EUVD-2026-88584
GHSA-frgp-cm59-pw23
Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain
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AccessCraft DKIM-Signature with ambiguous tags
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DeliveryDeliver message over SMTP
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Exploitdkim_process_set tokenizes tags divergently
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ExecutionOpenDKIM returns favorable auth result
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ImpactSpoofed mail passes DKIM/DMARC
Vulnerability Assessment
| Weakness |
CWE
ASVS 5.0
V6 - Authentication IP-independent access control
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| Exploitation Conditions | No special conditions beyond a running OpenDKIM verifier (up to 2.11.0) processing inbound mail - remote unauthenticated exploitation via a crafted DKIM-Signature header delivered over SMTP against default configurations of OpenDKIM used as a Postfix/Sendmail milter. The realistic value is limited: the desync only affects the authentication verdict of the single crafted message, and actual damage depends on downstream reliance on OpenDKIM's DKIM result (e.g., DMARC enforcement, anti-phishing filtering). No fix exists since the vendor did not respond, which prolongs exposure. |
| Impact | This is a genuine but low-to-medium priority integrity issue whose CVSS-4.0 base of 5.5 (AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:L) accurately reflects a remote, unauthenticated, low-complexity flaw with only limited integrity/availability impact and no confidentiality loss. The single-source vendor (VulDB) rates it 5.5; my independent assessment lands near 5.4, so severity is not inflated. The E:P flag plus the public write-up (xuyongzhe-vt.github.io opendkim-tag-parser-desync.html) confirm publicly available exploit/PoC material, but there is NO EPSS score and NO CISA KEV listing in the input, and no SSVC data was supplied - so there is no evidence of active or widespread exploitation. The real-world concern is a parser desynchronization (CWE-348, use of less-trusted source): OpenDKIM's tag tokenizer parses a DKIM-Signature differently than other verifiers, so a crafted signature can obtain a favorable authentication verdict OpenDKIM should have rejected. That undermines DKIM's anti-spoofing guarantee for downstream anti-phishing/DMARC decisions. The 'Information Disclosure' tag conflicts with the actual impact (this is an integrity/authentication-result issue, not a data leak) - the CVSS VI:L reflects integrity, not confidentiality. Net: a real but bounded issue amplified by OpenDKIM's very wide deployment, tempered by no confirmed exploitation and a low-value single-message impact. |
| Exploit ScenarioDerived | An attacker sending mail over SMTP crafts a DKIM-Signature header whose tags are tokenized by OpenDKIM's dkim_process_set differently than by other DKIM verifiers, causing OpenDKIM to return a favorable authentication result for a forged or spoofed message. A public PoC/write-up exists describing the tag-parser desync, and because the vector is AV:N/PR:N/UI:N it requires no authentication or user interaction beyond delivering the message. The result is a spoofed email that passes DKIM checks and can bypass DMARC/anti-phishing filtering downstream. |
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Alert meAnalysisAIv1 – Sep 29, 2026
DKIM signature verification in OpenDKIM through 2.11.0 can be desynchronized by a crafted DKIM-Signature header, causing the tag tokenizer (dkim_process_set in dkim.c) to consume attribute values from a less trusted source than the signed data and return an attacker-influenced authentication verdict. Exploitation is remote and unauthenticated over SMTP against a standard Postfix/Sendmail milter deployment, but the impact is deliberately limited: the desync affects only the DKIM result of the single crafted message, so real damage depends on whether downstream DMARC enforcement or anti-phishing filtering blindly trusts OpenDKIM's verdict. Publicly available exploit code exists (the CVSS 4.0 vector carries E:P and the researcher published a technical write-up), there is no confirmed active exploitation in CISA KEV, and the vendor did not respond to the disclosure, so no vendor-released patch has been identified.
RemediationAI
No vendor-released patch identified at time of analysis, because the maintainers were contacted early and did not respond; there is therefore no fixed OpenDKIM version to upgrade to, and affected sites must rely on compensating controls. The most direct control is to reject or quarantine messages whose DKIM-Signature headers are malformed or ambiguous before they reach OpenDKIM, for example using Postfix header_checks/PCRE or an upstream milter to drop messages carrying more than one DKIM-Signature header or duplicate tags within a single signature (trade-off: some legitimate forwarders and mailing lists emit multiple signatures, so tuning may be required to avoid false positives). Do not treat OpenDKIM's Authentication-Results as authoritative for DMARC enforcement, anti-phishing or allow-listing while unpatched; layer an independent verification path such as Rspamd, dkimpy or the SpamAssassin DKIM plugin, or degrade DKIM results to informational until a fix lands (trade-off: additional infrastructure cost and possible divergence between verifiers). Because the exploit is public, prioritize monitoring and logging of DKIM-Signature parsing anomalies and alert on messages where the reported DKIM outcome conflicts with the visible header, so that any spoofing attempt is detected rather than silently trusted; consult https://vuldb.com/vuln/411034 and https://xuyongzhe-vt.github.io/share/opendkim-tag-parser-desync.html for the disclosure details and watch for a vendor release above 2.11.0.
Affected VersionsENISA
Affected Packages
Source: OSV (Open Source Vulnerabilities)
Affects your stack
Affects 1 package in euvd
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Technical ContextAI · show details
OpenDKIM is the reference milter-based DKIM (DomainKeys Identified Mail) signer/verifier used with MTAs such as Postfix and Sendmail. DKIM signatures are expressed as a DKIM-Signature header containing space-separated tag=value pairs (v, a, b, bh, d, h, s, etc.), which dkim.c parses and stores via routines including dkim_process_set (the 'Tag Tokenizer'). The flaw is classified as CWE-348, Use of Less Trusted Source: manipulation of the tag stream (for example duplicate tag names or multiple/conflicting DKIM-Signature headers) causes the tokenizer to select or retain a value originating from a less trusted portion of the input rather than the signed material, producing a parser/verifier desynchronization (as reflected in the published write-up's 'tag-parser-desync' title) and an unreliable authentication result. Both the assessed CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N) and the vendor CVSS 4.0 base of 5.5 (AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:L) agree on remote, unauthenticated, low-complexity exploitation with no confidentiality loss and only limited integrity and availability impact confined to the message being verified. No CPE string was supplied in the feed data; the affected component is the dkim.c tag-processing path in OpenDKIM up to and including version 2.11.0.
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Severity by source
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N/E:P
Remote unauthenticated tag-parser desync yields a limited integrity impact (forged message gains favorable DKIM verdict); no confidentiality, and no genuine availability loss.
Primary rating from Vendor (vuldb).
CVSS VectorVendor: vuldb
DescriptionCVE.org
A security flaw has been discovered in Trusted Domain Project OpenDKIM up to 2.11.0. The impacted element is the function dkim_process_set of the file dkim.c of the component Tag Tokenizer. Performing a manipulation results in use of less trusted source. The attack may be initiated remotely. The exploit has been released to the public and may be used for attacks. The vendor was contacted early about this disclosure but did not respond in any way.
Threat Intelligence Overview
NVD
CVE.org
EPSS
Vulnrichment
VulDB
How this decision was reached
The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.
Exploit-DB