application only requested a small chunk of data.
Affected usages ---------------
Applications and libraries using urllib3 version 2.5.0 and earlier to stream large compressed responses or content from untrusted sources.
stream(), read(amt=256), read1(amt=256), readchunked(amt=256), readinto(b) are examples of urllib3.HTTPResponse method calls using the affected logic unless decoding is disabled explicitly.
Remediation -----------
Upgrade to at least urllib3 v2.6.0 in which the library avoids decompressing data that exceeds the requested amount.
If your environment contains a package facilitating the Brotli encoding, upgrade to at least Brotli 1.2.0 or brotlicffi 1.2.0.0 too. These versions are enforced by the urllib3[brotli] extra in the patched versions of urllib3.
Credits -------
The issue was reported by @Cycloctane.
Supplemental information was provided by @stamparm during a security audit performed by 7ASecurity and facilitated by OSTIF.
Severity: High 8.9 / 10 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H CVE ID: CVE-2025-66471 Weaknesses: Weakness CWE-409
Credits -------
@illia-v illia-v Coordinator @pquentin pquentin Remediation reviewer @sethmlarson sethmlarson Analyst @Cycloctane Cycloctane Finder @stamparm stamparm Finder
It was discovered that urllib3 incorrectly handled certain character sequences. A remote attacker could possibly use this issue to perform CRLF injection.
USN-3990-1 fixed a vulnerability in urllib3. This update provides the corresponding update for Ubuntu 14.04 ESM. Original advisory details: It was discovered that urllib3 incorrectly stripped certain characters from requests. A remote attacker could use this issue to perform CRLF injection. (CVE-2019-11236)
It was discovered that urllib3 incorrectly removed Authorization HTTP headers when handled cross-origin redirects. This could result in credentials being sent to unintended hosts. This issue only affected Ubuntu 16.04 LTS, Ubuntu 18.04 LTS and Ubuntu 18.10. (CVE-2018-20060) It was discovered that urllib3 incorrectly stripped certain characters from requests. A remote attacker could use this issue to perform CRLF injection. (CVE-2019-11236) It was discovered that urllib3 incorrectly handled situations where a desired set of CA certificates were specified. This could result in certificates being accepted by the default CA certificates contrary to expectations. This issue only affected Ubuntu 18.04 LTS, Ubuntu 18.10, and Ubuntu 19.04. (CVE-2019-11324)