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Python Urllib3 vulnerabilities

22 vulnerabilities

Known vulnerabilities in Python Urllib3, newest first, with CVSS severity, EPSS, KEV listings, ransomware use and the affected versions.

CVE-2018-20060debianpython-urllib3+47y ago11-12-2018 17:00

urllib3 before version 1.23 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.CVE-2018-25091debianpython-urllib3+43y ago15-10-2023 00:00

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).CVE-2021-28363alpinepy3-urllib3+75y ago15-03-2021 00:00

The urllib3 library 1.26.x before 1.26.4 for Python omits SSL certificate validation in some cases involving HTTPS to HTTPS proxies. The initial connection to the HTTPS proxy (if an SSLContext isn't given via proxy_config) doesn't verify the hostname of the certificate. This means certificates for different servers that still validate properly with the default urllib3 SSLContext will be silently accepted.CVE-2021-33503chainguardpy3.13-scanner-test-scanner-test-python-vulnerability-fixed+3575y ago29-06-2021 10:55

An issue was discovered in urllib3 before 1.26.5. 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.CVE-2016-9015PyPIurllib3+329y ago11-01-2017 16:00

Versions 1.17 and 1.18 of the Python urllib3 library suffer from a vulnerability that can cause them, in certain configurations, to not correctly validate TLS certificates. This places users of the library with those configurations at risk of man-in-the-middle and information leakage attacks. This vulnerability affects users using versions 1.17 and 1.18 of the urllib3 library, who are using the optional PyOpenSSL support for TLS instead of the regular standard library TLS backend, and who are using OpenSSL 1.1.0 via PyOpenSSL. This is an extremely uncommon configuration, so the security impact of this vulnerability is low.CVE-2025-66471alpinepy3-urllib3+27510mo ago05-12-2025 16:06

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.CVE-2025-66418alpinepy3-urllib3+33510mo ago05-12-2025 16:02

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.CVE-2025-50181chainguardairflow-2+2501y ago19-06-2025 01:08

urllib3 is a user-friendly HTTP client library for Python. Prior to 2.5.0, it is possible to disable redirects for all requests by instantiating a PoolManager and specifying retries in a way that disable redirects. By default, requests and botocore users are not affected. An application attempting to mitigate SSRF or open redirect vulnerabilities by disabling redirects at the PoolManager level will remain vulnerable. This issue has been patched in version 2.5.0.CVE-2025-50182chainguardairflow-2+1621y ago19-06-2025 01:42

urllib3 is a user-friendly HTTP client library for Python. Starting in version 2.2.0 and prior to 2.5.0, urllib3 does not control redirects in browsers and Node.js. urllib3 supports being used in a Pyodide runtime utilizing the JavaScript Fetch API or falling back on XMLHttpRequest. This means Python libraries can be used to make HTTP requests from a browser or Node.js. Additionally, urllib3 provides a mechanism to control redirects, but the retries and redirect parameters are ignored with Pyodide; the runtime itself determines redirect behavior. This issue has been patched in version 2.5.0.CVE-2020-26137alpinepy3-urllib3+1486y ago29-09-2020 00:00

urllib3 before 1.25.9 allows CRLF injection if the attacker controls the HTTP request method, as demonstrated by inserting CR and LF control characters in the first argument of putrequest(). NOTE: this is similar to CVE-2020-26116.CVE-2020-7212debianpython-urllib3+36y ago06-03-2020 19:14

The _encode_invalid_chars 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 percent_encodings array contains all matches of percent encodings. It is not deduplicated. For a URL of length N, the size of percent_encodings 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 percent_encodings were deduplicated, the time to compute _encode_invalid_chars would be O(kN), where k is at most 484 ((10+6*2)^2).CVE-2024-37891chainguardairflow+2452y ago17-06-2024 19:18

urllib3 is a user-friendly HTTP client library for Python. When using urllib3's proxy support with `ProxyManager`, the `Proxy-Authorization` header is only sent to the configured proxy, as expected. However, when sending HTTP requests *without* using urllib3's proxy support, it's possible to accidentally configure the `Proxy-Authorization` header even though it won't have any effect as the request is not using a forwarding proxy or a tunneling proxy. In those cases, urllib3 doesn't treat the `Proxy-Authorization` HTTP header as one carrying authentication material and thus doesn't strip the header on cross-origin redirects. Because this is a highly unlikely scenario, we believe the severity of this vulnerability is low for almost all users. Out of an abundance of caution urllib3 will automatically strip the `Proxy-Authorization` header during cross-origin redirects to avoid the small chance that users are doing this on accident. Users should use urllib3's proxy support or disable automatic redirects to achieve safe processing of the `Proxy-Authorization` header, but we still decided to strip the header by default in order to further protect users who aren't using the correct approach. We believe the number of usages affected by this advisory is low. It requires all of the following to be true to be exploited: 1. Setting the `Proxy-Authorization` header without using urllib3's built-in proxy support. 2. Not disabling HTTP redirects. 3. Either not using an HTTPS origin server or for the proxy or target origin to redirect to a malicious origin. Users are advised to update to either version 1.26.19 or version 2.2.2. Users unable to upgrade may use the `Proxy-Authorization` header with urllib3's `ProxyManager`, disable HTTP redirects using `redirects=False` when sending requests, or not user the `Proxy-Authorization` header as mitigations.CVE-2019-11324canonicalubuntu_linux+47y ago18-04-2019 00:00

The urllib3 library before 1.24.2 for Python mishandles certain cases where the desired set of CA certificates is different from the OS store of CA certificates, which results in SSL connections succeeding in situations where a verification failure is the correct outcome. This is related to use of the ssl_context, ca_certs, or ca_certs_dir argument.CVE-2019-11236debianpython-urllib3+47y ago15-04-2019 00:00

In the urllib3 library through 1.24.1 for Python, CRLF injection is possible if the attacker controls the request parameter.CVE-2026-21441alpinepy3-urllib3+1869mo ago07-01-2026 22:09

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 `preload_content=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 `preload_content=False`. If upgrading is not immediately possible, disable redirects by setting `redirect=False` for requests to untrusted source.CVE-2026-44432chainguardairflow-2+2525mo ago13-05-2026 15:17

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.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) on the client side. This vulnerability is fixed in 2.7.0.CVE-2026-44431chainguardairflow-2+3305mo ago13-05-2026 15:20

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.connection_from_url().urlopen(..., assert_same_host=False) still forward these sensitive headers. This vulnerability is fixed in 2.7.0.CVE-2026-97687chainguardairflow-2+18420d ago29-09-2026 15:30

urllib3 is an HTTP client library for Python. From 1.26.0 until 2.8.0, the proxy_ssl_context, proxy_assert_hostname, proxy_assert_fingerprint, ssl_context, cert_reqs, verify_mode, use_forwarding_for_https=True, and CERT_NONE configuration paths fail to remain separated because target-server TLS settings are incorrectly applied to the HTTPS proxy connection. The trigger is that an application uses an HTTPS proxy and configures target-server TLS settings that must remain separate from the proxy TLS handshake, including HTTPS forwarding with target-specific identity or credentials. Applying cert_reqs=CERT_NONE can overwrite proxy_ssl_context.verify_mode in place, and the mutation persists so later connections reusing the same context may connect to the HTTPS proxy without certificate verification. The attack mechanism is that an attacker intercepts and impersonates the HTTPS proxy after the effective proxy policy accepts the attacker's certificate. The impact is that the attacker can observe or modify forwarded traffic or receive a target TLS client certificate, while CONNECT tunneling still preserves the separate end-to-end target TLS connection. This issue is fixed in version 2.8.0.CVE-2026-97689chainguardairflow-2+18620d ago29-09-2026 15:36

urllib3 is an HTTP client library for Python. From 1.10.3 until 2.8.0, the HTTPResponse.read_chunked and HTTPResponse.stream methods can allocate unbounded memory because the streaming chunk parser buffers the chunk-size field until newline or EOF without a length bound. The trigger is that a malicious server returns Transfer-Encoding: chunked followed by a very long run of bytes without a newline. The attack mechanism is that a malicious HTTP server sends a very long unterminated chunk-size line. The impact is that unbounded memory allocation can exhaust the client process. This issue is fixed in version 2.8.0.CVE-2026-97688chainguardairflow-2+18420d ago29-09-2026 15:32

urllib3 is an HTTP client library for Python. From 2.6.2 until 2.8.0, HTTPResponse.stream and HTTPResponse.read_chunked can enter an infinite loop because the Deflate decoder retains trailing bytes as unconsumed input after reaching end-of-stream and repeatedly decodes them without progress. The issue occurs when an untrusted server sends a chunked Deflate response whose decoded body exceeds a positive finite chunk size and whose encoded body has trailing bytes, specifically a response with Transfer-Encoding: chunked and Content-Encoding: deflate, content decoding enabled, and the positive finite amt=N streaming chunk size. The attack mechanism is that a malicious server returns a compressed chunked response with trailing bytes after the Deflate stream. The impact is excessive CPU usage and a request that does not complete, and network read timeouts do not interrupt the loop because no further socket read occurs. This issue is fixed in version 2.8.0.