Details

urllib3 handles redirects and retries using the same mechanism, which is controlled by the Retry object. The most common way to disable redirects is at the request level, as follows:

resp = urllib3.request("GET", "https://httpbin.org/redirect/1", redirect=False)
print(resp.status)

##### 302

However, it is also possible to disable redirects, for all requests, by instantiating a PoolManager and specifying retries in a way that disable redirects:

import urllib3

http = urllib3.PoolManager(retries=0)  # should raise MaxRetryError on redirect
http = urllib3.PoolManager(retries=urllib3.Retry(redirect=0))  # equivalent to the above
http = urllib3.PoolManager(retries=False)  # should return the first response

resp = http.request("GET", "https://httpbin.org/redirect/1")

However, the retries parameter is currently ignored, which means all the above examples don't disable redirects.

Affected usages

Passing retries on PoolManager instantiation to disable redirects or restrict their number.

By default, requests and botocore users are not affected.

Impact

Redirects are often used to exploit SSRF vulnerabilities. An application attempting to mitigate SSRF or open redirect vulnerabilities by disabling redirects at the PoolManager level will remain vulnerable.

Remediation

You can remediate this vulnerability with the following steps:

• Upgrade to a patched version of urllib3. If your organization would benefit from the continued support of urllib3 1.x, please contact sethmichaellarson@gmail.com to discuss sponsorship or contribution opportunities.
• Disable redirects at the request() level instead of the PoolManager() level.

Severity

• CVSS Score: 5.3 / 10 (Medium)
• Vector String: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N

References

• https://github.com/urllib3/urllib3/security/advisories/GHSA-pq67-6m6q-mj2v
• https://github.com/urllib3/urllib3/commit/f05b1329126d5be6de501f9d1e3e36738bc08857
• https://nvd.nist.gov/vuln/detail/CVE-2025-50181
• https://github.com/urllib3/urllib3/releases/tag/2.5.0
• https://github.com/advisories/GHSA-pq67-6m6q-mj2v

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

Details

urllib3 supports being used in a Pyodide runtime utilizing the JavaScript Fetch API or falling back on XMLHttpRequest. This means you can use Python libraries to make HTTP requests from your browser or Node.js. Additionally, urllib3 provides a mechanism to control redirects.

However, the retries and redirect parameters are ignored with Pyodide; the runtime itself determines redirect behavior.

Affected usages

Any code which relies on urllib3 to control the number of redirects for an HTTP request in a Pyodide runtime.

Impact

Redirects are often used to exploit SSRF vulnerabilities. An application attempting to mitigate SSRF or open redirect vulnerabilities by disabling redirects may remain vulnerable if a Pyodide runtime redirect mechanism is unsuitable.

Remediation

If you use urllib3 in Node.js, upgrade to a patched version of urllib3.

Unfortunately, browsers provide no suitable way which urllib3 can use: XMLHttpRequest provides no control over redirects, the Fetch API returns opaqueredirect responses lacking data when redirects are controlled manually. Expect default browser behavior for redirects.

Severity

• CVSS Score: 5.3 / 10 (Medium)
• Vector String: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N

References

• https://github.com/urllib3/urllib3/security/advisories/GHSA-48p4-8xcf-vxj5
• https://github.com/urllib3/urllib3/commit/7eb4a2aafe49a279c29b6d1f0ed0f42e9736194f
• https://nvd.nist.gov/vuln/detail/CVE-2025-50182
• https://github.com/urllib3/urllib3/releases/tag/2.5.0
• https://github.com/advisories/GHSA-48p4-8xcf-vxj5

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

Details

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 preload_content=False and ensure that resp.headers["content-encoding"] contains a safe number of encodings before reading the response content.

Severity

• CVSS Score: 8.9 / 10 (High)
• Vector String: 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

References

• https://github.com/urllib3/urllib3/security/advisories/GHSA-gm62-xv2j-4w53
• https://github.com/urllib3/urllib3/commit/24d7b67eac89f94e11003424bcf0d8f7b72222a8
• https://nvd.nist.gov/vuln/detail/CVE-2025-66418
• https://github.com/advisories/GHSA-gm62-xv2j-4w53

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

Details

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), read_chunked(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

• CVSS Score: 8.9 / 10 (High)
• Vector String: 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

References

• https://github.com/urllib3/urllib3/security/advisories/GHSA-2xpw-w6gg-jr37
• https://github.com/urllib3/urllib3/commit/c19571de34c47de3a766541b041637ba5f716ed7
• https://nvd.nist.gov/vuln/detail/CVE-2025-66471
• https://github.com/advisories/GHSA-2xpw-w6gg-jr37

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

Details

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.

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.

However, 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 (high CPU usage and large memory allocations for decompressed data; CWE-409).

Affected usages

Applications and libraries using urllib3 version 2.6.2 and earlier to stream content from untrusted sources by setting preload_content=False when they do not disable redirects.

Remediation

Upgrade to at least urllib3 v2.6.3 in which the library does not decode content of redirect responses when preload_content=False.

If upgrading is not immediately possible, disable redirects by setting redirect=False for requests to untrusted source.

Severity

• CVSS Score: 8.9 / 10 (High)
• Vector String: 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

References

• https://github.com/urllib3/urllib3/security/advisories/GHSA-38jv-5279-wg99
• https://github.com/urllib3/urllib3/commit/8864ac407bba8607950025e0979c4c69bc7abc7b
• https://nvd.nist.gov/vuln/detail/CVE-2026-21441
• https://lists.debian.org/debian-lts-announce/2026/01/msg00017.html
• https://github.com/advisories/GHSA-38jv-5279-wg99

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

Details

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.

urllib3 can perform decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). When using the streaming API since version 2.6.0, the library decompresses only the necessary bytes, enabling partial content consumption.

However, urllib3 before version 2.7.0 could still decompress the whole response instead of the requested portion in two cases:

1. During the second HTTPResponse.read(amt=N) call when the response was decompressed using the official Brotli library.
2. When HTTPResponse.drain_conn() 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; CWE-409) on the client side.

Affected usages

Applications and libraries using urllib3 versions earlier than 2.7.0 may be affected when streaming compressed responses from untrusted sources in either of these cases, unless decompression is explicitly disabled:

1. A response encoded with br is read incrementally with at least two HTTPResponse.read(amt=N) or HTTPResponse.stream(amt=N) calls while using the official Brotli library.
2. HTTPResponse.drain_conn() is called after response decompression has already started.

Remediation

Upgrade to at least urllib3 version 2.7.0 in which the library:

1. Is more efficient for reads with Brotli.
2. Always skips decompression for HTTPResponse.drain_conn().

If upgrading is not immediately possible, the following workarounds may reduce exposure in specific cases:

1. For the Brotli-specific issue only, switch from brotli to brotlicffi until you can upgrade urllib3; the official Brotli package is affected because of https://github.com/google/brotli/issues/1396.
2. If your code explicitly calls HTTPResponse.drain_conn(), call HTTPResponse.close() instead when connection reuse is not important.

Credits

The Brotli-specific issue was reported by @​kimkou2024.
HTTPResponse.drain_conn() inefficiency was reported by @​Cycloctane.

Severity

• CVSS Score: 8.9 / 10 (High)
• Vector String: 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

References

• https://github.com/urllib3/urllib3/security/advisories/GHSA-mf9v-mfxr-j63j
• https://github.com/advisories/GHSA-mf9v-mfxr-j63j

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

Details

Impact

When following cross-origin redirects for requests made using urllib3’s high-level APIs, such as urllib3.request(), PoolManager.request(), and ProxyManager.request(), sensitive headers — Authorization, Cookie, and Proxy-Authorization (defined in Retry.DEFAULT_REMOVE_HEADERS_ON_REDIRECT) — are stripped by default, as expected.

However, cross-origin redirects followed from the low-level API via ProxyManager.connection_from_url().urlopen(..., assert_same_host=False) still forward these sensitive headers.

Affected usage

Applications and libraries using urllib3 versions earlier than 2.7.0 may be affected if they allow cross-origin redirects while making requests through HTTPConnection.urlopen() instances created via ProxyManager.connection_from_url().

Remediation

Upgrade to urllib3 version 2.7.0 or later, in which sensitive headers are stripped from redirects followed by HTTPConnection.

If upgrading is not immediately possible, avoid using this low-level redirect flow for cross-origin redirects. If appropriate for your use case, switch to ProxyManager.request().

Severity

• CVSS Score: 8.2 / 10 (High)
• Vector String: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

References

• https://github.com/urllib3/urllib3/security/advisories/GHSA-qccp-gfcp-xxvc
• https://github.com/advisories/GHSA-qccp-gfcp-xxvc

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

v2.7.0

Compare Source

=======================

Security

Addressed high-severity security issues.
Impact was limited to specific use cases detailed in the accompanying
advisories; overall user exposure was estimated to be marginal.

• Decompression-bomb safeguards of the streaming API were bypassed:

  1. When HTTPResponse.drain_conn() was called after the response had been
     read and decompressed partially.
  2. During the second HTTPResponse.read(amt=N) or
     HTTPResponse.stream(amt=N) call when the response was decompressed
     using the official Brotli <https://pypi.org/project/brotli/>__ library.

  See GHSA-mf9v-mfxr-j63j <https://github.com/urllib3/urllib3/security/advisories/GHSA-mf9v-mfxr-j63j>__
  for details.

• HTTP pools created using ProxyManager.connection_from_url did not strip
  sensitive headers specified in Retry.remove_headers_on_redirect when
  redirecting to a different host.
  (GHSA-qccp-gfcp-xxvc <https://github.com/urllib3/urllib3/security/advisories/GHSA-qccp-gfcp-xxvc>__)

Deprecations and Removals

• Used FutureWarning instead of DeprecationWarning for better
  visibility of existing deprecation notices. Rescheduled the removal of
  deprecated features to version 3.0.
  (#&#8203;3763 <https://github.com/urllib3/urllib3/issues/3763>__)
• Removed support for end-of-life Python 3.9.
  (#&#8203;3720 <https://github.com/urllib3/urllib3/issues/3720>__)
• Removed support for end-of-life PyPy3.10.
  (#&#8203;4979 <https://github.com/urllib3/urllib3/issues/4979>__)
• Bumped the minimum supported pyOpenSSL version to 19.0.0.
  (#&#8203;3777 <https://github.com/urllib3/urllib3/issues/3777>__)

Bugfixes

• Fixed a bug where HTTPResponse.read(amt=None) was ignoring decompressed
  data buffered from previous partial reads.
  (#&#8203;3636 <https://github.com/urllib3/urllib3/issues/3636>__)
• Fixed a bug where HTTPResponse.read() could cache only part of the
  response after a partial read when cache_content=True.
  (#&#8203;4967 <https://github.com/urllib3/urllib3/issues/4967>__)
• Fixed HTTPResponse.stream() and HTTPResponse.read_chunked() to handle
  amt=0.
  (#&#8203;3793 <https://github.com/urllib3/urllib3/issues/3793>__)
• Updated _TYPE_BODY type alias to include missing Iterable[str],
  matching the documented and runtime behavior of chunked request bodies.
  (#&#8203;3798 <https://github.com/urllib3/urllib3/issues/3798>__)
• Fixed LocationParseError when paths resembling schemeless URIs were
  passed to HTTPConnectionPool.urlopen().
  (#&#8203;3352 <https://github.com/urllib3/urllib3/issues/3352>__)
• Fixed BaseHTTPResponse.readinto() type annotation to accept
  memoryview in addition to bytearray, matching the
  io.RawIOBase.readinto contract and enabling use with
  io.BufferedReader without type errors.
  (#&#8203;3764 <https://github.com/urllib3/urllib3/issues/3764>__)