Threat Intelligence Database

urllib3 versions 2.6.2 through 2.7.0 contain a vulnerability in their streaming API when handling HTTP responses with both chunked transfer encoding and Deflate content encoding. Under specific conditions, the decompression process can enter an infinite loop due to trailing bytes after the end of the Deflate stream, causing excessive CPU usage and preventing request completion. This affects applications streaming compressed responses with automatic content decoding enabled. The issue is fixed in urllib3 version 2.8.0.

### Impact urllib3's [streaming API](https://urllib3.readthedocs.io/en/2.7.0/advanced-usage.html#streaming-and-i-o) 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](https://pypi.org/project/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](https://pypi.org/project/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](https://pypi.org/project/brotli/) to [brotlicffi](https://pypi.org/project/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.

CVE-2025-50181 is a vulnerability in the osc-cli product related to urllib3's handling of redirects when retries are disabled on PoolManager instantiation. The retries parameter intended to disable redirects at the PoolManager level is ignored, causing redirects to remain enabled. This can leave applications vulnerable to SSRF or open redirect attacks if they rely on disabling redirects at this level. The vulnerability affects specific versions of osc-cli and related packages. A patch is available to fix this issue.

A vulnerability in the idna.encode() function of the kimi-cli package allows specially crafted inputs to bypass the previous fix for CVE-2024-3651, causing excessive resource consumption and potential denial-of-service. The issue arises from insufficient early rejection of overly long inputs, leading to expensive processing. Versions prior to 3.14 do not implement the improved input length checks. The vulnerability has a medium severity with a CVSS score of 5.3. Official patches are available starting from version 3.14, which implement early input length rejection to mitigate the issue.

### Impact urllib3's [streaming API](https://urllib3.readthedocs.io/en/2.6.2/advanced-usage.html#streaming-and-i-o) 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](https://urllib3.readthedocs.io/en/2.6.2/user-guide.html#retrying-requests) by setting `redirect=False` for requests to untrusted source.

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. ## Affected usages We believe the number of usages affected by this advisory is low. It requires all of the following to be true to be exploited: * Setting the `Proxy-Authorization` header without using urllib3's built-in proxy support. * Not disabling HTTP redirects. * Either not using an HTTPS origin server or for the proxy or target origin to redirect to a malicious origin. ## Remediation * Using the `Proxy-Authorization` header with urllib3's `ProxyManager`. * Disabling HTTP redirects using `redirects=False` when sending requests. * Not using the `Proxy-Authorization` header.

### 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()`.

### Impact A specially crafted argument to the `idna.encode()` function could consume significant resources. This may lead to a denial-of-service. ### Patches The function has been refined to reject such strings without the associated resource consumption in version 3.7. ### Workarounds Domain names cannot exceed 253 characters in length, if this length limit is enforced prior to passing the domain to the `idna.encode()` function it should no longer consume significant resources. This is triggered by arbitrarily large inputs that would not occur in normal usage, but may be passed to the library assuming there is no preliminary input validation by the higher-level application. ### References * https://huntr.com/bounties/93d78d07-d791-4b39-a845-cbfabc44aadb