CVE-2026-97688
MediumCVSS 6.9Exploitation Probability (EPSS)
Low risk20th percentile - higher than 20% of all known CVEs
Summary
In urllib3 (versions 2.6.2 through 2.8.0), the HTTPResponse.stream and HTTPResponse.read_chunked methods 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.
Risk Assessment
A malicious server can cause excessive CPU usage and a request that does not complete, and network read timeouts do not interrupt the loop. This can lead to resource exhaustion and denial of service in applications using urllib3.
Recommendation
Update urllib3 to version 2.8.0 or later. Monitor CPU usage and unusual HTTP request hangs in applications using this library.
Other vulnerabilities in urllib3
See all- CVE-2026-97689High
urllib3 versions from 1.10.3 until 2.8.0 contain an unbounded memory allocation vulnerability. The HTTPResponse.read_chunked and HTTPResponse.stream methods buffer the chunk-size field without a length bound until a newline or EOF is encountered. A malicious HTTP server can send a very long unterminated chunk-size line, leading to exhaustion of the client process memory.
- CVE-2026-97687High
urllib3 versions from 1.26.0 until 2.8.0 fail to keep target-server TLS settings separate from the HTTPS proxy connection. 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 are affected. Setting cert_reqs=CERT_NONE can overwrite proxy_ssl_context.verify_mode in place, and the mutation persists so later connections may connect to the HTTPS proxy without certificate verification.
- CVE-2026-44432High
The urllib3 HTTP client library for Python versions 2.6.0 to before 2.7.0 contains a vulnerability where the entire response is decompressed instead of the requested portion during the second HTTPResponse.read(amt=N) call when using the official Brotli library, or after HTTPResponse.drain_conn() is called after partial read. This can lead to excessive resource consumption (high CPU usage and memory allocation) on the client side.
- CVE-2026-21441High
The urllib3 Python library versions 1.22 through 2.6.2 incorrectly decompresses entire HTTP redirect response bodies even when using the streaming API. This enables a decompression bomb attack leading to excessive client resource consumption.
- CVE-2025-66471High
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. 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 decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation, resulting in excessive resource consumption (high CPU usage and massive memory allocation).
- CVE-2025-66418High
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.
Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS