CVE-2023-39326: Denial of service via chunk extensions in net/http

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2023-12-06 · updated 2026-06-17

Description

A malicious HTTP sender can use chunk extensions to cause a receiver reading from a request or response body to read many more bytes from the network than are in the body. A malicious HTTP client can further exploit this to cause a server to automatically read a large amount of data (up to about 1GiB) when a handler fails to read the entire body of a request. Chunk extensions are a little-used HTTP feature which permit including additional metadata in a request or response body sent using the chunked encoding. The net/http chunked encoding reader discards this metadata. A sender can exploit this by inserting a large metadata segment with each byte transferred. The chunk reader now produces an error if the ratio of real body to encoded bytes grows too small.

Affected products

Go: before 1.20.12; from 1.21.0-0 before 1.21.5 — fixed in 1.20.12, 1.21.5

How to fix

Upgrade to a fixed version: Go 1.20.12, 1.21.5. Check the vendor advisory for mitigations.

References

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