CVE-2026-39820 Quadratic string concatentation in consumeComment in net/mail
Attackers can cause CPU exhaustion and memory allocation issues in Go applications by sending crafted inputs to net/mail parsing functions.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-39820?
CVE-2026-39820 is a high-severity denial of service vulnerability in Golang Go, published on 7 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Well-crafted inputs reaching ParseAddress, ParseAddressList, and ParseDate were able to trigger excessive CPU exhaustion and memory allocations.
Which versions are affected by CVE-2026-39820?
2 open source packages ship an affected version, including golang and stdlib. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
golang Bitnami | All versions | 1.25.10, 1.26.3 |
stdlib Go | All versions | 1.25.10, 1.26.3 |
Linux distribution packages (16)
kata-containers Red Hat:openshift:4.19::el9 | All versions | 0:3.31.0-5.rhaos4.19.el9 |
microshift Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-ai-model-serving Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-ai-model-serving-release-info Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-gateway-api Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-gateway-api-release-info Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-greenboot Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-low-latency Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-multus Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-multus-release-info Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-networking Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-observability Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-olm Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-olm-release-info Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-release-info Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
microshift-selinux Red Hat:openshift:4.19::el9 | All versions | 0:4.19.43-202608141837.p0.gcd3decf.assembly.4.19.43.el9 |
Products include Golang Go.
How do I fix CVE-2026-39820?
Upgrade golang to a fixed version for your release line: 1.25.10 or 1.26.3.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-39820 being exploited?
There is no known exploitation of CVE-2026-39820 in the wild and no public exploit code.
Other vulnerabilities in Golang Go
- CVE-2020-0601 Microsoft Windows CryptoAPI Spoofing
- CVE-2026-27143 Missing bound checks can lead to memory corruption in safe Go in cmd/compile
- CVE-2024-24790 Unexpected behavior from Is methods for IPv4-mapped IPv6 addresses in net/netip
- CVE-2023-39320 Arbitrary code execution via go.mod toolchain directive in cmd/go
- CVE-2023-24540 Improper handling of JavaScript whitespace in html/template
Sources
Also known as AZL-98772, BELL-CVE-2026-39820, BIT-golang-2026-39820, CLEANSTART-2026-DV13191, CLEANSTART-2026-IX22065, CLEANSTART-2026-KS11045 and 809 more.