Unrated

HTTP Response Header Injection in undici cookie parser

IdentifiersCVE-2026-9679CWE-113

CVE-2026-9679 affects undici's cookie parsing logic, specifically functions such as parseSetCookie, parseCookie, and getSetCookies. The vulnerable behavior is in parseSetCookie, which percent-decodes cookie values via qsUnescape, converting encoded sequences such as %0D%0A, %00, %3B, and %3D into literal control or delimiter bytes. This behavior is inconsistent with RFC 6265 section 5.4 and browser behavior, which do not decode Set-Cookie values in this way. As a result, if an application parses an attacker-controlled upstream Set-Cookie header using undici and then forwards the parsed cookie value into a downstream HTTP response header, the decoded CR/LF and related bytes can be used to inject arbitrary response headers. The issue was introduced in undici 7.0.0 via PR #3789.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation can lead to downstream HTTP response header injection. An attacker controlling an upstream response can inject arbitrary headers such as Set-Cookie, Location, or Cache-Control into the application's response to clients. This can enable session fixation, open redirect, and cache poisoning, and may also undermine application security controls that rely on trusted response header construction.

Mitigation

If you can’t patch tonight, do this now.

If immediate upgrade is not possible, do not forward values returned by parseSetCookie, parseCookie, or getSetCookies directly into downstream response headers. Sanitize or reject parsed cookie values before reuse in headers, specifically stripping or blocking CR, LF, NUL, ';', and '=' bytes and avoiding any header construction path that treats upstream cookie values as trusted header content.

Remediation

Patch, then assume compromise.

Upgrade undici to a fixed release. The provided content identifies fixed versions as v7.28.0 and v8.5.0, and indicates a patched 6.x line; however, the content is inconsistent on the exact 6.x fixed version, listing both v6.26.0 and v6.27.0. Based on the available information, upgrade to the appropriate latest patched release in the deployed major branch, specifically v7.28.0 or v8.5.0, and verify the correct fixed 6.x version from the vendor advisory before upgrading 6.x deployments.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

NodejsUndiciapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

6 sources tracked across advisories and community write-ups. News coverage will land here when it surfaces.

6 SOURCESView more in app

No news coverage yet. Advisories and community discussion only.

What this page doesn’t show

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Exposure mapping

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Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity6

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES

MITRE CVE

cve.org · CVE-2026-9679

NVD

nvd.nist.gov/vuln/detail/CVE-2026-9679

MITRE CWE · CWE-113

cwe.mitre.org