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 [Python] output_buffer_limit is not fully respected google/brotli#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.

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

Pillow did not limit the amount of GZIP-compressed data read when decoding a FITS image, making it vulnerable to decompression bomb attacks. A specially crafted FITS file could cause unbounded memory consumption, leading to denial of service (OOM crash or severe performance degradation).

Patches

The amount of data read is now limited to the necessary amount.
Fixed in Pillow 12.2.0 (PR #9521).

Workarounds

Avoid Pillow >= 10.3.0, < 12.2.0
Only open specific image formats, excluding FITS.

Impact

Processing a malicious PSD file could lead to memory corruption, potentially resulting in a crash or arbitrary code execution.

Patches

Patched version: 12.2.0

Pillow 12.1.1 addressed CVE-2026-25990 by adding checks for tile extents in PSD image decoding/encoding to prevent an out-of-bounds write. However, the bounds checks computed tile extent sums using types susceptible to integer overflow, meaning a PSD image with carefully chosen tile dimensions could produce values that wrap around and bypass the checks, still triggering an out-of-bounds write in src/decode.c and src/encode.c. The fix avoids adding extents together before comparison.

Workarounds

Use any version but affected versions: >= 10.3.0, < 12.2.0

Resources

• Fix: Avoid overflow by not adding extents together python-pillow/Pillow#9520
• Original issue: CVE-2026-25990 (Pillow 12.1.1)