Last updated 18 August 2025

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.

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 preloadcontent=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 preloadcontent=False.

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

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.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; 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.drainconn() 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.drainconn().

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.drainconn(), call HTTPResponse.close() instead when connection reuse is not important.

Credits

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

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.DEFAULTREMOVEHEADERSONREDIRECT) — are stripped by default, as expected.

However, cross-origin redirects followed from the low-level API via ProxyManager.connectionfromurl().urlopen(..., assertsamehost=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.connectionfromurl().

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().

Cookie HTTP header isn't stripped on cross-origin redirects

The encodeinvalidchars function in util/url.py in the urllib3 library 1.25.2 through 1.25.7 for Python allows a denial of service (CPU consumption) because of an inefficient algorithm. The percentencodings array contains all matches of percent encodings. It is not deduplicated. For a URL of length N, the size of percentencodings may be up to O(N). The next step (normalize existing percent-encoded bytes) also takes up to O(N) for each step, so the total time is O(N^2). If percentencodings were deduplicated, the time to compute encodeinvalidchars would be O(kN), where k is at most 484 ((10+62)^2).

Last updated 25 August 2025

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

- CVE-2021-33503 - JVNVU#92413403 (English) - JVNVU#92413403 (Japanese) - urllib3 v1.26.5

For more information If you have any questions or comments about this advisory: Ask in our community Discord Email sethmichaellarson@gmail.com

Last updated 31 October 2024

In the urllib3 library through 1.24.1 for Python CRLF injection is possible if the attacker controls the request parameter.

A flaw was found in python-urllib3. The HTTPConnection.request() does not properly validate CRLF sequences in the HTTP request method, potentially allowing manipulation of the request by injecting additional HTTP headers. The highest threat from this vulnerability is to confidentiality and integrity.

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.

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:

python 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:

python 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.

urllib3 before 1.24.2 does not remove the authorization HTTP header when following a cross-origin redirect (i.e., a redirect that differs in host, port, or scheme). This can allow for credentials in the authorization header to be exposed to unintended hosts or transmitted in cleartext. NOTE: this issue exists because of an incomplete fix for CVE-2018-20060 (which was case-sensitive).

A flaw found in the urllib3. The urllib3 is a user-friendly HTTP client library for Python. urllib3 previously wouldn't remove the HTTP request body when an HTTP redirect response using status 301, 302, or 303 after the request had its method changed from one that could accept a request body (like POST) to GET as is required by HTTP RFCs. Although this behavior is not specified in the section for redirects, it can be inferred by piecing together information from different sections and we have observed the behavior in other major HTTP client implementations like curl and web browsers. Because the vulnerability requires a previously trusted service to become compromised in order to have an impact on confidentiality we believe the exploitability of this vulnerability is low. Additionally, many users aren't putting sensitive data in HTTP request bodies, if this is the case then this vulnerability isn't exploitable. Both of the following conditions must be true to be affected by this vulnerability: 1. Using urllib3 and submitting sensitive information in the HTTP request body (such as form data or JSON) and 2. The origin service is compromised and starts redirecting using 301, 302, or 303 to a malicious peer or the redirected-to service becomes compromised. This issue has been addressed in versions 1.26.18 and 2.0.7 and users are advised to update to resolve this issue. Users unable to update should disable redirects for services that aren't expecting to respond with redirects with redirects=False and disable automatic redirects with redirects=False and handle 301, 302, and 303 redirects manually by stripping the HTTP request body.

References: https://github.com/urllib3/urllib3/security/advisories/GHSA-g4mx-q9vg-27p4 https://www.rfc-editor.org/rfc/rfc9110.html#name-get https://github.com/urllib3/urllib3/commit/4e98d57809dacab1cbe625fddeec1a290c478ea9