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Full vulnerability report · 2026
CVE-2026-56390High confidence

Arbitrary Output Location Change in GNU Bison

GNU · Bison

4.6MediumCVSS 4.0
Recommended action
Patch only the product branches with a verified fix

Medium technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority. Verified remediation exists for at least one product or source, but 21 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityMediumOperational priority:Medium, unchanged from published severity.unchanged

Evidence used

  • No CISA KEV confirmation is currently recorded.
  • EPSS is 0.18% for the current model date.

Compensating controls

  • Validate the affected product branch and deploy the verified fixed release.
  • Restrict local access and enforce least privilege on affected hosts.
  • Monitor vendor guidance and exploitation sources for a material change.

Verification

  1. Confirm that the asset runs GNU Bison and falls inside the recorded affected range.
  2. Verify the installed build against the product-specific fixed version after deployment.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: As exposure requiresRemediation target: Within 365 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Cross-source reconciliation

Remediation availability differs by product scope

Verified remediation exists for at least one product or source, but 21 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Distribution package intelligence

Release-specific package status

Debian, ubuntu findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.

5 package states
Package result overrides the generic status

BlackTree has verified remediation for at least one product or source, but the relevant distribution still reports no fixed package for 4 affected package states shown here. Treat those rows as affected with no fix until that distribution publishes a fixed version.

Repository candidate not checked

A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.

Distribution releaseSource packageVendor stateFixed versionEvidence
Debian trixietrixie · sourcebisonAffected, no fix publishedDebian currently tracks this release as open.Not published in this feedDebian Security Tracker ↗Source updated 6 Oct 2026
Debian bookwormbookworm · sourcebisonAffected, no fix publishedDebian currently tracks this release as open.Not published in this feedDebian Security Tracker ↗Source updated 6 Oct 2026
Debian forkyforky · sourcebisonAffected, no fix publishedDebian currently tracks this release as open.Not published in this feedDebian Security Tracker ↗Source updated 6 Oct 2026
Debian sidsid · sourcebisonAffected, no fix publishedDebian currently tracks this release as open.Not published in this feedDebian Security Tracker ↗Source updated 6 Oct 2026
Ubuntu 24.04 LTSnoble · standard archivebisonUnder evaluationCanonical reports that the package might be affected and still needs evaluation or fixing.Not published in this feedCanonical Ubuntu Security ↗Source updated 5 Oct 2026
Direct vendor intelligence

Authoritative vendor CSAF and VEX advisories

Structured product status and remediation from the issuing vendor. Product-state explanations are always visible; large lists can be searched or downloaded.

1 current
CVE-2026-56390 · CSAF 2.0 · revision 3 · finalRed Hat Product Securitybison: GNU Bison: Arbitrary file overwrite via grammar-defined output paths
17 known affected

The vendor explicitly identifies these products as affected by this CVE.

  • bison-devel as a component of Red Hat Enterprise Linux 10
  • bison-runtime as a component of Red Hat Enterprise Linux 10
  • bison.src as a component of Red Hat Enterprise Linux 10
  • bison-devel as a component of Red Hat Enterprise Linux 6
  • bison-runtime as a component of Red Hat Enterprise Linux 6
  • bison.src as a component of Red Hat Enterprise Linux 6
  • bison-devel as a component of Red Hat Enterprise Linux 7
  • bison-runtime as a component of Red Hat Enterprise Linux 7
  • bison.src as a component of Red Hat Enterprise Linux 7
  • bison-devel as a component of Red Hat Enterprise Linux 8
  • bison-runtime as a component of Red Hat Enterprise Linux 8
  • bison.src as a component of Red Hat Enterprise Linux 8
Summary
A flaw was found in GNU Bison. When processing attacker-supplied grammar, the software improperly handles grammar-defined output paths. This allows an attacker to use directives such as %output and %header to specify arbitrary file paths. Consequently, generated files can be directed to any writable location on the filesystem, potentially overwriting existing files and leading to data integrity issues.
Remediation
Fix deferred
Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2026-50277

No EUVD known-exploited evidence

ENISA has published the identifier mapping but no EUVD description has been stored yet.

EUVD state
Present in the current official mapping
Known exploitation
Not present in the current ENISA EUVD known-exploited dataset. This is not proof of no exploitation.
ENISA score
Not supplied in the stored EUVD record
Advisory evidence
No linked advisory details stored yet
Recommended actionPatch only the product branches with a verified fix

Medium technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority. Verified remediation exists for at least one product or source, but 21 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
01

What, why and how

GNU Bison improperly handles grammar‑defined output paths. Grammar directives such as %output and %header allow specifying file paths, which are accepted without restriction and override caller‑supplied output options. When processing attacker-supplied grammar, this behavior allows directing generated files to arbitrary writable locations on the filesystem, potentially overwriting existing files accessible to the Bison process. Maintainers of this project were notified about this vulnerability, and fixed the issue in commit 8d101c19d4d9aaedf83a448c925513742d4efcf0. However, they did not provide vulnerable version range. Version 3.8.2 was tested and confirmed as vulnerable, other versions were not tested but might also be vulnerable.

What

GNU Bison improperly handles grammar‑defined output paths. Grammar directives such as %output and %header allow specifying file paths, which are accepted without restriction and override caller‑supplied output options. When processing attacker-supplied grammar, this behavior allows directing generated files to arbitrary writable locations on the filesystem, potentially overwriting existing files accessible to the Bison process. Maintainers of this project were notified about this vulnerability, and fixed the issue in commit 8d101c19d4d9aaedf83a448c925513742d4efcf0. However, they did not provide vulnerable version range. Version 3.8.2 was tested and confirmed as vulnerable, other versions were not tested but might also be vulnerable.

Why

The product allows user input to control or influence paths or file names that are used in filesystem operations.

How

An attacker operating through local access may attempt exploitation when the stated preconditions are met. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

What

GNU Bison improperly handles grammar‑defined output paths. Grammar directives such as %output and %header allow specifying file paths, which are accepted without restriction and override caller‑supplied output options. When processing attacker-supplied grammar, this behavior allows directing generated files to arbitrary writable locations on the filesystem, potentially overwriting existing files accessible to the Bison process. Maintainers of this project were notified about this vulnerability, and fixed the issue in commit 8d101c19d4d9aaedf83a448c925513742d4efcf0. However, they did not provide vulnerable version range. Version 3.8.2 was tested and confirmed as vulnerable, other versions were not tested but might also be vulnerable.

Why

The product allows user input to control or influence paths or file names that are used in filesystem operations.

How

An attacker operating through local access may attempt exploitation when the stated preconditions are met. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
No confirmed evidence

No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path
local access → External Control of File Name or Path → cause the confidentiality, integrity or availability impact described by the vendor
Attack surface
Local
Privileges required
None: unauthenticated exploitation is possible
User interaction
Active interaction required
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Not a CVSS 4.0 base metric
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-73 ↗

CWE-73: External Control of File Name or Path. The product allows user input to control or influence paths or file names that are used in filesystem operations.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:A/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N

Common Vulnerability Scoring System 4.0: the compact vector below is decoded into plain language.

AVLocalAttack vector: The attacker needs local access to the vulnerable system.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.ATNoneAttack requirements: No additional deployment or execution condition is required.PRNonePrivileges required: The attacker does not need an account or existing privileges.UIActiveUser interaction: A user must take a deliberate action for exploitation to succeed.VCNoneVulnerable-system confidentiality: No direct loss is represented by this metric.VILowVulnerable-system integrity: A successful attack can cause a limited loss.VANoneVulnerable-system availability: No direct loss is represented by this metric.SCNoneSubsequent-system confidentiality: No direct loss is represented by this metric.SILowSubsequent-system integrity: A successful attack can cause a limited loss.SANoneSubsequent-system availability: No direct loss is represented by this metric.
Post-exploitation / living off the land
The issue can support a local privilege or sandbox boundary transition; normal system utilities may then be available in the gained context.
UnauthenticatedCWE-73
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

Cyber Security Agency of Singapore · English · CSA-SB-20260805Security Bulletin 5 Aug 2026

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 5 Aug 2026, published on 5 August 2026. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2026-029804GNU ProjectのBisonにおけるファイル名やパス名の外部制御に関する脆弱性

GNU Bisonは、文法で定義された出力パスを適切に処理しません。%outputや%headerなどの文法ディレクティブはファイルパスの指定を可能にしますが、これらは制限なく受け入れられ、呼び出し元が指定した出力オプションを上書きしてしまいます。攻撃者が提供する文法を処理する際、この挙動により生成されたファイルをファイルシステム上の任意の書き込み可能な場所に出力でき、Bisonプロセスがアクセス可能な既存ファイルを上書きする恐れがあります。本プロジェクトのメンテナはこの脆弱性を通知され、commit 8d101c19d4d9aaedf83a448c925513742d4efcf0で問題を修正しました。ただし、影響を受けるバージョン範囲は提供されていません。バージョン3.8.2はテストされて脆弱であることが確認されており、他のバージョンはテストされていませんが脆弱である可能性があります。

Official advisory ↗
CERT Polska · Polish · CVE-2026-56389CERT Polska w social mediach

CERT Polska otrzymał zgłoszenie o podatnościach w oprogramowaniu GNU Bison i koordynował proces ujawniania informacji. Podatność CVE-2026-56389 : Oprogramowanie GNU Bison umożliwia wykonanie dowolnego zewnętrznego programu podczas generowania raportów HTML z powodu nieprawidłowej obsługi zmiennych konfiguracyjnych zdefiniowanych w gramatyce. Plik gramatyki może nadpisać plik wykonywalny używany w kroku transformacji XML do HTML za pomocą %define tool.xsltproc , który jest akceptowany bez żadnych ograniczeń i przekazywany bezpośrednio do execvp() . Uruchomienie bison --html na dostarczonej przez atakującego gramatyce umożliwia wykonanie dowolnego programu z uprawnieniami procesu Bison. Podatność ta została naprawiona w commicie 3169c1e7a2c6acc4c59dfcf8b089896d6881925b. Podatność CVE-2026-56390 : GNU Bison nieprawidłowo obsługuje ścieżki wyjściowe zdefiniowane w gramatyce. Dyrektywy gramatyki takie jak %output i %header umożliwiają określanie ścieżek plików, które są akceptowane bez ograniczeń i nadpisują opcje wyjściowe dostarczone przez wywołującego. Podczas przetwarzania gramatyki dostarczonej przez atakującego, takie zachowanie umożliwia kierowanie wygenerowanych plików do dowolnych zapisywalnych lokalizacji w systemie plików, potencjalnie nadpisując istniejące pliki dostępne dla procesu Bison. Podatność ta została naprawiona w commicie 8d101c19d4d9aaedf83a448c925513742d4efcf0. Opiekunowie tego projektu zostali poinformowani o podatnościach, jednakże nie potwierdzili zakresu podatnych wersji. Wersja 3.8.2 została przetestowana i potwierdzona jako podatna, inne wersje nie były testowane, ale mogą również być podatne.

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Fix availability varies by product
Affected
Fixed
An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
Action
Use the product-specific evidence above. Patch only products with a verified fixed release, and keep every affected or under-investigation state without a matching fix in the remediation queue.
Workaround
No verified workaround is recorded. Limit untrusted access and use least privilege until authoritative guidance is available.
04

Evidence and provenance

Published 29 Jul 2026 · Last source change 29 Jul 2026, 12:15 UTC · CWE-73 · External Control of File Name or Path

CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2026-50277
Product sourceVendor CSAF · Red Hat Product Security
Remediation sourceVendor CSAF · Red Hat Product Security
CWE sourceCNA
NVD statusNVD enriched

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Affected versionsThe structured affected or fixed version information changed.
    Before
    bison-devel as a component of Red Hat Enterprise Linux 10; bison-runtime as a component of Red Hat Enterprise Linux 10; bison.src as a component of Red Hat Enterprise Linux 10; bison-devel as a component of Red Hat Enterprise Linux 6; bison-runtime as a component of Red Hat Enterprise Linux 6; bison.src as a component of Red Hat Enterprise Linux 6; bison-devel as a component of Red Hat Enterprise Linux 7; bison-runtime as a component of Red Hat Enterprise Linux 7; bison.src as a component of Red Hat Enterprise Linux 7; bison-devel as a component of Red Hat Enterprise Linux 8; bison-runtime as a component of Red Hat Enterprise Linux 8; bison.src as a component of Red Hat Enterprise Linux 8; bison-devel as a component of Red Hat Enterprise Linux 9; bison-runtime as a component of Red Hat Enterprise Linux 9; bison.src as a component of Red Hat Enterprise Linux 9; openshift/ose-rhel-coreos-9 as a component of Red Hat OpenShift Container Platform 4
    After
    bison-devel as a component of Red Hat Enterprise Linux 10; bison-runtime as a component of Red Hat Enterprise Linux 10; bison.src as a component of Red Hat Enterprise Linux 10; bison-devel as a component of Red Hat Enterprise Linux 6; bison-runtime as a component of Red Hat Enterprise Linux 6; bison.src as a component of Red Hat Enterprise Linux 6; bison-devel as a component of Red Hat Enterprise Linux 7; bison-runtime as a component of Red Hat Enterprise Linux 7; bison.src as a component of Red Hat Enterprise Linux 7; bison-devel as a component of Red Hat Enterprise Linux 8; bison-runtime as a component of Red Hat Enterprise Linux 8; bison.src as a component of Red Hat Enterprise Linux 8; bison-devel as a component of Red Hat Enterprise Linux 9; bison-runtime as a component of Red Hat Enterprise Linux 9; bison.src as a component of Red Hat Enterprise Linux 9; openshift/ose-rhel-coreos-8 as a component of Red Hat OpenShift Container Platform 4; openshift/ose-rhel-coreos-9 as a component of Red Hat OpenShift Container Platform 4
    Red Hat Product Security ↗
  2. Vendor guidanceAuthoritative vendor guidance changed: added patch: cgit.git.savannah.gnu.org/commit?id=8d101c19d4d9aaedf83a448c925513742d4efcf0; added remediation: cgit.git.savannah.gnu.org/commit?id=8d101c19d4d9aaedf83a448c925513742d4efcf0.
    Before
    no authoritative guidance recorded
    After
    patch: cgit.git.savannah.gnu.org/commit?id=8d101c19d4d9aaedf83a448c925513742d4efcf0 · remediation: cgit.git.savannah.gnu.org/commit?id=8d101c19d4d9aaedf83a448c925513742d4efcf0
    cvd@cert.pl ↗
  3. Affected versionsThe structured affected or fixed version information changed.
    Before
    3.8.2 · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    After
    3.8.2 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
    CNA ↗
  4. Remediation statusRemediation status changed from Awaiting fix to Patch available.
    Before
    Awaiting fix
    After
    Patch available
    cvd@cert.pl ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Fixed release or patch reference recorded
ENISA EUVD · EUVD-2026-50277↗Vendor CSAF · Red Hat Product Security↗Patch or update reference↗CVE authority or vendor reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2026-56390 · cve.blacktree.nl