zlib 1.2.11 through 1.3.2 Heap Buffer Overflow via Stale gzwrite Pointer After Failed Write (CVE-2026-76844)
Published Last modified Sources: Red Hat, Debian, Msrc Update Guide, OSV, CVE.org, Hersteller, EUVD
What is affected, what to do
A flaw was found in zlib. When an underlying write operation fails, the gz_write() function exits without properly resetting an internal stream pointer, leaving it referencing stale memory.
An exploit is publicly available. The available sources name no fixed version. Check the vendor guidance in the references below.
- Affected
- zlib
lte 1.2.11..1.3.2
- zlib
Not sure whether and how you are affected? Ask a question about this vulnerability
Metrics
8.3
XIEM Effective CVSS
Highest available CVSS rating from NVD and vendor data.
Source: Vendor (CNA), CVSS v4
7.4
CVSS v3
Severity High on the scale from 0 to 10.
Source: Red Hat, Debian, Msrc Update Guide, OSV, CVE.org, Hersteller, EUVD
0.6 %
EPSS score
Low — model estimates < 1% exploitation likelihood.
Source: FIRST EPSS, percentile 48.8
Show all metrics
- Severity
- High
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 83.38 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- PoC (publicly reported)
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 8.3 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 7.4
CVSS v4
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 8.3
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 0.6 %
- Low — model estimates < 1% exploitation likelihood.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 48.8
- High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- —
- Published
- CISA assessment (SSVC)
- Exploitation: proof of concept · not automatable · Technical impact: partial
- Source: CISA Vulnrichment, as of 26 August 2026
CWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-787
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
A flaw was found in zlib. When an underlying write operation fails, the gz_write() function exits without properly resetting an internal stream pointer, leaving it referencing stale memory. A subsequent write call can derive an offset from this stale pointer and write past the boundary of a heap allocation, resulting in a heap-based buffer overflow. An attacker who can trigger write errors during compression operations could exploit this vulnerability to execute arbitrary code or cause an application crash leading to a Denial of Service (DoS).
Source: redhat_csaf, Debian Security Advisories, Microsoft Security Update Guide, OSV (Open Source Vulnerabilities), NVD (NIST), cvelistv5, GitHub Security Advisories, ENISA EUVD
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| zlib | zlib |
|
|
| npm | webpack-dev-middleware |
|
|
Vendors and products
Affected operating systems
| Linux | debian | zlib | trixie |
Weakness classes (CWE)
CWE-787Out-of-bounds Write
Base
The product writes data past the end, or before the beginning, of the intended buffer.
Entry at MITRE CWE-787
Public exploit references
Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.
| Reference tagged as exploit | gist.github.com (opens in a new tab) | PoC (publicly reported)reported, not verified | — |
References & sources
- github.comwebpack/webpack-dev-middlewareProduct
- github.comwebpack/webpack-dev-middleware/blob/v8.1.1/src/middleware.jsTechnical description
- GitHub Security Advisoriesadvisories/GHSA-wr3j-pwj9-hqq6related
- vulncheck.comVulnCheck Advisory: webpack-dev-middleware Path Traversal via Offset Slice on a Non-Slash-Terminated publicPathThird-party advisory
- github.comwebpack/webpack-dev-middleware/security/advisories/GHSA-wr3j-pw…web
- NVD (NIST)vuln/detail/CVE-2024-29180advisory
- github.comwebpack/webpack-dev-middleware/commit/189c4ac7d2344ec132a4689e7…web
- github.comwebpack/webpack-dev-middleware/commit/9670b3495da518fe667ff3428…web
- github.comwebpack/webpack-dev-middleware/commit/e10008c762e4d5821ed699034…web
- github.comwebpack/webpack-dev-middleware/blob/7ed24e0b9f53ad1562343f9f517…web
- github.comwebpack/webpack-dev-middleware/blob/7ed24e0b9f53ad1562343f9f517…web
- github.comwebpack/webpack-dev-middleware/releases/tag/v5.3.4web
- github.comwebpack/webpack-dev-middleware/releases/tag/v6.1.2web
- github.comwebpack/webpack-dev-middleware/releases/tag/v7.1.0web
- github.comCVEProject/cvelistV5/tree/main/cves/2024/29xxx/CVE-2024-29180.j…advisory
- github.comwebpack/webpack-dev-middleware/security/advisories/GHSA-g84c-rx…web
- NVD (NIST)vuln/detail/CVE-2026-76844advisory
- github.comwebpack/webpack-dev-middleware/pull/2404web
- github.comwebpack/webpack-dev-middleware/commit/13344e78b26d88fe9b7c8381a…web
- github.comwebpack/webpack-dev-middleware/commit/cefccbac330c29052fc6dffe8…web
Changes to this CVE
Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).
Record modifiedVulnCheck
CVSS V4.0: AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N
CVSS V4.0: before AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N, after —
CVSS V3.1: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H
CVSS V3.1: before AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N, after —
Weakness type: CWE-787
Description added or changed
Show 22 original changes
Individual changes of this event as published by NVD Type Action Before After Description changed webpack-dev-middleware 5.3.4, 6.1.2 through 6.1.3, 7.1.0 before 7.4.6, and 8.0.0 before 8.3.0 contains a path traversal vulnerability in getFilenameFromUrl that bypasses the fix for CVE-2024-29180: a publicPath without a trailing slash is matched by string prefix, and the '..' guard runs before that prefix is stripped. Unauthenticated attackers can request a path such as /assets../secret to read files one directory above the output directory when the middleware serves from the physical filesystem through writeToDisk or a custom outputFileSystem.zlib 1.2.11 through 1.3.2 contains a heap buffer overflow: after an underlying write() fails, gz_write() returns without resetting strm.next_in, leaving it pointed at the caller's buffer. A later gz* write call then derives a position from the stale pointer and writes past a heap allocation; any write() failure reaches it, including EPIPE on a blocking descriptor, and in versions before 1.3.1.2 the failed write must be followed by a gzclearerr() call.CVSS added —AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:XCVSS removed AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X—CVSS added —AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:HCVSS removed AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N—Weakness type (CWE) added —CWE-787Weakness type (CWE) removed CWE-22—Affected versions (publisher) added —Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/76xxx/CVE-2026-76844.json">CVE-2026-76844</a>Reference added —https://gist.github.com/thesmartshadow/e0b9481792afb7c31e86fee1ff084490Reference added —https://github.com/madler/zlibReference added —https://github.com/madler/zlib/blob/v1.3.2/gzwrite.c#L393Reference added —https://github.com/madler/zlib/commit/df84af25dc1942490e1d1c899a07619152a46148Reference added —https://www.vulncheck.com/advisories/zlib-1.3.1.2-through-1.3.2-heap-buffer-overflow-via-gz-vacateReference removed https://github.com/advisories/GHSA-wr3j-pwj9-hqq6—Reference removed https://github.com/webpack/webpack-dev-middleware—Reference removed https://github.com/webpack/webpack-dev-middleware/blob/v8.1.1/src/middleware.js—Reference removed https://github.com/webpack/webpack-dev-middleware/commit/13344e78b26d88fe9b7c8381a3860e248a52359b—Reference removed https://github.com/webpack/webpack-dev-middleware/commit/cefccbac330c29052fc6dffe85fd292ea2ac782b—Reference removed https://github.com/webpack/webpack-dev-middleware/releases/tag/v7.4.6—Reference removed https://github.com/webpack/webpack-dev-middleware/releases/tag/v8.3.0—Reference removed https://github.com/webpack/webpack-dev-middleware/security/advisories/GHSA-g84c-rxfj-3j2c—Reference removed https://www.vulncheck.com/advisories/webpack-dev-middleware-path-traversal-via-offset-slice-on-a-non-slash-terminated-publicpath—Source identifier: disclosure@vulncheck.com
Record modifiedVulnCheck
Description added or changed
New references:- https://github.com/webpack/webpack-dev-middleware/commit/13344e78b26d88fe9b7c8381a3860e248a52359b
- https://github.com/webpack/webpack-dev-middleware/commit/cefccbac330c29052fc6dffe85fd292ea2ac782b
- https://github.com/webpack/webpack-dev-middleware/releases/tag/v7.4.6
- https://github.com/webpack/webpack-dev-middleware/releases/tag/v8.3.0
and 1 more reference
Show 7 original changes
Individual changes of this event as published by NVD Type Action Before After Description changed webpack-dev-middleware resolves a request to a local file in getFilenameFromUrl by testing the request pathname against a traversal guard and then slicing it at a fixed character offset. The guard, UP_PATH_REGEXP applied to path.normalize(`./${pathname}`), only matches ".." that stands as a whole path segment, while the containment test is the string comparison pathname.startsWith(publicPathPathname) and the file path is built as path.join(outputPath, pathname.slice(publicPathPathname.length)). When the configured publicPath has no trailing slash, a request such as GET /assets../.env against publicPath /assets yields the pathname /assets../.env, whose only dot-dot sits inside the segment "assets.." and so passes the guard, but the offset slice cuts within that segment and hands "../.env" to path.join, resolving one directory above outputPath. Reading a file from that path requires the middleware to be backed by the physical filesystem, which happens when writeToDisk is true or a custom outputFileSystem is supplied, since the default memfs volume holds only build output. Traversal depth is limited to a single directory because a separately delimited dot-dot segment is collapsed during URL parsing before the guard runs. The default publicPath value of "auto" resolves to "/" and is not affected. This is an incomplete fix for CVE-2024-29180: the guard and offset slice were introduced by that fix and are present in every release from 5.3.4, 6.1.2 and 7.1.0 onward.webpack-dev-middleware 5.3.4, 6.1.2 through 6.1.3, 7.1.0 before 7.4.6, and 8.0.0 before 8.3.0 contains a path traversal vulnerability in getFilenameFromUrl that bypasses the fix for CVE-2024-29180: a publicPath without a trailing slash is matched by string prefix, and the '..' guard runs before that prefix is stripped. Unauthenticated attackers can request a path such as /assets../secret to read files one directory above the output directory when the middleware serves from the physical filesystem through writeToDisk or a custom outputFileSystem.Affected versions (publisher) added —Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/76xxx/CVE-2026-76844.json">CVE-2026-76844</a>Reference added —https://github.com/webpack/webpack-dev-middleware/commit/13344e78b26d88fe9b7c8381a3860e248a52359bReference added —https://github.com/webpack/webpack-dev-middleware/commit/cefccbac330c29052fc6dffe85fd292ea2ac782bReference added —https://github.com/webpack/webpack-dev-middleware/releases/tag/v7.4.6Reference added —https://github.com/webpack/webpack-dev-middleware/releases/tag/v8.3.0Reference added —https://github.com/webpack/webpack-dev-middleware/security/advisories/GHSA-g84c-rxfj-3j2cSource identifier: disclosure@vulncheck.com
Record modifiedVulnCheck
New references:- https://github.com/advisories/GHSA-wr3j-pwj9-hqq6
- https://github.com/webpack/webpack-dev-middleware
- https://github.com/webpack/webpack-dev-middleware/blob/v8.1.1/src/middleware.js
- https://www.vulncheck.com/advisories/webpack-dev-middleware-path-traversal-via-offset-slice-on-a-non-slash-terminated-publicpath
Show 8 original changes
Individual changes of this event as published by NVD Type Action Before After Reference added —https://github.com/advisories/GHSA-wr3j-pwj9-hqq6Reference added —https://github.com/webpack/webpack-dev-middlewareReference added —https://github.com/webpack/webpack-dev-middleware/blob/v8.1.1/src/middleware.jsReference added —https://www.vulncheck.com/advisories/webpack-dev-middleware-path-traversal-via-offset-slice-on-a-non-slash-terminated-publicpathReference removed https://github.com/advisories/GHSA-wr3j-pwj9-hqq6—Reference removed https://github.com/webpack/webpack-dev-middleware—Reference removed https://github.com/webpack/webpack-dev-middleware/blob/v8.1.1/src/middleware.js—Reference removed https://www.vulncheck.com/advisories/webpack-dev-middleware-path-traversal-via-offset-slice-on-a-non-slash-terminated-publicpath—Source identifier: disclosure@vulncheck.com
Record modifiedCISA ADP
CISA assessment (SSVC): exploitation: proof of concept, automatable: no
Show the original change
Individual changes of this event as published by NVD Type Action Before After SSVC assessment added —{"id":"CVE-2026-76844","role":"CISA Coordinator","options":[{"exploitation":"poc"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-26T18:22:33.313523Z"}Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0
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<a href="https://intel.neosec.eu/en/cve/CVE-2026-76844"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-76844.svg?lang=en" alt="CVE-2026-76844 – NEOSEC Intel" height="20"></a>